participants of the dartmouth biology fsp aculty …...dartmouth faculty from the department of...

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PARTICIPANTS OF THE DARTMOUTH BIOLOGY FSP 2014

FACULTY RYAN G. CALSBEEK MATTHEW P. AYRES CELIA Y. CHEN

LAB COORDINATOR CRAIG D. LAYNE

GRADUATE ASSISTANTS JESSICA V. TROUT-HANEY VIVEK V. VENKATARAMAN

UNDERGRADUATES

ALLEGRA M. CONDIOTTE MAYA H. DEGROOTE ANN M. FAGAN NATHANAEL J. FRIDAY EMILY R. GOODWIN LARS-OLAF HOEGER ALEXA E. KRUPP

AARON J. MONDSHINE REBECCA C. NOVELLO CLAIRE B. PENDERGRAST ELLEN R. PLANE AMELIA L. RITGER ZACHARY T. WOOD

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Dartmouth Studies in Tropical Ecology, Vol. 24 (2014) Dartmouth College runs an annual 9-10 week ecological field research program in Costa Rica and the Caribbean. Manuscripts from the research projects in this program have been published in the annual volume “Dartmouth Studies in Tropical Ecology” since 1989. Copies are held in the Dartmouth library and in Costa Rica at the San Jose office of the Organization for Tropical Studies (OTS/OET), at the OTS field stations at Palo Verde, Las Cruces and La Selva, at the Cuerici Biological Station, at the Sirena Station of the Corcovado National Park, and at the Monteverde Biological Station. On Little Cayman Island, there are copies at the Little Cayman Research Center. Dartmouth faculty from the Department of Biological Sciences, along with two Ph.D. students from Dartmouth’s Ecology and Evolutionary Biology graduate program, advise ca. 15 advanced undergraduate students on this program. The order of authorship on papers is usually alphabetical or haphazard, because all authors contribute equally on projects. For each paper there is a faculty editor (indicated after the author listing), who takes responsibility for defining the revisions, and decides on the acceptability of manuscripts for publication. Graduate student Teaching Assistants are also heavily involved as mentors at every stage, from project design to final manuscript. We thank the Costa Rican Ministry of the Environment and Energy (MINAE) for permission to conduct research in Costa Rica’s extraordinary national parks. The Organization for Tropical Studies (OTS/OET) has provided essential support for our program for over 30 years, taking care of most of our logistical needs in Costa Rica, always to high standards of quality and reliability. We thank OTS staff at the Palo Verde and La Selva Biological Stations, and at the Wilson Botanical Garden at Las Cruces, for all their services rendered efficiently, politely and in good spirit. Staff at the Santa Rosa and Corcovado National Parks have also been gracious in accommodating and assisting us. We thank Carlos Solano at the Cuerici Biological Station for his depth of knowledge and inspiration. We are grateful to the staff of the Monteverde Biological Station for access to their facilities, and for making us so comfortable when we arrive late, dirty, hungry and tired from Santa Rosa. On Little Cayman Island, the Little Cayman Research Center (LCRC), operated by the Central Caribbean Marine Institute, is our base for the entire coral reef ecology segment of the program. Expert LCRC staff run the lab, provide accommodations and food, operate research vessels and take care of SCUBA diving logistics and safety. On the Dartmouth campus, the Off Campus Programs Office, under the Associate Dean of International and Interdisciplinary Studies, deals with administration and emergency services and provides an essential lifeline to remote locations in rare times of need. We are grateful for the generous financial support of the Biology Foreign Studies Program from Dorothy Hobbs Kroenlein. If you have questions about this volume or the program, contact the Biological Sciences Department at Dartmouth College, Hanover New Hampshire, USA. Currently, the Biology Foreign Studies Program Director is Mstthew Ayres at [email protected] and the administrative assistant is Sherry L. Finnemore, [email protected].

Matt Ayres

Hanover NH, USA 21 November 2014

mailto:[email protected]

mailto:[email protected]

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Schedule for Costa Rica

Date Day Location Morning Afternoon Evening

6-Jan Mon To San Jose Travel Travel Arrive in evening

7-Jan In San Jose OTS, InBIO free: shopping etc. Group dinner in SJ

8-Jan To Palo Verde Travel Orientation Lec: Intro CR ecol (RC,Zach)

9-Jan At PV Orientation Research Questions. Lec: Avian Ecol (JTH, Becca)

10-Jan At PV FIP-1 (ant-acacia) Writing clinic, Stat lab (VV) Data analysis/synthesis

11-Jan At PV FIP-2 ArthLab (JTH). Peer-review clinic FIP-1 symposium. Writing.

12-Jan Sun At PV FIP-2 Lec: DivCoex (RC,Nate ); Peer Review Writing. FIP-1 ms due

13-Jan At PV SIP-1 plan/proposals Lec: Primatesl (VV, Lexa). Data /Writ. FIP-2 seminars. Writing

14-Jan At PV SIP-1 SIP-1; Peer Review Writing. FP-2 ms due.

15-Jan At PV SIP-1 SIP-1/analysis. Revisions. SIP-1/anal. Revs.

16-Jan At PV River trip SIP-1 symposium. Writing. Peer Review Writing: SIP-1 ms due

17-Jan To Santa Rosa Travel/walk Orientation. Lec:Turtles(RC); VertLab (VV).Field: Sea turtle nesting

18-Jan At SR Lec: Mngrv(NL) Exploration Field: Sea turtle nesting

19-Jan Sun To Monteverde Travel Orientation Writing (revisions)

20-Jan At MV Orientation SIP-2 planning Lec: Behav (RC,Adam)

21-Jan At MV SIP-2 pilot /props SIP-2 Lec: His/Orig (RC,Allegra)

22-Jan At MV SIP-2 SIP-2 Analysis.Writing (rev.). Peer Review

23-Jan At MV SIP-2 Analysis/synthesis SIP-2 symposium

24-Jan At MV Writing Writing SIP-2 ms due. Bat Jngl Writing

25-Jan At MV All final mss due Exploration Free

26-Jan Sun To Cuerici Travel Travel/Orientation Lec: Coevol 1 (MA, Emily)

27-Jan At Cuerici Trip to Paramo Orientation Writing lab 1 (MA)

28-Jan At Cuerici SIP-3 planning / proposals SIP-3 pilot SIP-3 final proposals

29-Jan At Cuerici SIP-3 SIP-3 Lec: Coevol 2 (HtH, Annie)

30-Jan At Cuerici SIP-3 SIP-3 Analysis, writing

31-Jan At Cuerici Analysis, writing SIP-3 symposium Writing SIP-3 ms due

1-Feb To La Palma Exploration at Cuerici Travel to La Palma Sirena preparation

2-Feb Sun To Sirena Hike to Sirena Hike to Sirena Natural history reports from hike

3-Feb At Sirena Orientation SIP-4 plan Lec: Social insects (VV, Claire)

4-Feb At Sirena SIP-4 SIP-4 Lec: Plant-Herb (MA, Aaron)

5-Feb At Sirena SIP-4 SIP-4. Adios Hannah. Analysis

6-Feb To Las Cruces Hike out of Sirena Travel to Las Cruces Analysis. Writing

7-Feb At Las Cruces Orientation. Plant lab. Analysis. Writing SIP-4 symposium.

8-Feb At Las Cruces Writing. Botany. Writing. Botany. Writing lab 2 (MA)

9-Feb Sun At Las Cruces Writing. Botany. Writing. Botany. Writing

10-Feb At Las Cruces Writing. Botany. Writing. Botany practicum. Discussion: Why Science?

11-Feb To La Selva Travel Travel Analysis. Writing

12-Feb At La Selva Orientation SIP-5 plan/pilot Lec: Aquatic Ecology (JTH, Maya)

13-Feb At La Selva SIP-5 SIP-5 Writing. Night walk.

14-Feb At La Selva SIP-5 Agroecology field trip Lec: Ecosystems (MA, Ellen)

15-Feb At La Selva SIP-5 SIP-5 Paper (Lars). Analysis. Writing. Walk.

16-Feb Sun At La Selva SIP-5 Analysis. Writing Paper (Amelia). Analysis. Writing. Walk.

17-Feb At La Selva Analysis. Writing. Analysis. Writing SIP-5 symposium.

18-Feb At La Selva Final revisions to all CR mss. Travel to SJ Lec: Cons. Biology (MA)

19-Feb To San Jose Exploration Travel to SJ Free

20-Feb To Little Cayman until 10 March

FIP = Faculty ini tiated project; SIP = Student ini tiated project

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LITTLE CAYMAN 2014 SCHEDULE

Date Morning Afternoon Evening 20-Feb

Arrive from CR

Main orientation and safety information – Rob, Celia, Lowell, Greg

21-Feb

Orientation Get BCD and regulators assigned from Lowell Discussion: program to date and expectations for LC segment. Lecture: Coral biology lecture & Reef morphology (CC)

SCUBA –shore dive at Cumber’s Cave (check dive and snorkel) Discussion: Natural History, Queen Conch

Critique: Coral biology (Lars-Olaf Hoeger) Critique: Coral Disease (Zach Wood)

22-Feb

General natural history

Lecture: Algae and Sea & lab (CC) Preston Bay - Snorkel to see invertebrates and collect algae followed

Queen Conch Project Critique: Seagrass beds (Emily Goodwin) Discussion: Natural History

23-Feb

Queen Conch project

Queen Conch Project Queen Conch Data collection

Work on Queen Conch data analysis and write up

24-Feb

Project 1 starts

9am Queen Conch Project Presentation Lecture: Invertebrates (CC) Critique: Invertebrates (Aaron Mondshine)

Project 1 exploration

Critique: Sponges (Rebecca Novello) Project 1 idea discussion

25-Feb

SCUBA

Project 1 brainstorm Finalize project 1 idea, design, and group members Lecture: Zooplankton (JTH) Critique: Zooplankton (Allegra Condiotte)

26-Feb

Project 1 Proposal Presentation Lecture: Reef Nekton (CC) Critique: Fish ecology (Ellen Plane)

Project 1 Zooplankton lab (JTH)

Critique: Herbivory (Amelia Ritger 5PM)

27-Feb

Lecture: Fish behavior (VVV) Project 1

Project 1

Critique: Fish Behavior (Nathanael Friday) Critique: Fish Biology (Annie Fagan)

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28-Feb

Project 1 SCUBA

2:30 pm Skype with John Bothwell, Senior Research Officer, Cayman Environment Centre

Lecture: : Coral reefs and climate change (CC) Project 1 – data analysis, write methods, stats help RR: Karaoke for those interested

Date Morning Afternoon Evening 1-Mar Dia libre

(OFF)

Beach Cleanup 11AM

Relax/Cookout Lecture Conservation and Management (CC) Critique: Food webs (Adam Schneider

2-Mar **Project 1 PRESENTATIONS**

4 PM Project 1 – 1st Draft due Lecture: Lionfish and coral nursery, Katie Lohr

Critique: Coral Reef Bleaching (Claire Pendergast) Project 2 brainstorming Project 1 editing and revision

3-Mar Project 2 starts

Project 2 brainstorming and pilot

Project 2 1pm - Project 2 idea discussion

Finalize project 2 idea, design, and group members

4-Mar 8 AM Proposal 2 Presentation

Project 2 4 PM Project 2 proposal DUE

Critique: Fish and Climate Change (Maya DeGroote) SCUBA Night Dive

5-Mar Project 2

Project 2 Lecture: Mangroves (CC) Discussion: Graduate School/Career

6-Mar Project 2

Project 2 Critique: Mangroves (Lexi Krupp) Project 2 – writing and analysis **Project 2 Presentation**

7-Mar SCUBA

Project 2 **Project 2 1st draft DUE @ 12 pm**

Project 2 refine drafts RR: Karaoke for those interested

8-Mar Mardi Gras parade Project 2 refine drafts

Project 2 refine drafts

9-Mar Breakfast Brunch

Clean up science equipment/field sites Project 2 final draft due

Revisions & copy editing of all projects Individual meetings

Cookout and Bonfire

(pending wind)

Individual meetings Clean up & pack

10-Mar

Depart LC 8:20 or 10:00 am

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PAPERS FOR STUDENT PRESENTATIONS: COSTA RICA

Site Lecture Student Student Paper Staff

PV Intro CR Ecol Zach

McCain, C. 2009. Vertebrate range sizes indicate that mountains may be 'higher' in the tropics. Ecology Letters 12:550-560. (*See also: Janzen, D. H. 1967. Why mountain passes are higher in the tropics. American Naturalist 101:230-243.)

Ryan

PV Primate Ecol Lexi

Fedigan, L. M., S. D. Carnegie, and K. M. Jack. 2008. Predictors of reproductive success in female white-faced capuchins (Cebus capucinus). Amer. J. Physical Anthropology 137:82-90.

Vivek

PV Avian Ecol Becca

Botero, C. A., R. J. Rossman, L. M. Caro, L. M. Stenzler, I. J. Lovette, S. R. de Kort, and S. L. Vehrencamp. 2009. Syllable type consistency is related to age, social status and reproductive success in the tropical mockingbird. Animal Behaviour 77:701-706.

Jess

PV Behav Adam Irwin D.E. et al. 2001. Speciation in a ring. Nature 409, 333-337. Ryan

PV Diversity & Coexistence

Nate Molino, J.-F. and D. Sabatier. 2001. Tree diversity in tropical rain forests: a validation of the intermediate disturbance hypothesis. Science 294:1702-1704.

Ryan

MV History & Origins Allegra Janzen, D. H. 1981. Neotropical anachronisms: the

fruits the gomphotheres ate. Science 215:19-27. Ryan

Cuer Coevol 1 Emily

Ramirez, S. R., T. Eltz, M. K. Fujiwara, G. Gerlach, B. Goldman- Huertas, N. D. Tsutsui, and N. E. Pierce. 2011. Asynchronous diversification in a specialized plant-pollinator mutualism. Science 333:1742-1746.

Matt

Cuer Coevol 2 Annie Herre, E. 1993. Population-structure and the evolution of virulence in nematode parasites of fig wasps. Science 259:1442-1445.

Hannah

Corc Social Insects Claire

Wray, M. K., H. R. Mattila, and T. D. Seeley. 2011. Collective personalities in honeybee colonies are linked to colony fitness. Animal Behaviour 81:559-568.

Vivek

Corc Plant-Herbiv Aaron

Kursar T. A., et al. 2009. The evolution of antiherbivore defenses and their contribution to species coexistence in the tropical tree genus Inga. PNAS 106:18073-18078.

Matt

LaSelva Aquatic Ecol Maya

Pringle, C., T. Hamazaki. 1998. The role of omnivory in a neotropical stream: Separating diurnal and nocturnal effects. Ecology 79:269-280.

Jess

LaSelva Ecosystems Ellen Higgins S. I., S. Scheiter. 2012. Atmospheric CO2 forces abrupt vegetation shifts locally, but not globally. Nature 488:209-212.

Matt

LaSelva Wildcard Lars

Caldera, E. J., C. R. Currie. 2012. The population structure of antibiotic-producing bacterial symbionts of Apterostigma dentigerum ants: Impacts of coevolution and multipartite symbiosis. American Naturalist 180:604-617.

Matt

LaSelva Cons. Biology Amelia

Becker, C. G., K. R. Zamudio. 2011. Tropical amphibian populations experience higher disease risk in natural habitats. PNAS 108:9893- 9898.

Matt

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PAPERS FOR STUDENT PRESENTATIONS: LITTLE CAYMAN

Student Lecture Paper Allegra Condiotte Zooplankton Heidelberg, K. B., K. P. Sebens, and J. E. Purcell. 2004. Composition and sources of near reef zooplankton on a Jamaican forereef

along with implications for coral feeding. Coral Reefs 23:263-276. Maya DeGroote Fish biology Miller, G.M., S. Watson, S., J.M. Donelson, M.I. McCormick, P.L.

Munday. Parental environment mediates impacts of increased carbon dioxide on a coral reef fish. Nature Climate Change 2: 858-861.

Annie Fagan Fish biology Gerlach, G., J. Atema, M. J. Kingsford, K. P. Black, and V. Miller-Sims. 2007. Smelling home can prevent dispersal of reef fish larvae. Proceedings of the National Academy of Sciences of the United States of America 104:858-863.

Nathanael Friday Fish behavior Grutter, A. S., J. M. Murphy, and J. H. Choat. 2003. Cleaner fish drives local fish diversity on coral reefs. Current Biology 13:64-67.

Emily Goodwin Seagrass beds Tewfik, A., J. Rasmussen, and K. S. McCann. 2005. Anthropogenic enrichment alters a marine benthic food web. Ecology 86:2726-2736.

Lars-Olaf Hoeger Coral biology Wild, C., M. Huettel, A. Klueter, S. G. Kremb, M. Y. M. Rasheed, and B. B. Jorgensen. 2004. Coral mucus functions as an energy carrier and particle trap in the reef ecosystem. Nature 428:66-70.

Alexa Krupp Mangroves Mumby, P. J., A. J. Edwards, J. E. Arias-Gonzalez, K. C. Lindeman, P. G. Blackwell, A. Gall, M. I. Gorczynska, A. R. Harborne, C. L. Pescod, H. Renken, C. C. C. Wabnitz, and G. Llewellyn. 2004. Mangroves enhance the biomass of coral reef fish communities in the Caribbean. Nature 427:533-536.

Aaron Mondshine Invertebrates Carpenter, R. C. and P. J. Edmunds. 2006. Local and regional scale recovery of Diadema promotes recruitment of scleractinian corals. Ecology Letters 9:268-277.

Becca Novello Sponges De Goeij, J.M., D. vab Oevelen, M.J.A. Verimeij, R. Osinga, J.J. Middelburg, A.F.P.M. de Goeij, W. Admiraal. 2013. Surviving in a marine desert: the sponge loop retains resources within coral reefs. Science 342: 108-110.

Claire Pendergast Coral bleaching Grottoli, A.G., L.J. Rogriguez and J.E. Palardy. 2006. Heterotrophic plasticity and resilience in bleached corals. Nature 440: 1186-1189.

Ellen Plane

Fish ecology McMahon, K.W., M.L. Berumen, and S.R. Thorrold. Linking habitat mosaics and connectivity in a coral reef seascape. Proceedings of the National Academy of Sciences 109: 15372-15376.

Amelia Ritger Herbivory Dixson, D. L. and M. E. Hay. 2012. Corals Chemically Cue Mutualistic Fishes to Remove Competing Seaweeds. Science 338:804-807.

Adam Schneider Food webs Mumby, P. J., C. P. Dahlgren, A. R. Harborne, C. V. Kappel, F. Micheli, D. R. Brumbaugh, K. E. Holmes, J. M. Mendes, K. Broad, J. N. Sanchirico, K. Buch, S. Box, R. W. Stoffle, and A. B. Gill. 2006. Fishing, trophic cascades, and the process of grazing on coral reefs. Science 311:98-101.

Zach Wood Coral disease Patterson, K. L., J. W. Porter, K. E. Ritchie, S. W. Polson, E. Mueller, E. C. Peters, D. L. Santavy, and G. W. Smiths. 2002. The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata. Proceedings of the National Academy of Sciences of the United States of America 99:8725-8730.

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MAPS: COSTA RICA AND LITTLE CAYMAN ISLAND

http://www.nationsonline.org/oneworld/map/costa-rica-map.htm

http://www.southerncrossclub.com/cayman-islands/

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Table of Contents

Participants of the 2014 FSP

i

Note from Professor Ayres

ii

Costa Rica 2014 Schedule

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Little Cayman 2014 Schedule

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Papers for Student Presentations in Costa Rica

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Papers for Student Presentations in Little Cayman

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Maps viii

Palo Verde

The ant, Pseudomyrmex spinicola, responds to variation in reward level by the bullhorn acacia, Acacia collinsii. Maya H. DeGroote, Aaron J. Mondshine, Rebecca C. Novello, and Amelia L. Ritger

1

Forgetful Flavicornis: acacia ants do not react more vigorously to repeated attacks. Allegra M. Condiotte, Lars-Olaf Höger, Alexa E. Krupp, Ellen R. Plane, and Adam N. Schneider

4

Explaining the distribution of tropical Myrmeleontidae antlions: contributions of habitat, prey availability, and feeding dynamics. Ann M. Fagan, Nathanael J. Friday, Emily R. Goodwin, Claire B. Pendergrast, and Zachary T. Wood

6

Explaining the distribution of wetland macroinvertebrates: contributions of habitat, prey availability and feeding dynamics. Allegra M. Condiotte, Nathanael J. Friday, Aaron J. Mondshine, Adam N. Schneider, and Zachary T. Wood

9

Behavioral thermoregulation based on coloration in tropical butterflies. Maya H. DeGroote, Alexa E. Krupp, Rebecca C. Novello, Claire B. Pendergrast, and Amelia L. Ritger

12

Factors affecting vigilance behavior in the primates, Alouatta palliata

and Cebus capucinus. Ann M. Fagan, Emily R. Goodwin, Lars-Olaf 16

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Höger, and Ellen R. Plane Ontogeny and predatory efficiency in Centruroides margaritatus. Ann M. Fagan, Emily R. Goodwin, Aaron J. Mondshine, Ellen R. Plane, and Amelia L. Ritger

19

Wetland macrophyte diversity affects fish abundance, morphology, and species richness. Allegra M. Condiotte, Maya H. DeGroote, Alexa E. Krupp, Adam N. Schneider, and Zachary T. Wood

22

Patterns of nutrient distribution between soil and aboveground plant biomass in tropical forests do not hold at the understory level. Nathanael J. Friday, Lars-Olaf Höger, Rebecca C. Novello, and Claire B. Pendergrast

25

Monteverde

Orchidaceae diversity in tropical primary and secondary forest. Ann M. Fagan, Alexa E. Krupp, Aaron J. Mondshine and Amelia L. Ritger

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Thermoregulatory cost of dominance behaviors in hummingbirds. Allegra M. Condiotte, Emily R. Goodwin, Rebecca C. Novello, and Adam N. Schneider

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Optimizing leaf inclination angles: water shedding and photosynthesis in cloud forest understory plants. Maya H. DeGroote, Claire B. Pendergrast, and Ellen R. Plane

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Forest structure and epiphyte richness. Nathanael J. Friday, Lars-Olaf Höger, and Zachary T. Wood

40

Cuerici

Differential mobbing in tropical highland birds. Ann M. Fagan, Emily R. Goodwin, and Aaron J. Mondshine

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Diversity or dominance: synchronous masting as a reproductive strategy in Quercus costaricensis montane forests. Maya H. DeGroote and Claire B. Pendergrast

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Insectivorous bats alter foraging decisions based on risk of visual predation. Alexa E. Krupp, Rebecca C. Novello, and Ellen R. Plane

52

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Spatial patterning in the abundance of floral mites phoretic on hummingbirds. Lars-Olaf Höger and Zachary T. Wood

56

Female mate choice and male lekking behavior in Drosophila. Amelia L. Ritger

61

Size distribution and aggression in rainbow trout, Oncorhynchus mykiss. Allegra M. Condiotte, Nathanael J. Friday, and Adam N. Schneider

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Corcovado

Aquatic community composition of the Río Claro estuary. Allegra M. Condiotte, Nathanael J. Friday, Lars-Olaf Höger, Aaron J. Mondshine, Ellen R. Plane, and Amelia L. Ritger

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Constraints on resource transport in Atta colombica. Maya H. DeGroote, Ann M. Fagan, Emily R. Goodwin, Adam N. Schneider, and Zachary T. Wood

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The spatial dispersion of immature and adult tropical cicadas. Alexa E. Krupp, Rebecca C. Novello, and Claire B. Pendergrast

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La Selva

Spatial distribution of Lycosidae spiders within and across habitats. Alexa E. Krupp, Aaron J. Mondshine, Claire B. Pendergrast, Amelia L. Ritger, and Adam N. Schneider

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The rich get redder: delayed greening in Welfia regia palms. Allegra M. Condiotte, Ann M. Fagan, Nathanael J. Friday, Ellen R. Plane, and Zachary T. Wood

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Log preference in Lepidophyma flavimaculatum: shelter, hunting ground, or both? Maya H. DeGroote, Lars-Olaf Höger, and Rebecca C. Novello

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Behavioral thermoregulation in facultatively poikilothermic sloths (Bradypus Variegatus and Choloepus hoffmanni). Emily R. Goodwin

99

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Little Cayman

Patterns in coral assemblages of a patch reef system. Maya H. DeGroote, Emily R. Goodwin, Aaron J. Mondshine, and Ellen R. Plane

105

Grazing and epibionts in marine turtle grass beds. Allegra M. Condiotte, Nathanael J. Friday, Lars-Olaf Höger, and Zachary T. Wood

110

Burrow occupation and movement in Echinometra lucunter. Ann M. Fagan, Rebecca C. Novello and Claire B. Pendergrast

114

A. palmata colony health and size affect fish abundance, not species diversity. Alexa E. Krupp, Amelia L. Ritger, and Adam N. Schneider

120

The role of heterotypic schooling in foraging behavior of parrotfish Emily R. Goodwin and Aaron J. Mondshine

124

Salt pond minnows acclimatize to salinity. Rebecca C. Novello, Amelia L. Ritger, and Zachary T. Wood

128

Energy allocation varies throughout growth and between sexes of Pterois volitans. Maya H. DeGroote and Alexa E. Krupp

132

Impact of protected areas on spiny lobster (Panulirus argus) distribution and population structure. Allegra M. Condiotte, Nathanael J. Friday, and Claire B. Pendergrast

136

My anemone, my rules: niche partitioning in anemone communities. Lars-Olaf Höger, Ellen R. Plane, and Adam N. Schneider

143

Dartmouth Studies in Tropical Ecology 2014

1

THE ANT, PSEUDOMYRMEX SPINICOLA, RESPONDS TO VARIATION IN REWARD LEVEL BY THE BULLHORN ACACIA, ACACIA COLLINSII

MAYA H. DEGROOTE, AARON J. MONDSHINE, REBECCA C. NOVELLO AND AMELIA L. RITGER

Project Designer: Jessica Trout-Haney Faculty Editor: Ryan Calsbeek

Abstract: In the coevolution of plants and herbivores, plants must balance energy among growth, reproduction, and defense. The mutualism between the bullhorn acacia tree, Acacia collinsii, and its protective ant, Pseudomyrmex spinicola, provides a useful model to examine plant investment in defense systems. Given the acacia's significant energetic input in producing carbohydrate- and protein-rich ant rewards in the form of extrafloral nectaries and Beltian Bodies, we hypothesized that ants would be sensitive to variations in reward level. To test our hypothesis, we measured ant movement and ant abundance after applying treatments of varying artificial reward to three acacia trees. Ants were not significantly affected by changes in reward level. Although our results were non-significant they suggest that ants may be sensitive to variation in nutritional rewards by host plants. This would indicate that acacia reward levels are the product of an energetic tradeoff driven by plant-herbivore interactions. Key words: mutualism, plant defense theory, Pseudomyrmex spinicola, rewards, Vachellia collinsii INTRODUCTION In the evolutionary arms race between plants and herbivores, plants must carefully balance energetic inputs among growth, reproduction and defense (Frederickson et al 2012). Plants can adopt a variety of defense strategies including chemical defenses, such as toxins and digestibility reducers, and mechanical defenses, such as thorns and trichomes. Additionally, many plants recruit other organisms for defense against herbivores in the form of mutualisms (Bronstein 1998; Song and Feldman 2013).

One such mutualism occurs in the tropical dry forest of Central America between the bullhorn acacia tree, Acacia collinsii, and the ant, Pseudomyrmex spinicola. The tree provides ants with nutrition in the form of carbohydrate- and protein-rich extrafloral nectaries (EFNs) and Beltian Bodies, as well as shelter in the form of thorns on its stems (Wood 2005). Typically, the EFNs provide the ants with 3-11% sugar solution (Heil 2004). In return, ants provide the tree with protection against herbivorous predators (Janzen 1966; Coley and Barone 1996) by quickly responding to physical disturbances on the plant and attacking the source of disturbance (Young et al 2008). Since evolution favors those plants that are able to minimize cost and maximize defense, the acacia tree should expend energy to provide ants with rewards only if the energy investment truly increases protection by the ants (Mazancourt et al 2001). If ants were insensitive to reward

level, we would expect to see the acacia plant minimizing energy investment into reward. However, since acacias produce various nutrient- and energy-rich rewards, we hypothesized that the ants would be sensitive to variation in reward, adjusting their activity based on reward level.

To test this hypothesis, we simulated varying levels of reward by acacia plants and measured ant sensitivity to this variation. We predicted that there would be a positive relationship between reward level and both ant abundance on a leaf and ant movement to and from a leaf. METHODS We haphazardly selected three acacias of similar size and ant species in dry shady areas of an acacia grove in Palo Verde National Park, Costa Rica on January 10, 2014 at 0830. For each tree, we chose three leaves of comparable height on the main stem to receive one of three treatments: (1) high reward, (2) a control, or (3) no reward. For the high reward treatment we applied thirty droplets of a 70% sugar solution on leaflets near the base of the leaf and tapped the base several times. We used a 70% solution in order to simulate a higher potency reward than that naturally produced by EFNs. For the control treatment, we applied thirty droplets of tap water and tapped the base of the leaf in an identical manner. For the no reward treatment, we applied thirty droplets of tap water and clipped the EFNs located at the base of the each selected leaf. Tapping was included in the

Palo Verde

2

control and high reward treatments to standardize for any potential disturbance response due to clipping the EFNs in the no reward treatment. Before we applied the treatments, we recorded a baseline count of ant movement, which we defined as the number of ants that passed over the EFN in a two-minute interval. We disregarded directionality for all measurements of ant movement. After treating each leaf, we waited one hour to eliminate any immediate ant response to herbivory. After one hour we measured ant movement and ant abundance on each experimental leaf. We defined ant abundance as the number of ants present on a leaf. We used both change in ant movement and abundance after treatment as indicators of ant sensitivity to reward.

We performed ANOVAs to test differences in ant abundance and change in ant movement across treatments. We also performed a two-tailed matched-pair t-test to determine whether any treatments resulted in a significant change in ant movement. All analyses were performed using JMP® 10 statistical software. RESULTS

We found a non-significant difference in ant abundance across reward treatments (ANOVA, F2,8 = 1.93, P = 0.23). Ant abundance was greatest in the high reward treatment and lowest in the no reward treatment. We found no significant difference in change in ant movement across treatments (ANOVA, Fig. 2, F2,8 = 2.14, P = 0.20). None of the treatments resulted in a significant change in movement (two-tailed matched-pair t-

test, high reward: t2 = -1.56, P = 0.26; control: t2 = 1.89, P = 0.20; no reward: t2 = -1.56, P = 0.86). DISCUSSION We found that ants did not significantly change their movement based on host rewards. However, there was a trend towards decreased ant movement in response to high reward (Fig. 2). Decreased ant movement on high reward treatments could be due to increased sugar concentration compelling ants to remain on a leaf. Ant abundance might therefore be a better proxy for ant sensitivity than ant movement. Furthermore, change in ant movement in the no reward treatment varied greatly, suggesting there may be other consequences to mechanically removing of EFNs from the stem (e.g., a chemical response to clipping).

We found a non-significant trend in ant abundance that indicates ants are potentially sensitive to reward (Fig. 1). Ant abundance was greatest on leaves with high reward and lowest on leaves with no reward. If this trend in ant abundance is valid and ants are sensitive to alterations in reward levels, then the bullhorn acacia tree may be caught in an energetic tradeoff. This would suggest that acacias are producing an intermediate level of nutritional reward that satisfies the ants’ needs and lies within the plant’s energy budget. However, since our findings were non-significant, it is also possible that ants are not actually sensitive to variations in reward, or that they are only sensitive to repeated applications of reward for longer periods of time. Long-term

Figure 2. There was no significant difference in change in ant movement across treatments (ANOVA, F2,8 = 2.14, P = 0.20). Ant movement was calculated as the number of ants passing over the extra-floral nectary at the base of the specified leaf in either direction.


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experimental tests of ant sensitivity remain important in better understanding how energetic tradeoffs drive patterns in plant-herbivore interactions. ACKNOWLEDGEMENTS We would like to thank Jess Trout-Haney for her insight on our project. We would also like to thank

the 2014 Dartmouth Biology FSP group for peer reviewing our paper. AUTHOR CONTRIBUTIONS All authors contributed equally in the experiment and writing process.

LITERATURE CITED Coley, P.D. and J.A. Barone. 1996. Herbivory and

plant defenses in tropical forests. Annual Review of Ecology and Systematics 27: 305-335.

Bronstein, J.L. 1998. The contribution of ant-plant protection studies to our understanding of mutualism. Biotropica 30: 150-161.

de Mazancourt, C., M. Loreau, and U. Dieckmann. 2001. Can the evolution of plant defense lead to plant herbivore mutualism. The American Naturalist 158: 109-123.

Frederickson, M.E., A. Ravenscraft, G.A. Miller, L.M. Arcila Hernandez, G. Booth, and N.E. Pierce. 2012. The direct and ecological costs of an ant-plant symbiosis. The American Naturalist 179: 768-778.

Heil, M., B. Baumann, R. Krüger, and K.E. Linsenmair. 2004. Main nutrient compounds in food bodies of Mexican Acacia-ant plants. Chemoecology 14: 45-52.

Heil, M., M. Gonzalez-Teuber, L.W. Clement, S.

Kautz, M. Verhaagh, and J.C.S Bueno. 2009. Divergent investment strategies of acacia myrmecophytes and the coexistence of mutualists and exploiters. PNAS 106: 18091-18096.

Janzen, D.H. 1966. Coevolution of mutualism between ants and acacias in Central America. Evolution 20: 249-275.

Song, Z., and M.W. Feldman. 2013. Plant-animal mutualism in biological markets: evolutionary and ecological dynamics driven by non-heritable phenotypic variance. Theoretical Population Biology 88: 20-30.

Young, H., L. Fedigan, and J. Addicott. 2008. Look before Leaping: Foraging Selectivity on Capuchin Monkeys on Acacia Trees in Costa Rica. Oecologia 155: 85-92.

Wood, W.F. 2005. A comparison of mandibular gland volatiles from ants of the bull horn acacia, acacia collinsii. Biochemical Systematics and Ecology 33: 651-658.

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FORGETFUL FLAVICORNIS: ACACIA ANTS DO NOT REACT MORE VIGOROUSLY TO REPEATED ATTACKS

ALLEGRA M. CONDIOTTE, LARS-OLAF HÖGER, ALEXA E. KRUPP, ELLEN R. PLANE, AND

ADAM N. SCHNEIDER

Project Designer: Vivek Venkataraman Faculty Editor: Ryan G. Calsbeek

Abstract: Concentrated herbivore damage to one area of a plant generally has more deleterious effects on plant fitness than a comparable amount of damage dispersed throughout the plant. Using the ant-acacia defense mutualism as a model, we address the question of whether Pseudomyrex flavicornis will respond more vigorously to a second attack at the site of recent damage than to an attack at a previously undamaged site. We simulated herbivory on acacia trees to determine whether there was higher ant recruitment to branches receiving multiple attacks. We found no significant relationship between repetitive disturbance and the strength of the ant response, suggesting that acacia ants have not evolved to protect their host plant more vigorously against concentrated herbivory. Key words: Acacia collinsii, herbivory, mutualism, Pseudomyrmex flavicornis INTRODUCTION Plant fitness is often reduced more by concentrated herbivory to one area than by an equivalent amount of damage dispersed throughout the plant (Mauricio et. al 1993). It should therefore be advantageous for plants to avoid repeated damage to the same area.

In the ant-acacia mutualism, ants defend the tree against competition and herbivory and are rewarded with shelter and nourishment provided by the tree (Cronin 1996). Mechanical disturbance to Acacia collinsii induces a chemical response that recruits Pseudomyrex flavicornis to attack potential herbivores (Agrawal 1998), thus discouraging herbivory on A. collinsii.

Plants are capable of modifying their defensive strategies in response to changing external stimuli: in plants with ant mutualists, many different cues, both mechanical and chemical, result in ant recruitment and defense (Agrawal and Rutter 1998). In the ant-acacia relationship, ants could reduce the severity of a concentrated attack by increasing the number of ants responding to a recent attack site upon subsequent disturbances.

The goal of this study was to determine whether P. flavicornis protect against concentrated damage to their host plant by responding more intensely to repeat attacks. We predicted that P. flavicornis would respond more vigorously to a second attack at a site of recent damage than to an attack at a previously undamaged site.

METHODS We measured P. flavicornis activity on A. collinsii trees in Palo Verde National Park, Costa Rica on January 10, 2014 from 08:30 to 10:30.

We haphazardly chose five sites consisting of two adjacent trees. We chose one branch with similar orientation, height, and sun exposure on each tree and randomly designated one as control and one as experimental. We recorded baseline activity levels at both branches by counting the number of ants that passed a designated point 5-10 cm from the distal end of the branch in either direction for one minute.

For experimental branches in the first trial, we clipped five terminal leaves with scissors and then tapped the branch with the scissors close to the end of the branch five times to create further mechanical disturbance. At the control branches, we made five snipping noises with scissors next to the end of the branch to control for sound disturbance. Ten seconds after each treatment, we again recorded ant activity. We waited one hour to allow ant activity to return to pre-disturbance levels. Upon return, we recorded a second baseline activity level. For the second trial we carried out an identical attack on both control and experimental branches and measured ant activity as above. Statistical Analyses and Modeling

We conducted a matched pairs t-test on the first and second baseline activity levels of experimental branches. We calculated the effect of prior


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disturbance by comparing the percent change in ant response (post-attack minus second baseline) between control and treatment (previously attacked) trees. We used JMP11 software (SAS Institute, Cary, NC) to perform a 2-tailed t-test comparing the difference in means between these two groups.

RESULTS Simulated herbivory produced a significant increase in ant activity (t9 = -3.33, P = 0.01) with a mean response of 9.3 ± 1.4 (±1SE) ants moving to and from the site of disturbance. There was no

significant difference between baseline ant counts on experimental branches in the first trial compared to the second (t4 = 0.95, P = 0.40).

The percent change in ant response between branches that were previously attacked and control branches was not significant (t9 = 1.48, P = 0.18; Fig. 1). The mean number of ants responding to attack during the second trial was 21±7.4 (±1SE) for previously attacked branches and 26±10.3

(±1SE) for control branches. DISCUSSION The average number of ants responding to attacks administered during the second trial did not differ between first attack and repeat attack. Ants did not have an elevated response in previously attacked areas, suggesting that the A. collinsii and P. flavicornis association does not have a “memory” for concentrated herbivory.

Although the attacks clearly stimulated an increase in ant activity, after the hour wait between trials, activity had reverted to pre-attack levels. Therefore the second attack was not affected by lingering activity induced by the first attack. Since we observed increased activity immediately after an attack, future studies may benefit from a reduced wait time between the simulated attack and the monitoring of defensive response.

Our results suggest that in the ant-acacia mutualism, the benefits of the ants mitigating a concentrated attack do not outweigh the cost of increased defense. It is, however, unclear for which partner the energetic costs outweigh the benefits. It is also possible that the ants interpret a branch with repeated damage as a compromised resource, and that a low-resource branch is no longer worth the extra energy expense of increased defense. These findings emphasize the need for further study on the distribution of energy costs between mutualistic partners. ACKNOWLEDGEMENTS We would like to thank Vivek Venkataraman for his guidance in planning and executing this study. AUTHOR CONTRIBUTIONS All authors contributed equally in the experiment and writing process.

LITERATURE CITED Agrawal, A.A. 1998. Leaf damage and associated

cues induce aggressive ant recruitment in a neotropical ant-plant. Ecology 79: 2100-2112.

Agrawal, A.A. and Rutter, M.T. 1998. Dynamic anti-herbivore defense in ant-plants: the role of induced responses. Oikos 83: 227-236.

Conin, G. 1996. Between-species and temporal variation in Acacia-ant-herbivore interactions.

Biotropica 30: 135-139.

Janzen, D.H. 1966. Coevolution of mutualism between ants and acacias in Central America. Evolution 20: 249-275.

Mauricio, R., Bowers, M.D., and Bazzaz, F.A. 1993. Pattern of leaf damage affects fitness of the annual plant Raphanus sativus (Brassicaceae). Ecology 74: 2066-2071.

Figure 1: No significant difference in ant response was observed between control and experimental treatments.

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EXPLAINING THE DISTRIBUTION OF TROPICAL MYRMELEONTIDAE ANTLIONS: CONTRIBUTIONS OF HABITAT, PREY AVAILABILITY, AND FEEDING DYNAMICS

ANN M. FAGAN, NATHANAEL J. FRIDAY, EMILY R. GOODWIN, CLAIRE B. PENDERGRAST, AND ZACHARY T. WOOD

Project Designer: Ryan Calsbeek Faculty Editor: Ryan G. Calsbeek

Abstract: Predator distributions are generally limited by prey availability and habitat suitability. We compared the importance of prey preference and substrate type in determining the distribution of the Myrmeleontidae antlion in Palo Verde National Park. We hypothesized that substrate type was the primary driver of trap site selection, trap size, and predation success for larval antlions. To test this hypothesis, we censused antlion traps across different substrate types and experimentally assessed antlion substrate preference. We tested antlion capture success across a range of trap sizes in sandy substrate to determine trap size’s impact on prey capture success. Antlions significantly preferred sandy substrate over hard and loose soil. Antlion capture success increased with trap size, while prey species had no effect. This indicates that antlion predation is more strongly limited by suitable substrate for efficient trap construction than by prey type or abundance. Antlions and other predators who modify their environment to catch their prey are strongly limited by habitat conditions.

Key words: Myrmeleontidae, species distributions, predator-prey dynamics, burrower-soil interactions

INTRODUCTION

Species distributions are linked to resource availability and community interactions, such as predation, prey availability, and habitat type (Gatti and Farji-Brener 2002). Predatory organisms aim to optimize food intake through a variety of predation strategies including ambush and trap-building. Organisms whose predation strategies necessitate a specific habitat type may be more limited by habitat availability than prey availability.

Bull Horn Acacia (Vachellia collinsii; Seigler and Ebinger 2005) are hosts in a mutualism (Janzen 1966) with red acacia ants (Pseudomyrmex spinicola). One of the facets of the ant-acacia mutualism is the eradication of competing vegetation under the acacia trees by the ants, leaving the soil dry and hard. A common predator on ants is the larval antlion (Myrmeleontidae spp.), which captures its prey by constructing and hiding inside conical traps in the ground (Coelho 2001). Though ants are extremely abundant in acacia groves, antlion traps are rarely found there (Farji-Brener et al. 2008). This suggests antlion distribution is not primarily driven by prey availability.

As antlions are dependent on constructed traps for prey capture, the substrate in which the trap is built may affect capture success. Due to the high energetic cost of constructing traps in hard soils

(Fertin 2006), antlions build smaller traps there (Farji-Brener 2003). However, in a finer substrate such as sand, antlions can build larger and more efficient traps (Fertin and Casas 2006). Greater capture success in sandy substrates suggests that substrate type may have an effect on antlion distribution.

We tested the hypothesis that substrate type would be more important than prey availability in driving antlion distribution. We predicted that antlions would prefer sandy substrates for trap construction and that larger traps would increase prey capture success.

METHODS

Antlion Trap Census

We studied two 60 meter transects in Palo Verde National Park: one along a sandy roadside and one through a stand of V. collinsii. We counted the number of antlion traps in 10 randomly selected 1-meter square quadrats for both transects.

Capture Dynamics

To test capture success and efficiency of antlions, we collected ten P. spinicola and ten ants of other species. P. spinicola were collected from V. collinsii branches, and the other species were collected from fallen logs and tree snags. We categorized each ant by size class (small < 0.5 cm, medium 5-1.0 cm, large >1.0 cm). We then


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selected 20 antlion traps, measured their diameter, and randomly dropped either a P. spinicola or other species of ant into the trap. We recorded capture success (whether or not an antlion killed its prey) and latency to attack. Substrate Preference

We collected 16 antlions from roadside traps. We collected sandy substrate from roadsides and hard-packed soil from V. collinsii groves, which we ground into finer particles. To mimic the packed soil found under V. collinsii, we created a moldable mix of soil and water, which we then dried in the sun. Some of the dried soil was then reground and sieved to a consistency similar to the sand. In all cases, we added equal amounts of water to each substrate to avoid confounding the treatments. We then evenly divided 16 containers between pairs of substrates. Eight arenas provided the choice between sand and hard packed soil, while the other eight contained sand and ground soil. We placed one antlion in each arena on the border between the two substrates and allowed the antlions 24 hours to build traps. We recorded the substrate chosen and the trap diameter of all constructed traps.

Statistical Analyses

We used a two-tailed t-test to determine whether antlion trap densities differed across substrates in our census. We used a nominal logistic regression to assess the antlions’ preference for sand versus soil substrates. A second nominal logistic regression was used to analyze the contribution of ant size and ant species to antlion feeding success. Finally, we used a nominal logistic regression to predict antlion feeding success based on trap diameter. We used JMP 11.0 (SAS Institute, Inc. 2013) for all statistical analyses.

RESULTS

Substrate Preference

Antlions constructed traps in higher densities in sandy roadsides (mean ± SE = 1.90 ± 0.89 m-2) than in V. collinsii groves (mean ± SE = 0 ± 0 m-2) (F1,18= 4.58, P= 0.046; Figure 1). In preference experiments, antlions preferred sandy substrates (chi-square= 4.92, P= 0.026), but this preference was not significantly affected by the choices provided (chi-square= 4.31, P= 0.34, df= 2, Figure 2).

Capture Dynamics

Antlion capture success was not influenced by ant type (chi-square = 0; P = 1, df = 1), but was influenced by ant size, with larger ants more likely to escape antlion traps than smaller ants (chi-square = 12.98; P = 0.0015, df = 2; Figure 3). Ants placed in larger traps were less likely to escape (chi-square = 4.31; P = 0.038, df = 1; Figure 4), indicating that larger traps led to more successful feedings.

Figure 1. Antlion trap density was significantly higher in roadside sandy sites than hard-packed acacia soil sites (F1,18 = 4.58, P = 0.046). Error bars represent standard error.

Figure 2. Antlions prefer sand over any type of soil (chi-square = 4.92, P = 0.026, df = 1).

Figure 3. Larger ants (>1.0 cm) were more successful at escaping antlion traps than small (<0.5 cm) and medium (0.5-1.0 cm) ants (chi-square = 12.98; P = 0.0015, df = 2). Error bars represent standard error.

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Figure 4. Antlions in larger traps had higher success rates in killing ants. Heavy line represents median, box represents upper and lower quartiles, and whiskers represent range.

DISCUSSION We investigated the importance of substrate type to antlion distribution. We found a significant preference for sandy substrate in antlion trap sites (Figs. 1 and 2). Previous research has shown that traps built in sand are bigger (Fertin and Casas 2006). We found that larger traps led to more successful feedings (Figure 4). Therefore, construction in sandy soils leads to traps with greater prey capture success. This explains the greater density of antlion traps on roads than in V.

collinsii groves, despite the lower abundance of ants on roads (Crowley 1999). We observed no difference in antlion capture success between prey species, confirming that antlion success is not related to prey type. Therefore, our findings on substrate preference and capture success are consistent with our hypothesis that substrate type is more important than prey availability in driving antlion distribution.

Many predators are limited in their distribution and abundance by the quantity and variety of prey resources. However, when a predation strategy requires a specific environmental resource, this may force predator populations to forgo broader distribution opportunities. ACKNOWLEDGEMENTS We would like to thank R. Calsbeek for the initial experimental design, V. Venkataraman and J. Trout-Haney for their guidance and feedback, the students of the Dartmouth FSP 2014 for their assistance with the peer review process, and the staff of Palo Verde National Park for their support. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Burgess, M.G. 2009. Sub-optimal pit

construction in predatory antlion larvae (Myrmeleon sp.) Journal of Theoretical Biology 260: 379-385.

Coelho, J.R. 2001. The natural history and ecology of antlions (Neuroptera: Myrmeleontidae). Ex Scientia 7: 3-12.

Crowley, P.H. and M.C. Linton. 1999. Antlion foraging: Tracking prey across space and time. Ecology 80: 2271-2282.

Farji-Brener, A.G., D. Carvajal, M.G. Gei, J. Olano, J.D. Sanchez. 2008. Direct and indirect effects of soil structure on the density of an antlion larva in a tropical dry forest. Ecological Entomology 33: 183-188.

Fertin, A. and J. Casas. 2006. Efficiency of antlion trap construction. The Journal of Experimental Biology 209: 3510-3515.

Gatti, M.G. and A. G. Farji-Brener. 2002. Low density of ant lion larva (Myrmeleon crudelis) in ant-Acacia clearings: high predation risk or inadequate substrate? Biotropical 34: 458-462.

Janzen, D.H. 1966. Coevolution of Mutualism Between Ants and Acacias in Central America. Evolution 20: 246-275.

Seigler, D.S. and J.E. Ebinger. 2005. New combinations in the genus Vanchellia (Fabaceae: Mimosoideae) from the new world. Phytologia 87: 139-178.


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EXPLAINING THE DISTRIBUTION OF WETLAND MACROINVERTEBRATES: CONTRIBUTIONS OF HABITAT, PREY AVAILABILITY, AND FEEDING DYNAMICS

ALLEGRA M. CONDIOTTE, NATHANAEL J. FRIDAY, AARON J. MONDSHINE, ADAM N. SCHNEIDER,

AND ZACHARY T. WOOD

Project Designer: Jessica V. Trout-Haney Faculty Editor: Ryan G. Calsbeek

Abstract: In wetlands, macrophyte beds create a dense structure that can harbor invertebrates from predation and ultraviolet radiation. Predation pressures on invertebrates can influence their size distributions and cause mass migrations into and out of shelter. Using a tropical marsh system in Palo Verde, Costa Rica, we tested the hypothesis that invertebrates would be larger and more abundant in macrophyte-covered areas of the marsh due to a refuge effect. We also assessed whether invertebrate prey underwent horizontal diel migrations to avoid predators. We surveyed invertebrates in three microhabitats (tall macrophytes, short macrophytes, and open water) during morning, afternoon, and night. Invertebrates were more abundant under macrophyte cover, and the invertebrate predator-to-herbivore ratio was greatest in the afternoon. These results suggest that macrophytes provide a refuge for invertebrates, and that larger invertebrate predators are most active at midday. The higher prevalence of invertebrates under macrophyte cover emphasizes the importance of aquatic plant populations to marsh community functions. Key words: aquatic invertebrates, predator-herbivore interactions, horizontal migrations, macrophytes

INTRODUCTION Trophic interactions in aquatic systems can alter community dynamics and organism morphology (Skoglund et al. 2013). When predators select prey by size they can cause a shift in prey size distributions, especially when predators are much larger than their prey. When fish consume zooplankton, they typically consume the largest and most conspicuous individuals first.

Diel migrations can provide zooplankton with a spatio-temporal refuge from predation. In shallow wetlands where vertical migration is impossible, zooplankton often exhibit diel horizontal migration (DHMs). When predators (e.g., sunfish) rely on visual cues, most zooplankton are found in open water during the night and under macrophyte cover or lower in the water column during the day (Pasternak et al. 2006).

Zooplankton migrate in the opposite direction in the presence of nocturnal predators such as the phantom midge chaoborus (Minto et al. 2010). The spatial heterogeneity of herbivores caused by migrations and refugia means that grazing may be limited to specific areas at different times of day.

We tested the hypothesis that invertebrates would be more abundant and larger in macrophytic refugia. We also predicted that

invertebrates would undergo horizontal migrations based on predator presence, with zooplankton migrating from the tall macrophytes during the day into the open at night. METHODS On 11 January 2014 we surveyed a 24-meter transect crossing three microhabitats (tall macrophytes, short macrophytes, and open water) in the marsh in Palo Verde National Park, Costa Rica. We took three integrated water column zooplankton tows (vertical net sweeps through the entire water column) per microhabitat. Along the transect, the first eight meters were dominated by Thalia geniculata (height > 2 meters), followed by a second eight-meter microhabitat comprised of short macrophytes, primarily Eichornia crassipes and Salvinia auriculata. The last eight meters of each transect were in open water.

We collected samples at three times during the day: at 8:30 AM, at 4:00 PM, and at 8:30 PM. We rinsed the samples with ethanol and used a dissecting microscope to count, identify, and measure all invertebrates found in the samples. We identified the invertebrates taxonomically to family, and classified them as either a predator or a grazer accordingly.

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Statistical Analysis

We used analyses of variance (ANOVA) in R statistical analysis software to test for effects of macrophyte cover and time of day on invertebrate size and density. For all ANOVAs we used Tukey’s Honestly Significant Differences (HSD) test to analyze differences between groups post hoc. We used JMP11 software to conduct contingency analyses on predator/herbivore and plankton/non-plankton groups between microhabitats and collection times. RESULTS Invertebrates were 7.2 times more abundant under macrophytes than in open areas (Figure 1) (F2,24 = 5.89, P = 0.0083). Invertebrate densities in tall and short macrophyte areas were not significantly different (Tukeys HSD, P = 1.0). The length of aquatic invertebrates between tall and short macrophytes did not differ (Fig. 2, t90 = -0.087, P = 0.93).

The predator/herbivore ratio in all areas was highest in the afternoon (Fig. 2, 2 = 8.30, df = 2, P = 0.016). However, neither aquatic invertebrate abundance nor length changed with time of collection (F2,24 = 0.54, P = 0.59 and F2,94 = 0.76, P = 0.47 respectively; Fig. 3).

DISCUSSION

Aquatic invertebrates were more abundant in macrophyte beds than in open water. This might be the consequence of increased protection in the structurally complex plant habitat. Time of day did not influence overall aquatic invertebrate abundance. However, the relative abundance of

Figure 1. Invertebrate abundance in macrophyte-covered microhabitats was higher than in open water (F2,24 = 5.89, P = 0.0083). Invertebrate abundance between tall and short macrophyte cover was not significantly different (Tukeys HSD, P = 1.0).

Figure 2. Predator/grazer composition differed with respect to collection time (2 = 8.30, P = 0.016, df = 2). The predator:grazer ratio increased in afternoon samples.Figure 1. There was a non-significant relationship between reward level and ant abundance (ANOVA, F2,8 = 1.93, P = 0.23). Ant abundance was calculated as the number of ants on the specified leaf an hour after treatment.

Figure 3. Mean length (cm) for aquatic invertebrates did not change significantly with collection time for tall and short macrophyte cover (F2,94 = 0.76, P = 0.47).


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predators increased significantly during the afternoon.

At midday, herbivores could be more susceptible to UV radiation than predators, which are larger and have a lower surface area to volume ratio than the herbivores. Therefore, herbivores may seek shelter and leave the water column while predators remain in the water column. Fish, which typically eat invertebrate predators, may also be less active during midday, which could then allow invertebrate predators to move freely in the water column.

Higher densities of invertebrates in macrophyte beds indicate that macrophyte structures may influence essential community processes. For example, invertebrate herbivores regulate phytoplankton abundance, thereby filtering the marsh water. According to our results, most of the prey for fish and other predators live under the cover of macrophytes, demonstrating

that macrophytes shelter a major food reserve for predators at higher trophic levels and constitute an important trophic link between producers and top consumers. ACKNOWLEDGEMENTS We would like to thank Jessica Trout-Haney for her insight on our project. We would also like to thank the 2014 Dartmouth Biology FSP group for peer reviewing our paper. Additionally, we would like to thank Sergio for reassuring us that we would not be viciously mauled by voracious crocodiles. We would like to acknowledge Ryan Calsbeek for his melodic tunes, which assisted us in maintaining a stress-reduced environment conducive to thinking and reflecting on this paper. AUTHOR CONTRIBUTIONS All authors contributed equally in the experiment and writing process.

LITERATURE CITED Boeing, W.J., D.M. Leech, C.E. Williamson, S.

Cooke, and L. Torres. 2004. Damaging UV radiation and invertebrate predation: conflicting selective pressures for zooplankton vertical distribution in the water column of low DOC lakes. Oecologia 138: 603-612.

Bronmark, C. and L.A. Hansson. 2005. The Biology of Lakes and Ponds. Oxford University Press, Oxford, Britain.

Pasternak, A.F., V.N. Mikheev, and J. Wanzenbock. 2006. How plankton copepods avoid fish predation: from individual

responses to variations of the life cycle. Journal of Ichthyology 46: 220-226.

Skoglund, S., R. Knudsen, and P.A. Amundsen. 2013. Selective predation on zooplankton by pelagic Arctic charr, Salvelinus alpinus, in six subarctic lakes. Journal of Ichthyology 53: 849-855.

Minto, W.J., M.S. Arcifa, and A. Perticarrari. 2010. Experiments on the influence of

Chaoborus brasiliensis Theobald, 1901 (Diptera: Chaoboridae) on the diel vertical migration of microcrustaceans from Lake Monte Alegre, Brazil. Brazilian Journal of Biology 70: 25-35.

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BEHAVIORAL THERMOREGULATION BASED ON COLORATION IN TROPICAL BUTTERFLIES

MAYA H. DEGROOTE, ALEXA E. KRUPP, REBECCA C. NOVELLO, CLAIRE B. PENDERGRAST, AND AMELIA L. RITGER

Project Designer: Ryan Calsbeek Faculty Editor: Ryan Calsbeek

Abstract: Ectotherms acquire heat from environmental sources rather than from internal metabolic processes. Variable ambient temperatures require ectotherms such as butterflies to use behavioral mechanisms to regulate their body temperatures. In the tropical dry forest, sun and shade habitats are a major source of thermal variation. We predicted that light and dark butterflies would differ in their distribution across light conditions and use of postural mechanisms because of their varying heat absorption capacities. To test this hypothesis, we performed experimental and observational studies of body temperature and behavioral differences between light and dark butterflies. Our results showed that dark butterflies spent less time in the sun and closed their wings more often than light butterflies, suggesting that dark phenotypes depend more strongly on behavioral cooling mechanisms. Such differences may have important implications for the foraging capacities of butterflies as well as other ectotherms. Key words: Behavior, Posture, Thermoregulation, Tropical butterflies INTRODUCTION Ectotherms acquire heat from their environment and are therefore highly sensitive to variation in ambient temperature (Van Dyck 1997). Butterflies are ectotherms found across a large latitudinal gradient, which requires that they tolerate a broad range of thermal conditions. To maximize time available for foraging and mating, butterflies employ behavioral strategies to maintain a body temperature within the relatively narrow range (33-38˚C) necessary for flight (Huey and Kingsolver 1989). Butterfly wings absorb solar irradiance and transfer heat to the thorax through small veins. Two thermoregulatory behaviors commonly observed in butterflies are movement between shade and sun and wing position adjustment – opening wings to maximize absorbance of solar energy or closing wings to minimize absorbance and facilitate cooling (Heinrich 1972).

Tropical butterflies have developed a vast array of wing shapes and colorations to serve diverse purposes such as camouflage, mate attraction, and aposematic display (Basset et al. 2011). Despite such variation in morphology, optimal butterfly body temperature is consistent across species (Heinrich 1972). This pattern suggests that butterflies may achieve stable body temperatures by spatially distributing across various light conditions (e.g., sun versus shade) and changing thermoregulatory behaviors as a function of body coloration (Clench 1966).

We test the hypothesis that light and dark butterflies behaviorally thermoregulate in different ways. Specifically, based on the differing heat absorption capacities of light and dark structural colors, we predicted that light butterflies would spend relatively more time in the sun, while dark butterflies would spend relatively more time in the shade (Ball 2012). Moreover, we predicted that light butterflies would spend relatively more time with their wings open, while dark butterflies would spend more time with their wings closed. METHODS We conducted all research between 08:00 and 12:00 on January 11 and 12, 2014 in the tropical dry forest of Palo Verde National Park, Costa Rica. We classified Palo Verde butterfly species as light or dark based on the dominant shade of their wing pattern. Body temperature experiment We collected four light and four dark butterflies, asphyxiated the butterflies with acetone, and mounted them on a Styrofoam board using dissection pins. We placed the mounted butterflies in direct sunlight for eight minutes and recorded their temperatures with a Raytek Raynger MX temperature gun at two-minute intervals. We then placed them in the shade for eight minutes, repeating temperature measurements. We used these data to verify our underlying assumption that butterflies have higher body temperatures in the


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sun than in the shade irrespective of color. Given that our dataset was made up of multiple estimates of body temperature over time, we began our analyses using Repeated Measures ANOVA with each measure of body temperature as a set of dependent variables, and butterfly coloration and light treatment as independent variables. However, because there was no significant effect of the time component in the RMANOVA, we simplified all analyses and calculated a mean value for body temperature, then used this as the dependent variable in a two way ANOVA. We included butterfly coloration and light treatment to test for the effect of each variable on body temperature, and we tested for a butterfly-coloration light treatment interaction. Body temperature observational study We measured the body temperatures of stationary butterflies found in either the sun (n = 17) or shade (n = 26) using the Raytek temperature gun. We recorded and averaged three ambient temperature readings less than 25 cm from the individual. We then compared body temperature patterns between our observational and experimental studies. Spatial thermoregulation observational study To determine whether butterflies were spatially distributed based on color, we observed the total number of light and dark butterflies in sun (n = 155) and shade (n = 95) on both days of sampling.

Observers spent equal time in an open field and under a forested canopy, recording the color and light condition of all butterflies seen. We compared the proportions of time light and dark butterflies spent in each light condition using a chi-square test. Postural thermoregulation observational study To determine whether butterflies use changes in body posture to thermoregulate, we measured wing position of stationary butterflies and recorded ambient temperature with an Onset Hobo Pendant temperature logger between 08:00 and 12:00 on January 12, 2014. We defined wing positioning as either open, with wings touching dorsally, or closed, with wings outstretched laterally. We compared wing positions of light and dark butterflies throughout the day using a chi-square test. All analyses were performed using Microsoft® Excel and JMP® 10 statistical software. RESULTS Body temperature experiment All butterflies, regardless of wing color, were significantly warmer in the sun than in the shade (mean Tb (sun) = 48.2C (1.46) vs. mean Tb (shade) = 32.85C (0.16); ANOVA, F1,13 = 186.63, p < 0.0001; Fig. 1A). The two factor ANOVA revealed a significant interaction between butterfly coloration and light treatment (t3,12 = -2.52,

Figure 1. (A) There was a significant interaction between butterfly coloration and light treatment (t3,12 = -2.52, P = 0.02) indicating that although there was no difference in butterfly temperature in the shade (32.94°C ±0.95) dark versus 32.76°C ± 0.95 light), dark butterflies had warmer body temperatures in the sun (50.7°C ±0.95) relative to light butterflies (45.75°C ± 0.95). (B) Field observations showed that body temperature did not differ between light and dark butterflies regardless of light condition (ANOVA, F3,44 = 1.27, P = 0.30). Dashed lines indicate the optimal range of body temperatures for butterfly flight (33-38˚C).

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P=0.02) indicating that although there was no difference in butterfly temperature in the shade (32.94°C ±0.95 dark versus 32.76°C ±0.95 light), in the sun, dark butterflies had warmer body temperatures (50.7°C ±0.95) compared to light butterflies (45.75°C ±0.95). Body temperatures observational study Field observations showed that body temperature of butterflies did not differ in terms of either coloration or light condition (ANOVA, F3,44=1.27, P=0.30, Fig. 1B). Spatial thermoregulation observation study Light butterflies were more commonly found in the sun, and dark butterflies were more commonly found in the shade (2=29.071, df=1, P<0.0001, Fig. 2).

Postural thermoregulation observational study Wing posture differed significantly between stationary dark and light butterflies (P=0.002, Fig. 3) but did not vary by temperature (P=0.66) or time (P=0.28). All stationary dark butterflies had closed wings, whereas light butterflies varied in their wing posture (Fig. 3). DISCUSSION We have shown that the thermal environment predicts the spatial distribution and postural behaviors of butterflies based on wing color. We observed a difference between butterfly

temperatures in the sun and shade treatments of the experiment (Fig. 1A) but not in our field observations (Fig. 1B). This suggests that butterfly behavior influences their body temperature. In the field, butterfly body temperature remained near the ideal metabolic range (Fig. 1B), while in the experiment, butterflies in the sun reached lethal temperatures (Fig. 1A).

We also found that light butterflies were significantly more likely than dark butterflies to be found in the sun (Fig. 2). This suggests that butterfly coloration influences spatial distribution. Wing posture also differed between dark and light butterflies, supporting the hypothesis that heat absorption capacity motivates posturing behavior.

Our findings suggest that dark tropical butterflies have a physiological disadvantage in the sun due to their susceptibility to overheating. This cost is mitigated by behavioral strategies such as spending more time in the shade. Although dark coloration is costly in terms of thermoregulation, it may be advantageous in terms of camouflage (Sun et al 2013).

Tradeoffs in coloration and thermoregulatory behavioral strategies may also affect foraging capacity, a major determinant of butterfly fitness. By extending the number of hours during which butterflies can forage, thermoregulatory strategies offer significant fitness gains (Clench 1966). Morphological adaptations and behavioral thermoregulation allow butterflies and other ectotherms to tolerate a diversity of habitats and environmental conditions.

Figure 2. Light butterflies were significantly more likely to be found in sunny areas, and dark butterflies were more likely to be found in shady areas (2=29.07, df=1, P<0.0001, df=1).

Figure 3. Wing posture was significantly different between stationary dark and light butterflies (2=9.92, P=0.0016, df=1). Dark butterflies were never observed with open wings.


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ACKNOWLEDGEMENTS We would like to thank Jessica Trout-Haney and Ryan Calsbeek for their help on statistical methods and insight into the delicate world of the butterfly. We would also like to thank the 2014 Dartmouth Biology FSP group or peer reviewing our paper. We would like to acknowledge the butterflies of

Palo Verde National Station for being very understanding of the sacrifice of their friends in the name of science. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Basset, Y., R. Eastwood, S. Legi, D. Lohman, V.

Novotny, T. Treuer, S. Miller, G. Weiblen, N. Pierce, S. Bunyavejchewin, W. Sakchoowong, P. Kongnoo and M. Osorio-Arenas. 2011. Comparison of rainforest butterfly assemblages across three biogeographical regions using standardized protocols. Journal of Research on the Lepidoptera 44: 17-28.

Ball, Philip (May 2012). Nature's Color Tricks. Scientific American 306: 74–79.

Clench, H.K. 1966. Behavioral Thermoregulation in Butterflies. Ecology 46: 1021-1034.

Heinrich, B. 1972. Thoracic Temperatures of Butterflies in the Field Near the Equator. Comparative Biochemical Physiology 43: 459- 467.

Kingsolver, J.G. 1985. Butterfly

Thermoregulation: Organismic Mechanisms and Population Consequences. Journal of Research on the Lepidoptera 24: 1-20.

Schowalter, T.D. 2009. Insect Ecology: An Ecosystem Approach. Elselvier, Burlington, MA.

Sun, J.Y., B. Bhushan, J. Tong. 2013. Structural coloration in nature. RSC Advances 35: 14862-14889.

Wasserthal, L.T. 1975. The Role of Butterfly Wings in the Regulation of Body Temperature. Journal of Insect Physiology 21: 1921-1930.

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FACTORS AFFECTING VIGILANCE BEHAVIOR IN THE PRIMATES

ALOUATTA PALLIATA AND CEBUS CAPUCINUS

ANN M. FAGAN, EMILY R. GOODWIN, LARS-OLAF HÖGER, AND ELLEN R. PLANE

Project Design: Vivek V. Venkataraman Faculty editor: Ryan G. Calsbeek

Abstract: Animals use vigilance behaviors to detect potential threats and monitor social dynamics. According to the energy input-vigilance tradeoff hypothesis, energy intake is negatively correlated with vigilance because vigilance and feeding cannot be performed simultaneously. In larger groups, shared responsibility for predator detection tends to reduce per capita vigilance. However, it is unclear whether these trends should hold for primates because of their upright feeding behavior, manual dexterity, and complex social interactions. We monitored feeding and vigilance activity of Alouatta palliata and Cebus capucinus in Palo Verde National Park, Costa Rica. We tested the hypothesis that greater manual dexterity in C. capucinus would allow for more compatibility in vigilance and feeding than in A. palliata. Additionally, we predicted that social monitoring within primate groups would be a stronger driver of vigilance than predation risk. We found no evidence for an energy intake-vigilance tradeoff in either capuchin or howler monkeys. Individual vigilance increased with group size in capuchins, but not in howlers, suggesting that social monitoring is more important for capuchins. Our results suggest that feeding adaptations and social organization influence the allocation of vigilance in primates. Key words: Alouatta palliata, Cebus capucinus, energy input-vigilance tradeoff, group size effect on vigilance INTRODUCTION Vigilance behaviors allow animals to gauge predation risk and assess social dynamics. Vigilance, however, conflicts with other essential behaviors such as eating and resting; this tradeoff between energy maximization and vigilance is documented in multiple taxa (Houston et al. 1993). Group-living animals share responsibility for predator detection to reduce per capita energy spent on vigilance. In many species of birds and mammals, per capita vigilance decreases with increasing group size, a phenomenon known as the group size effect (Elgar 1989). However, in primates, evidence for both the energy input-vigilance (EI-V) tradeoff and the group size effect on vigilance is ambiguous. One explanation is that primates have the ability to feed upright and collect and process food with their hands, allowing them to simultaneously feed and monitor their surroundings (Treves 2000). The absence of the group size effect on vigilance in some free-ranging primates (Treves 2000), however, may be due to the importance of vigilance in social monitoring, or looking around at other members of the group (Hirsch 2002).

Free-ranging groups of both white-faced capuchins Cebus capucinus and mantled howlers Alouatta palliata inhabit the tropical dry forest of

Palo Verde National Park, Costa Rica. As highly dexterous frugivores/insectivores, capuchins feed primarily by picking up and holding food items (Tomblin & Cranford 1994). Folivorous howler monkeys are less dexterous, pulling branches to themselves and eating directly from the branch (Tomblin & Cranford 1994). Increased manual dexterity in capuchins may enhance their ability to simultaneously feed and observe their surroundings.

Because of this difference in feeding modes, we investigated the hypothesis that the EI-V tradeoff would be less pronounced in capuchins than in howlers. Additionally, we expected that increased conspecific vigilance through social monitoring among larger groups of primates would result in increased vigilance with group size. METHODS We monitored A. palliata and C. capucinus feeding activity in Palo Verde National Park, Costa Rica over a three day period from 10-13 January, 2014 during the hours of 06:00-12:00. Once a group was located, we recorded group size and, when possible, noted each individual as male, female, or juvenile (sex indistinguishable).


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We began opportunistic focal sampling (Altmann 1974) of feeding bouts and continued until view of the monkey’s face became obstructed, feeding activity terminated, or 180 seconds elapsed. We recorded “looks,” “scans,” and “bites” taken by monkeys. We defined both looks and scans as a change in head position; a “look” was an eye movement directed upwards or away for less than two seconds, whereas a “scan” was a sweeping gaze lasting more than two seconds. We recorded a “bite” when an individual placed food in its mouth or brought its mouth to a food source. One observer monitored behavior and dictated looks, bites, and scans to a second person, who recorded them. All behaviors were converted to rates. We stopped monitoring a group when monkeys were no longer feeding, a group fled, monkeys became obscured from view indefinitely, or we had monitored an individual for three feeding bouts. Statistical Analyses We performed linear regression to test the relationship between feeding rate (bites per second) to vigilance (looks plus scans per second). We also used linear regression to determine whether group size was correlated with vigilance behaviors per second. Group size was defined as the number of adults in each group. All statistical analyses were performed using JMP 11.0 (SAS Institute, Cary, NC). RESULTS Energy Input - Vigilance Tradeoff There was no significant relationship between feeding rates and vigilance rates in either capuchin groups (slope= 0.142 ± 0.06, P = 0.37, r2 = 0.04; Fig. 1) or howler groups (slope = 0.097 ± 0.03, P = 0.50, r2 = 0.02; Fig. 1). Group Size Effect on Vigilance As group size increased, capuchins showed higher rates of vigilance (slope = 0.016 ± 0.06, P = 0.05, r 2 = 0.19; Fig. 2), whereas vigilance rates of howler monkeys did not vary with group size (slope = 0.004 ± 0.02, P = 0.25, r2 = 0.25; Fig. 2).

Figure 1. Energy input was not significantly correlated with vigilance in either capuchins or howlers.

Figure 2. Vigilance increased significantly with group size in capuchins but not in howlers. DISCUSSION Differences in feeding behavior between capuchins and howlers do not appear to affect the EI-V tradeoff (Fig. 1). We found no evidence of an EI-V tradeoff in either species. This result is consistent with previous findings that feeding and vigilance are compatible in primates (Treves 2000). The presence of grasping hands in all primates, rather than interspecific differences in feeding behavior, may explain the lack of an EI-V tradeoff. We observed the importance of grasping hands in individuals of both species, who appeared to coordinate looks and scans with bites. In many other taxa (e.g. ungulates), the predominant use of the head/neck in food harvesting limits the ability to perform other behaviors simultaneously.

Capuchin group size was positively correlated with vigilance, but this relationship did not hold for howlers. Our data support the theory that in

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some group-living animals, vigilance is primarily conspecific (Hirsch 2002). Howler monkeys generally live in groups with a single-male harem structure (Wang and Milton 2003), while capuchins often form multi-male multi-female groups (Rose and Fedigan 1995). The presence of multiple males within a group could explain why we observed increased vigilance with group size in capuchins but not in howlers. Our results support the concept that group composition and social organization influence vigilance allocation.

This study provides evidence that the grasping hands of primates allow them to avoid sacrificing vigilance while feeding. Because the complex

social structures of primate groups require heightened conspecific vigilance relative to other taxa, primate multitasking is especially beneficial. ACKNOWLEDGEMENTS We would like to thank V. Venkataraman for the project concept, the students of the Dartmouth FSP 2014 for their help during the peer review process, the staff of Palo Verde National Park for their support, and the monkeys for cooperating. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Altmann, J. 1974. Observational study of

behavior: sampling methods. Behavior 49: 227- 267.

Elgar, M.A. 1989. Predator vigilance and group size in mammals and birds: a critical review of the empirical evidence. Biological Reviews 64: 13-33.

Hirsch, B.T. 2002. Social monitoring and vigilance behavior in brown capuchin monkeys (Cebus apella). Behavioral Ecology and Sociobiology 52: 458-464.

Houston, A.I., McNamara, J.M., and J.M.C., Hutchinson. 1993. General results concerning the trade-off between gaining energy and avoiding predation.

Philosophical Transactions: Biological Sciences 341: 375-397.

Rose, L.M., and Fedigan, L.M. 1995. Vigilance in white-faced capuchins, Cebus capucinus, in Costa Rica. Animal Behaviour 45: 63-70.

Tomblin, D.C., and Cranford, J.A. 1994. Ecological niche differences between Alouatta palliata and Cebus capucinus comparing feeding modes, branch use, and diet. Primates 35: 265-274.

Treves, A. 2000. Theory and methods in studies of vigilance and aggregation. Animal Behaviour 60: 711-722.

http://www.researchgate.net/journal/0340-5443_Behavioral_Ecology_and_Sociobiology

http://www.researchgate.net/journal/0340-5443_Behavioral_Ecology_and_Sociobiology


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ONTOGENY AND PREDATORY EFFICIENCY IN CENTRUROIDES MARGARITATUS

ANN M. FAGAN, EMILY R. GOODWIN, AARON J. MONDSHINE,

ELLEN R. PLANE, AND AMELIA L. RITGER

Faculty editor: R. Calsbeek

Abstract: Ontogenetic changes in morphology and predation strategies can impact how well predators capture their prey. In some species of scorpions, prey capture strategies change with body size. We hypothesized that scorpion age would be positively correlated with capture efficiency and negatively correlated with use of stingers, since older scorpions are more experienced and have larger chelae. Using body size as a proxy for age, we explored changes in predation strategies and capture efficiency with respect to age in the scorpion Centruroides margaritatus. We found a positive relationship between age and capture efficiency, although this finding may have been confounded by prey size. We found no relationship between age and use of stinger. This study reveals the need for further research on scorpion development and its effect on predation efficiency and strategy. Key words: ontogenetic variation, capture efficiency, predatory strategy, Centruroides margaritatus INTRODUCTION Adult animals are generally more successful predators than juveniles. This is often attributed to (1) morphological differences and (2) ontogenetic changes in prey capture strategy (Ringler 1983). As animal body size and age increase, foraging and predation efficiency also increase through either enhanced searching and hunting efficiency, or through increased prey handling efficiency (Mittelbach 1981, Breitwisch 1987, Ehlinger 1990).

Bark scorpions (Centruroides spp.) exhibit direct development, meaning scorplings have adult body proportions throughout growth (McReynolds 2012). Therefore, size is an appropriate proxy for age (Polis and McCormick 1986, McReynolds 2012).

Ontogenetic niche shifts in predatory behavior have been documented in other scorpions (Casper 1985, McReynolds 2012). Younger scorpions paralyze prey by stinging while older individuals are able to restrain their prey with chelae alone (Casper 1985; Figure 1). When older scorpions restrict stinger use, they conserve energy that would have been spent on venom production (Rein 1993). This behavioral change indicates that older scorpions should be more efficient predators.

We hypothesized that the Central American bark scorpion C. margaritatus would show ontogenetic variation in both predatory behavior and morphology. Specifically, we predicted that

older scorpions would (1) have higher overall prey capture efficiency because of their larger body size and (2) crush prey in their chelae instead of using their stingers to maximize predation success while minimizing energy input. METHODS Scorpion & Prey Collection We collected 15 scorpions and 15 crickets on the nights of 13-15 January 2014 in Palo Verde National Park, Costa Rica by searching roads and building perimeters after sunset. We placed the scorpions in individual containers, provided them with water, and starved them for 14-42 hours. We kept the prey in a separate container. We weighed each prey item and each scorpion prior to conducting our trials. Predation Trials We conducted predation trials in a dark area covered in black fabric to reduce ambient light. Scorpions and prey items were ranked from smallest to largest and paired accordingly to ensure that all scorpions received proportionally sized prey items. One scorpion container was placed in the trial area, and the trial began when the prey item was dropped into the container. A “strike” was defined as a rapid movement of the scorpion’s pedipalps towards its prey. “Capture” was defined as the final strike, after which the prey did not escape from the scorpion. We also

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recorded the total number of strikes, the total number of stings (which always occurred after the prey had been captured), and the cumulative time from the beginning of the trial to the time of prey capture. We used only red lighting inside the trial area to avoid confounding effects due to scorpion phototaxis.

Figure 1. Image of C. margaritatus showing measured morphological traits. Morphological Measurements After the predation trials, we sacrificed all scorpions by asphyxiating them with acetone. We then sexed each scorpion and measured carapace length of each individual (Figure 1). Statistical Analyses We used a linear regression to test for a relationship between carapace length and capture efficiency. We defined capture efficiency as the ratio of capture success (a binary variable scored 0 or 1) to the number of strikes. To test for a sex effect on size, we used a two-tailed t-test on mean carapace length by sex. We also used a linear regression to examine the relationship between prey mass and capture efficiency. We used the residuals from this test to conduct a second linear regression comparing carapace length and residual capture efficiency.

To test our hypothesis regarding age and stinger use, we only used trials where scorpions struck at least once, assigning a value of 1 to scorpions that successfully captured their prey using a sting and a value of 0 to those that captured prey with no sting. We then conducted a two-tailed t-test to compare mean carapace length between scorpions that stung and scorpions that did not sting. All statistical analyses were conducted using JMP 11.0 (SAS Institute, Cary, NC).

RESULTS Carapace length and capture efficiency were positively correlated (slope = 0.029 ± 0.16, P = 0.01, r2 = 0.55, Fig. 2). Carapace length did not differ between the sexes (t = 0.66, P = 0.52, df = 24). There was a significant positive correlation between prey mass and capture efficiency (slope = 2.49 ± 0.16, P = 0.01, r2 = 0.56). We found no relationship between carapace length and residual capture efficiency controlled for prey mass.

There was no significant difference in carapace length between individuals that stung their prey compared to those that did not (t = 0.17, P = 0.87, df = 6, Fig. 2).

Figure 2. Capture efficiency increased with scorpion carapace length (slope = 0.029 ± 0.16, P = 0.01, r2 = 0.55). We defined capture efficiency as capture success divided by the number of attempts (0 = no capture success, 1 = capture success).

Figure 3. There was no difference in mean carapace length between scorpions that stung prey and scorpions that did not sting (t = 0.17, P= 0.87, df = 6). DISCUSSION Age was not correlated with increased capture efficiency or decreased stinger use in C. margaritatus. There was no relationship between

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scorpion size and capture efficiency when controlling for prey mass. Additionally, we did not observe a significant relationship between predation strategy and scorpion age (Fig. 2). This result suggests that both young and old scorpions use their stingers to subdue their prey upon capture. This finding contradicts previous research indicating that older scorpions crush prey in their large chelae and use their stingers less frequently (Casper 1985) to conserve energetically costly venom (Nisani et al 2007)

A major caveat of the study is the likely presence of a confounding effect of prey size on both hypotheses, making it difficult to determine the relationship between age, capture efficiency, and predation strategy. Although we saw a positive correlation between body size and capture efficiency, this could be because larger crickets were easier to catch. In addition, a consistently high prey to scorpion ratio may have compelled

both young and old scorpions to use their stingers. Our results do not support the hypothesis that

C. margaritatus exhibits ontogenetic variation in morphology and predatory behavior. Such species-specific ontogenetic complexities require further exploration. ACKNOWLEDGEMENTS We would like to thank Sergio Padilla of the Organization for Tropical Studies for his advice on capturing and handling scorpions, the park staff for assisting us in capturing scorpions and J. Trout-Haney for her help in conducting predation trials. We would especially like to acknowledge R. Calsbeek for his valued advice on acceptable methods of randomization. Thanks for nothing. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Casper, G.S. 1985. Prey capture and stinging

behavior in the Emperor scorpion, Pandinus imperator (Koch) (Scorpiones, Scorpionidae). The Journal of Arachnology 13: 277-28.

Ehlinger, T.J. Habitat choice and phenotype- limited feeding efficiency in bluegill: individual differences and trophic polymorphism. Ecology 71: 886-896.

McReynolds, C.N. 2012. Ontogenetic shifts in microhabitat use, foraging and temporal activity for the striped bark scorpion Centruroides vittatus (Scorpiones: Buthidae). Euscorpius 144: 1-19.

Mittelbach, G.G. 1981. Foraging efficiency and body size: A study of optimal diet and habitat use by bluegills. Ecology 62: 1370–1386.

Nisani, Z., Dunbar, S.G., and Hayes, W.K.

2007. Cost of venom regeneration in Parabuthus transvaalicus (Arachnida: Buthidae). Comparative Biochemistry and Physiology Part A: Molecular and Integrative Physiology 147: 509-513.

Polis, G.A., and McCormick, S.J. Patterns of resource use and age structure among species of desert scorpion. Journal of Animal Ecology 55: 59-73.

Rein, J.O. 1993. Sting use in two species of Parabuthus scorpions (Buthidae). The Journal of Arachnology 21: 60-63.

Ringler, N.H. 1983. Variation in foraging tactics in fishes. Pages 159-171 in Predators and prey in fishes. Dr W. Junk. The Netherlands.

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WETLAND MACROPHYTE DIVERSITY AFFECTS FISH ABUNDANCE, MORPHOLOGY, AND RICHNESS

ALLEGRA M. CONDIOTTE, MAYA H. DEGROOTE, ALEXA E. KRUPP, ADAM N. SCHNEIDER AND ZACHARY

T. WOOD

Faculty Editor: Ryan Calsbeek Abstract: Structural complexity and heterogeneity can create diverse ecological niches. Habitat heterogeneity can explain localized species richness, as it provides a greater variety of niches for species to occupy. We tested whether edge habitats between macrophyte beds and open water would foster greater fish species richness in a marsh in Palo Verde National Park, Costa Rica. Fish species richness was highest at the edges of the macrophyte beds where the two habitats converge. We found the greatest fish abundance outside the macrophyte beds, and the largest fish within the beds. Our findings suggest that edge habitats are areas of higher habitat heterogeneity that enable increased species richness. Keywords: macrophytes, habitat diversity, species richness, tropical freshwater fish INTRODUCTION Habitat heterogeneity and structural complexity play a large role in shaping biological communities. Heterogeneity is an established explanation for species richness, as it enables species to specialize on particular niches (Rosenzweig 1992).

In aquatic environments, structural complexity facilitates higher fish species richness, as demonstrated in temperate marine rocky shore systems (Eriksson et al. 2006). Marsh plants also typically foster higher densities of invertebrate prey and limit predator accessibility (Condiotte et al. 2014). However, macrophytes can also limit fish movement and cause diel hypoxia. Emergent plants limit dissolved oxygen levels by drawing oxygen from the water column back into the atmosphere during respiration and decreasing mixing, thus reducing surface gas exchange (Valk 2012). Such tradeoffs may structure fish communities in the marsh ecosystem.

Our study addresses how structural complexity and heterogeneity influence fish biodiversity. We compared fish species richness, abundance, and size in a tropical marsh in three different microhabitats: open water, macrophyte edge, and complete macrophyte cover. We predicted that the edge habitat would harbor the highest fish species richness as a consequence of increased structural heterogeneity. We also expected fish abundance to be highest in the edge habitat, and fish size to be smallest in complete macrophyte cover due to the difficulty of maneuvering in areas with high

structural complexity. METHODS On January 14 and 15, 2014, between 8:30 and 10:00, we haphazardly placed six to eight minnow traps in each of three Palo Verde wetland microhabitats: areas lacking plants that were greater than a meter from the nearest macrophyte beds (open), habitats on the borders of the macrophytes (edge), and habitats fully enclosed by macrophytes that were over a meter from the edges (covered). We recorded water depth and macrophyte height at each site. Dominant macrophytes included Salvinia, Thalia geniculata, Neptunia natans, Cyperaceae Eleocharis, Ceratophyllum muricatum, Pistia stratiotes, and Eichhornia spp. We collected traps between 16:00 -17:30 and recorded fish species and size. Statistical Analyses and Modeling We used ANCOVA and ANOVA to examine effects of habitat and species on fish length, and of habitat type on abundance. We used a post hoc Tukey's HSD Test to compare ANOVA group means. All analyses were performed in R statistical analysis software. RESULTS We found four species of fish in edge habitats and three species of fish in both open water and macrophyte-covered areas. Banded tetras were dominant in open and edge habitats, and cichlids were dominant in the covered areas (Figure 1).


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Fish density was significantly affected by habitat type (Figure 2, ANOVA, F2,18 = 5.58, P =

0.013). Open and edge habitats had higher densities than the covered areas (p < 0.03 for both, Tukey’s HSD). There was no significant difference in fish density between open and edge habitats (P = 0.94, Tukey’s HSD test).

Fish were larger in macrophyte beds than in either the open water or edge area (Figure 3; ANOVA, F2,88 = 5.39, P = 0.0034). This habitat-only effect became nonsignificant when we considered the effects of species and the interaction of species with habitat (Table 1), indicating that size differences are likely caused

by naturally larger fish species residing in macrophytes (i.e., cichlids, Table 2).

DISCUSSION As predicted, we found higher fish species richness at the edge of the macrophyte zone than in either open or covered areas. It was impossible to compare fish species per trap across habitat site due to rarefaction limitations (i.e. some traps had very few species).

Contrary to our hypothesis, mean fish length was larger under complete macrophyte cover. This difference was mainly attributed to a species effect, as relatively large cichlids were dominant in the macrophyte beds (Fig. 3, Table 2). Both cichlid species in the marsh have laterally compressed bodies, which are optimal for navigating macrophytic habitats compared to the other fishes’ cylindrical body forms (Webb 1984).

We caught more fish in the open and edge habitats than under complete macrophyte cover. Although macrophyte beds offer increased food and protection, it appears the costs of daily hypoxia and high structural complexity in macrophytes outweigh the benefits for most fishes, thereby inhibiting their presence (Valk 2012, Fig. 2).

There are tradeoffs for organisms living in any habitat; by living at the intersection of several microhabitats, organisms may reap the benefits of multiple niches, allowing them to minimize tradeoff costs. Those organisms whose ideal

Table 1. ANCOVA results for fish length based on species and habitat. Most of the variation in length was explained by species in this model.

Effect df Mean sq F p

Species 4 6.108 8.467 <0.0001 Habitat 2 0.342 0.475 0.62 Species * Habitat 3 1.827 2.533 0.063 Residuals 81 0.721

Table 2. Mean size of fish species caught in minnow traps from open, edge, and covered sites.

Fish Common name Mean Length (cm) Standard Error

Rivulus uroflammeas K 4.55 1.55

Brachyrhaphis olomina L 4.42 0.286

Herotilapia multispinosa C1 4.857 0.637

Astyanax aeneus T 3.636 0.084

Parachromis dovii* C2 7 ---

Not included in the analysis because we caught only one individual.

Figure 1. Species richness was greatest in edge habitats, and fish abundance was greatest in edge and open habitats. Species codes described in Table 2. Dashed lines indicate species absence.

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niches exist primarily in either open water or macrophyte beds may take advantage of edge habitats, where both conditions are accessible.

We found that habitat heterogeneity, especially at edge habitats, favors increased species richness. This conclusion has implications for many marsh systems, where invasive macrophytes (e.g. water hyacinth) can reduce heterogeneity within microhabitats. Habitat homogenization disrupts species composition, potentially altering the greater ecosystem structure.

ACKNOWLEDGEMENTS We would like to thank Vivek Venkataraman and Jessica Trout-Haney for their assistance in data collection, and Jess in particular for swimming face-down in a crocodile-infested marsh. We would also like to thank the minnows who donated their time and energy to our research, and the brave minnow traps who sacrificed themselves in the name of science, and which are now presumably serving as take-out meals for crocodiles. AUTHOR CONTRIBUTIONS All authors contributed equally. Adam Schneider identified all fish by species.

LITERATURE CITED Condiotte, A.M., Schneider, A.N., Friday, N.J.,

Mondshine, A.J., Wood, Z.T. 2014. Wetland and Macrophyte Diversity Affects Fish Abundance, Morphology, and Richness. Dartmouth Studies in Tropical Ecology 2014, In Press

Eriksson, B.K., Rubach, A., and Hillebrand, H. 2006. Biotic habitat complexity controls species diversity and nutrient effects on net biomass production. Ecology 87: 246-254.

Mykra, H., Heino, J., Oksanen, J, and Muotka, T. 2011. The stability-diversity relationship in

stream macroinvertebrates: influences of sampling effects and habitat complexity. Freshwater Biology 56:1122-1132.

Valk, A. G. 2012. The Biology of Freshwater Wetlands. Oxford University Press, Oxford, Britain.

Rosenzweig, M. L.1992. Species diversity gradients: we know more and less than we thought. Journal of Mammology 73: 715-730.

Webb, P.W. 1984. Body form, locomotion, and foraging in aquatic vertebrates. American Zoologist 24: 107-120.

Figure 2. Fish density was significantly higher in open and edge habitats than covered areas (p<0.03 for both, Tukey’s Honestly Significant Differences test). There was no significant difference between fish density in open and edge habitat (P=0.94, Tukey’s HSD test).

Figure 3. Fish were significantly larger in the covered macrophyte habitat than in either the open water or edge habitat (Tukey HSD p < 0.003 for differences between open/edge and covered).


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PATTERNS OF NUTRIENT DISTRIBUTION BETWEEN SOIL AND ABOVEGROUND PLANT BIOMASS IN TROPICAL FORESTS DO NOT HOLD AT THE UNDERSTORY LEVEL

NATHANAEL J. FRIDAY, LARS-OLAF HÖGER, REBECCA C. NOVELLO, AND CLAIRE B. PENDERGRAST

Faculty Editor: Ryan Calsbeek

Abstract: Most nutrients in a tropical forest system are held in aboveground plant biomass, leaving soils relatively nutrient poor. We tested for this pattern in understory plants and topsoil, using microarthropod richness and abundance as indicators of soil quality. We found that microarthropod abundance was positively correlated with soil moisture levels but not with biomass of understory plants. We discuss our results in terms of the limitations of the common assumption that ecological patterns established at one scale hold at another. Key words: microarthropods, nutrient distribution, plant biomass, spatial scale INTRODUCTION Tropical forests support a large amount of plant biomass, and yet tropical soils are remarkably nutrient-poor (Whitaker 1975). Through adaptations such as the production of sclerophyllous leaves, root mats, and high root-shoot ratios, rainforest plants extract nearly all available nutrients from the soil and trap them in aboveground biomass, resulting in mineral-poor and infertile soils (Jordan and Herrera 1981). This pattern of nutrient distribution has been observed at the scale of canopy and soil interactions (Vitousek 1986); however, the dynamics of nutrient cycling at the smaller scale of topsoil and understory interactions are less well understood (Yan 2011).

Extraction of available soil nutrients by understory plants should leave few nutrients in the topsoil to support microarthropod life. Soil microorganisms play a vital role in sustaining and limiting plant productivity through the processing and consumption of soil nutrients (Van Straalen 1997). Soil microarthropod diversity and abundance are correlated with and therefore used as proxies for soil nutrient content (Stork and Eggleston 1992). We tested the hypothesis that microarthropod richness and abundance would be negatively correlated with understory plant biomass. METHODS We collected soil and plant samples in the tropical dry forest of Palo Verde National Park on January 14, 2014 between 09:00 and 12:00. We sampled

along a 60 meter transect that ran along a moisture gradient. At 3 m intervals along the transect, we randomly determined a distance (between 1 and 10 m) and direction from the transect. We then laid a 30 cm square quadrat at the determined location. We collected all aboveground plant biomass in each quadrat. Additionally, we took a total of five soil cores at haphazard locations within the quadrat, each 5 cm deep and 2 cm in diameter: one to measure soil moisture levels and four to measure microarthropod abundance and richness.

We weighed soil moisture cores and biomass samples for each site and dried them in a drying oven at 105˚C for 22 hours, by which time the largest samples had remained at a constant weight for three hours. We then recorded dry weight for each soil sample and calculated moisture content as the percent water by weight of the original sample. To extract microarthropods from the soil, we constructed Berlese funnels from plastic cups and mesh (Barberena-Arias et al. 2012). We filled each cup with 30 ml of water and hung the funnels under 150 watt fluorescent light bulbs for 12 hours. We then identified the microarthropods to order using a dissection scope.

We performed two multiple regression analyses using JMP10 analytical software. One analysis tested for relationships between aboveground plant biomass and microarthropod richness and abundance, correcting for soil moisture. The second analysis tested for relationships between soil moisture and microarthropod richness and abundance, correcting for aboveground plant biomass.

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RESULTS There was no relationship between aboveground plant biomass and either microarthropod order richness (Table 1, slope = 0.04 0.09, P = 0.67, r2 = 0.01) or microarthropod abundance (Table 2, slope = 0.04 0.13, P = 0.77, r2 = 0.01), even after accounting for soil moisture. However, soil moisture was positively correlated with microarthropod order richness (Fig. 1A, Table 1, slope = 48.79 15.95, P = 0.01, r2 = 0.46) and abundance (Fig. 1B, Table 2, slope = 55.80 22.69, P = 0.03, r2 = 0.36) when accounting for plant biomass.

DISCUSSION Our results suggest that the patterns of nutrient storage observed at the broader canopy level of tropical dry forests do not hold when applied to understory microhabitats. There was no relationship between understory plant biomass and either soil microarthropod abundance or order richness. Resource limitations, herbivore pressure, and niche exploitation opportunities vary widely with ecosystem type and scale. It is therefore likely that so il-plant nutrient economies within tropical forests differ depending on spatial scale and location. These results may also be explained by variables that we omitted, such as pore size and soil pH, both of which may influence microarthropod communities (Hågvar 1990, Vreeken-Buijs et al. 1998).

We found that soil moisture was positively correlated with both microarthropod abundance and order richness. Studies have shown that fluctuations in soil moisture have considerable influence on soil fauna (Frouz 2004), which facilitate mineralization and nutrient cycling (Cole 2004). Our findings suggest that under conditions of water stress, as found in a tropical dry forest, moisture is a primary driver of soil nutrient availability for both plant and microorganism communities. Future studies should explore in greater depth the impact of temporal and spatial variations of moisture on nutrient cycling,

Table 2. Soil moisture predicted microarthropod abundance, but aboveground plant biomass did not.

Table 1. Soil moisture predicted microarthropod order richness, but aboveground plant biomass did not.

Figure 1. Soil moisture was positively correlated with both (A) microarthropod order richness (multiple regressions analysis, slope = 48.79 15.95, P = 0.01, r2 = 0.46) and (B) abundance (slope = 55.80 22.69, P = 0.03, r2 = 0.36), accounting for plant aboveground biomass.


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microarthropod abundance and diversity, and trophic interactions at the understory level. ACKNOWLEDGEMENTS We would like to thank R. Calsbeek, V. Venkataraman and J. Trout-Haney for their guidance and feedback, the students of the

Dartmouth FSP 2014 for their assistance with the peer review process, and the staff of Palo Verde National Park for their support. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Barberena-Arias, M., G. Gonzalez, and E. Cuevas.

2012. Quantifying Variation of Soil Arthropods Using Different Sampling Protocols: Is Diversity Affected? Tropical Forests, Dr. Padmini Sudarshana (Ed.)

Cole, L., K. Dromph, V. Boaglio, R. Bardgett. 2004. Effect of density and species richness of soil mesofauna. Biology and Fertility of Soils 39: 337-343.

Frouz, J., A. Ali, J. Frouzova, R. Lobinske. 2004. Horizontal and vertical distribution of soil macroarthropods along a spatio-temporal moisture gradient in subtropical central Florida. Environmental Entomology 33: 1282-1295.

Hågvar, S. 1990. Reactions to soil acidification in microarthropods: Is competition a key factor? Biology and Fertility of Soils 9:178-181.

Jordan C.F., Herrera R. 1981. Tropical Rain Forests: Are Nutrients Really Critical? The American Naturalist 117: 167-180.

Montagnini, F. 2000. Accumulation in above-ground biomass and soil storage of mineral nutrients in pure and mixed plantations in a

humid tropical. Forest Ecology and Management 134: 257-270.

Stork, N. E. and P. Eggleton. 1992. Invertebrates

as Determinants and Indicators of Soil

Quality. American Journal of Alternative

Agriculture 7: 38-45.

Van Straalen, N.M. 1997. Biological indicators of soil health. pp. 235-264, Pankhurst, C. Doube, B. M.: Gupta, V. V. S. R., editors. Vrije Universiteit, Department of Ecology and Ecotoxicology, De Boelelaan 1087, 1081 HV Amsterdam, Netherlands.

Vitousek, P.M. and R.L. Sanford. 1986. Nutrient Cycling in Moist Tropical Forest. Annual Review of Ecological Systems 17: 137-67.

Vreeken-Buijs, M.J., Hssink, J., Brussaard L. 1998. Relationships of soil microarthropod biomass with organic matter and pore size distribution in soils under different land use. Soil Biology and Biochemistry 30: 97-106.

Yan, S., A.N. Singh, S. Fu, C. Liao, S. Wang, Y. Li, Y. Cui, and L. Hu. 2011. Soil fauna index for assessing soil quality. Soil Biology and Biochemistry 47: 158-165.

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ORCHIDACEAE DIVERSITY IN TROPICAL PRIMARY AND SECONDARY FOREST

ANN M. FAGAN, ALEXA E. KRUPP, AARON J. MONDSHINE, AND AMELIA L. RITGER

Faculty Editor: Ryan Calsbeek

Abstract: Higher biodiversity is consistently found in primary forests, which have more complex structures and established biological communities. In particular, disturbances associated with secondary forests may have more acute long-term effects on floral diversity than other taxa. Epiphytic orchids may be used as a measure of floral diversity because of their influence on ecosystem dynamics and sensitivity to disturbance. We examined orchid genera composition and diversity in the primary and secondary forests of Monteverde Cloud Forest Biological Reserve. We found that the secondary forest had higher overall orchid diversity than the primary forest, but that the diversity per branch by forest type was not significantly different. We suggest that higher orchid diversity is not due to light gap dynamics but rather other biotic factors. Keywords: biodiversity, primary forest, secondary forest, Orchidaceae INTRODUCTIONStructural differences between primary and secondary forests have important implications for patterns of biodiversity. New secondary forests contain light gaps that allow fast-growing heliophilic plants to flourish (Guariguata and Ostertag 2001), whereas primary forests contain complex structures that are able to sustain established, highly diverse communities (Martin et al 2004). More tree and liana species are unique to primary forests than species of all other taxa (Barlow et al 2007). Thus anthropogenic disturbances often associated with secondary forests may have more acute long-term effects on floral diversity relative to other taxa.

Epiphyte communities are major contributors to forest biodiversity and influence ecosystem dynamics by intercepting rainfall, absorbing nutrients and creating niche habitats (Batke 2012, Shashidhar and Kumar 2009). Epiphytic species composition differs greatly between primary and secondary forests (Barthlott et al 2000). In particular, epiphytic orchids can be used to understand the implications of forest disturbance because of their dependence on specific microhabitats and specialized pollinators, both of which are highly sensitive to disturbance (Hundera et al 2012). Orchids can also be used as indicators of biodiversity because of their diverse physiological adaptations to fit many niches and because of their intricate relationships with other plant and animal communities (Shashidhar and Kumar 2009).

To examine the relationship between forest type and floral diversity, we surveyed epiphytic orchids in a primary and secondary forest. We tested the hypothesis that orchid diversity would differ by forest type, predicting that the primary forest would contain the greatest orchid diversity. METHODS Data Collection We conducted all research between 0900 and 1700 on 22 and 23 January 2014 in the Monteverde Cloud Forest Biological Reserve, Costa Rica. Park scientists showed us the boundaries of the primary and secondary forests, noting that the secondary forest was disturbed approximately 30 years ago. For each forest type, we walked along research trails and off trail, haphazardly selecting fallen branches that held epiphytic orchids. Fallen branches served as proxies for standing trees because arboreal orchid species grow slowly and are able to continue growing on a fallen branch (Zotz 1998).

For each branch, we measured branch length, circumference, number of orchid species and number of individuals of each species. We also took four canopy photographs at each branch site and recorded tree density, defined as the number of trees above eye-level within a five-meter radius. Sample Processing We used ImageJ (NIH, Bethesda, MD) to analyze average percent canopy cover at each branch


29

location. We used both percent canopy cover and tree density as estimates of light availability. We also identified orchids to the genus and species level with the help of a local orchidologist. Statistical Analysis We calculated generic diversity within each forest type using Simpson's diversity index, which characterizes community biodiversity on a scale from 0 (infinite diversity) to 1 (no diversity). We then compared genus composition between the two forest types using a contingency analysis. We also conducted an ANOVA to compare canopy cover and tree density in both forest types. All analyses were performed using the Al Young Studios Biodiversity Calculator and JMP® 10 statistical software (SAS Institute, Cary, NC).

RESULTS Using the Simpson diversity index, we calculated a primary forest genera diversity of 0.435 and a secondary forest genera diversity of 0.291. However, we found no significant difference in orchid diversity on branches between

Figure 1. There was no significant difference in orchid genera diversity on branches between primary and secondary forests (t52=0.16, P=0.87). Genera diversity was calculated using the Simpsons diversity index, an index that ranks diversity from zero to one, zero being most diverse and one having no diversity.

Figure 3. Across forest type, there was no relationship between tree density and number of orchids per branch (A; slope = 0.02 ± 0.05, P = 0.73, r2 = 0.02) or number of genera per branch (B; slope = 0.004, P=0.68, r2 = 0.003).

Figure 2. There was no relationship between % canopy cover and number of orchids per branch (A; slope = -0.24 ± 0.15, P = 0.11, r2 = 0.0477) or number of genera per branch (B; slope= -0.01 ± 0.03, P = 0.67, r2 = 0.004).

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the primary (mean ± SE = 0.93 ± 0.05) and secondary (mean ± SE = 0.94 ± 0.05) forest (t52=0.16, P=0.87).

We found no significant difference in tree density between primary forest (mean ± SE = 61.11 ± 3.41 trees) and secondary forest (mean ± SE = 57.44 ± 3.47 trees; F1,53= 0.567, P = 0.46). We also found no significant difference in canopy cover between primary forest (mean ± SE = 20.69 ± 1.06 % canopy cover) and secondary forest (mean ± SE = 22.59 ±1.10 % canopy cover; F1,54 = 1.551, P = 0.22). Additionally, we found that the number of individual orchids per branch sampled between primary forest (mean ± SE = 4.62 ± 1.18 orchids) and secondary forest (mean ± SE=6.59 ± 1.22 orchids) was not significantly different (F1,54 = 1.35, P=0.25). Canopy cover had no significant effect on number of orchids (slope = -0.24 ± 0.15, P = 0.11, r2 = 0.05) or number of genera (slope= -0.01 ± 0.03, P = 0.65, r2 = 0.004) per branch across both forests (Figure 2A,B). Similarly, tree density had no significant effect on number of orchids found

per branch (slope = 0.02 ± 0.05 P = 0.73, r2 = 0.02) or number of genera per branch (slope = 0.004, P=0.68, r2 = 0.003) across forest type (Figure 3A,B). We also found a difference in genera composition between the primary and secondary forest (Figure 4). DISCUSSION Contrary to our hypothesis, diversity in the secondary forest was higher than in the primary forest. The secondary forest of our study area was logged until approximately 30 years ago. Depending on how complete this process was, the disturbance might have enabled a higher level of orchid diversity in the secondary forest than in the primary. However, this difference in diversity between forest types was not represented at the branch scale. This indicates that orchids are not evenly dispersed throughout the forest.

The difference in diversity between forest types may be explained by the intermediate disturbance hypothesis. This hypothesis posits that forest diversity may increase following non-

Figure 4. Genera composition was different between primary and secondary forests.


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catastrophic disturbances as new niches from light gaps are generated for previously excluded species (Molino and Sabatier 2001). Still, we found no difference in canopy cover or tree density between the two forest types, indicating that light availability may not be the mechanism driving increased orchid diversity in the secondary forest. Some structural differences between the primary and secondary forest of Monteverde may no longer exist; after multiple decades, secondary tropical forests can resemble old growth forests in many ways, including light availability (Martin 2004).

There may still be important biotic differences between the two forests that impact orchid community structure. Changes in community structure may be prompted by pollinator species composition, seed dispersal or recruitment limitations (Pauw & Bond 2011, Hubbel et al 1999). These biotic factors may explain the observed difference in orchid genus composition

between the primary and secondary forest (Figure 4).

Our study adds to the growing debate about the specific mechanisms driving changes in biodiversity. Although moderate disturbance may increase plant diversity in the tropical forest, this may not be a function of light availability. Understanding which factors drive biodiversity in forest ecosystems can inform future land use practices and conservation strategies. ACKNOWLEDGEMENTS We are indebted to Gabriel Barboza for his assistance in locating and identifying orchids and for his knowledge of the Monteverde cloud forest. We would also like to thank the Dartmouth FSP 2014 group for their help with the peer-editing process. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Barlow, J., Gardner, T. A., Araujo, I.S., Ávila-

Pires, T.C., Bonaldo, A. B., Costa, J.E., Esposito, M.C., Ferreira, L.V., Hawes, J., Hernandez, M.I.M., Hoogmoed, M.S., Leite, R.N., Lo-Man-Hung, N.F., Malcom, J.R., Martins, M.B., Mestre, L.A.M., Miranda-Santos, R., Nunes-Gutjahr, A.L., Overal, W.L, Parry, L., Peters, S.L., Riveiro-Junior, M.A., da Silva, M.N.F., da Silva Motta, C. and C.A. Peres.2007. Quantifying the biodiversity value of tropical primary, secondary, and plantation forests. Proceedings of the National Academy of Sciences of the United States of America 104: 18555-18560.

Barthlott, W., Schmit-Neuerburg, V., Nieder, J., and S. Engwald. 2001. Diversity and Abundance of Vascular Epiphytes: A Comparison of Secondary Vegetation and Primary Montane Rain Forest in the Venezuelan Andes. Plant Ecology 152: 145-156.

Batke, S.P. 2012. A preliminary survey of epiphytes in some tree canopies in Zambia and the Democratic Republic of Congo. African Journal of Ecology 50: 343-354.

Hubbell, S. P., Foster, R.B., O’Brien, S.T., Harms, K.E., Condit, R., Wechsler, B., Wright, S.J. and S. Loo de Lao. 1999. Light gap

disturbances, recruitment limitation, and tree diversity in a Neotropical forest. Science 283: 554-557.

Hundera, K., Aerts, R., Beenhouwer, M.D., Overveld, K.V., Helsen, K., Muys, B. and O. Honnay. 2013. Both forest framentation and coffee cultivation negatively affect epiphytic orchid diversity in Ethiopian moist evergreen afromontane forests. Biological Conversation 159: 285-291.

Hobbs, R.J., and Huenneke, L.F. 1992. Disturbance, diversity, and invasion- implications for conservations. Conservation Biology 6: 324-337.

Guariguata, M.R. and R. Ostertag. 2001. Neotropical secondary forest succession: changes in structural and functional characteristics. Forest Ecology and Management 148: 185-206.

Martin, P.H., Sherman, R.E., and Fahey, T.J. 2004. Forty years of tropical forest recovery from agriculture: structure and floristics of secondary and old-growth riprarian forests in the Dominican Republic. Biotropica 36: 297-317.

Molino, J.F. and D. Sabatier. 2001. Tree diversity in tropical rain forests: A validation of the intermediate disturbance hypothesis. Science 294: 1702-1704.

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Pauw, A. and W.J. Bond. 2011. Mutualisms matter: pollination rate limits the distribution of oil-secreting orchids. Oikos 120: 1531-1538.

Roberts, M.R. and F.S. Gilliam. 1995. Patterns and mechanisms of plant diversity in forested ecosystems: implications for forest management. Ecological Applications 5:

969-977. Shashidhar, K. S. and A. N. Arun Kumar. 2009.

Effect of Climate Change on Orchids and their Conservation Strategies. The Indian Forester 135.

Zotz, G. 1998. Demography of the epiphytic orchid, Dimerandra emarginata. Journal of Tropical Ecology 14: 725-741


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THERMOREGULATORY COST OF DOMINANCE BEHAVIORS IN HUMMINGBIRDS

ALLEGRA M. CONDIOTTE, EMILY R. GOODWIN, REBECCA C. NOVELLO, AND ADAM N. SCHNEIDER

Faculty editor: Ryan G. Calsbeek

Abstract: Small endotherms devote a large proportion of their energy to maintaining a constant body temperature. In hummingbirds, which have the highest per-mass metabolic rate of all vertebrates, feeding competition has a strong impact on behavior and foraging success and is frequently expressed through energetically expensive dominance displays. We hypothesized that in a warm tropical climate, dominance behaviors, which produce excess metabolic heat, would decrease as temperature increased. We tested this hypothesis by observing the behavior of hummingbirds at a single feeder in the Monteverde Cloud Forest Biological Reserve in Costa Rica. There were fewer dominance behaviors with increasing temperature, supporting our hypothesis and suggesting that the risk of overheating outweighs the competitive advantage of frequent dominance behaviors. Even in the tropics, where temperature ranges are narrow, temperature and other abiotic conditions may create physiological constraints that drive foraging strategies in small endotherms. Key words: endothermy, dominance behavior, hummingbirds, metabolism, thermoregulation INTRODUCTION Small endotherms must expend a high proportion of their energy intake in thermoregulation (Gass et al. 1999). Because of their small body size and energetically costly form of flight, hummingbirds have among the highest mass-specific metabolic rate of all vertebrates (Suarez and Gass 2002). Energy intake is a limiting factor on hummingbird reproduction, and fitness is largely dependent on foraging success (Ydenberg 1998). Foraging competition has a strong influence on hummingbird behavior, demonstrated by frequent inter- and intraspecific displays of dominance (Stiles and Wolf 1970).

These displays are valuable to ensuring access to feeding sites (Stiles and Wolf 1970), but they are also energetically costly and produce excess metabolic heat that could be detrimental at high ambient temperatures (González-Gómez et al. 2011). González-Gómez et al. (2011) proposed a model predicting that hummingbird aggression level follows a parabolic curve, dropping off at high and low temperatures (Fig. 1). Since ambient temperatures in the Monteverde Cloud Forest fall in the upper temperature ranges of their study, we hypothesize that the costs of excess metabolic heat within this range may drive observable patterns of dominance behavior. We tested the hypothesis that hummingbird dominance behaviors would

decrease as ambient temperature increased in the cloud forest.

METHODS We monitored hummingbird activity at a single hummingbird feeder at Cafe Colibri in the Monteverde Cloud Forest Biological Reserve, Costa Rica, from January 21 to 23, 2014. Hummingbirds were habituated to human presence in the feeding area, allowing us to observe them closely without disturbing their activities.

Figure 1. Thermal performance curve for hummingbird aggression level adapted from Gonzalez-Gomez et al. (2011). The curve represents the theoretical prediction and points represent empirical data using Sephanoides sephanoides.

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Hummingbirds at the feeders included the Brown Violet-ear (Colibri delphinae), Coppery-headed Emerald (Elvira cupreiceps), Green-crowned Brilliant (Heliothryx barroti), Green Violet-ear (Colibri thalassinus), Magenta-throated Woodstar (Calliphlox bryantae), Purple-throated Mountain Gem (Lampornis (castaneoverntris) cinereicauda), Striped-tailed Hummingbird (Eupherusa eximia), and Violet Sabrewing (Campylopterus hemileucurus). Using a HOBO temperature logger, we recorded ambient temperature at the birdfeeder for the duration of the study. We recorded a three-minute video of hummingbird activity every half hour between 05:30 and 17:30 each day using a digital video camera (Nikon D600). We viewed the footage at 25% original speed and counted all hummingbird visits to the feeder. We categorized all observed aggressive interactions as dominance displays, and classified them into three actions: vocalizations directed towards other birds (chirps), flying swoops made in the direction of another individual (chases), and chases ending in collisions (contacts). Statistical Analyses We performed a linear regression between total weighted dominance behaviors and ambient temperature. We weighted each of the three behaviors according to its energetic cost, with chirps scaled by a factor of 2, chases by a factor of 3, and contacts by a factor of 5 (González-Gómez et al. 2011). We corrected for number of visits to the feeder by saving the residuals from a regression of the number of visits versus weighted dominance. We performed all statistical analyses in JMP 10 (SAS Institute). RESULTS Time of day and temperature were significantly correlated (r = 0.69, P < 0.0001, N = 26), and we chose to use temperature over time of day in our analysis because of its direct and established implications for hummingbird thermoregulatory physiology. Ambient temperature significantly predicted dominance behaviors, when corrected for number of visits to the feeder (P = 0.04, r2 = 0.16; Fig. 2).

DISCUSSION Dominance behaviors are an important strategy in maximizing foraging success by facilitating access to feeding sites (Stiles and Wolf 1970), but they may come at a thermoregulatory cost. We found that ambient temperature significantly predicted hummingbird dominance (Fig. 2), suggesting that the risk of overheating may outweigh the benefits of frequent dominance behaviors at high ambient temperatures.

Temperatures at Monteverde fall between 13 and 17˚C, representing the upper end of the 5-20˚C range shown in the model proposed by González-Gómez et al. (2011, Fig. 1). Therefore, our results may fit with the trend predicted on the right side of their parabolic thermal performance curve.

The patterns of behavioral thermoregulation shown here may be expected in temperate endotherms that experience a wide range of temperatures. However this pattern is less well understood in areas such as the tropics where temperature ranges are much narrower. With their small body mass and high metabolic rate, it appears that hummingbirds may be forced to adjust their behavior in response to relatively small ambient temperature changes to maintain a stable body temperature. Therefore, the physiological constraints associated with abiotic conditions could have important implications for

Figure 2. Ambient temperature significantly predicted weighted dominance behaviors, corrected for number of visits to feeder (P = 0.04, r2 = 0.16).


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foraging strategies in small endotherms across latitudinal gradients. ACKNOWLEDGEMENTS We would like to thank R. Calsbeek for his invaluable advice on our project, V. Venkataraman

for lending us his camera, and Cafe Colibri for allowing us to conduct research on their property. AUTHOR CONTRIBUTIONS All authors contributed equally

LITERATURE CITED Gass, C.L., Romich, M.T., and Suarez, R.K. 1999.

Energetics of hummingbird foraging at low ambient temperature. Canadian Journal of Zoology 77: 314-320.

Gonzalez-Gomez, P.L., Ricote-Martinez, N.,Razeto-Barry, P., Cotoras, I.S., and Bozinovic, F. 2011. Thermoregulatory cost affects territorial behavior in hummingbirds: a model and its application. Behavioral Ecology and Sociobiology 65: 2141-2148.

Lima, S.L. 1991. Energy, predators and the behavior of feeding hummingbirds.

Evolutionary Ecology 5:220-230.

Stiles, F.G. and Wolf, L.L. 1970. Hummingbird territoriality at a tropical flowering tree. The Auk 87: 467-491. Suarez, R.K. and Gass, C.L. 2002. Hummingbird

foraging behavior and the relation between bioenergetics and behavior. Comparative Biochemistry and Physiology 133: 335-343.

Ydenberg, 1998. R.C. Behavioral decisions about foraging and predator avoidance. In Cognitive Ecology, ed. R. Dukas, 343-378. Chicago, IL: Chicago Univ. Press.

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OPTIMIZING LEAF INCLINATION ANGLES: WATER SHEDDING AND PHOTOSYNTHESIS IN CLOUD FOREST UNDERSTORY PLANTS

MAYA H. DEGROOTE, CLAIRE B. PENDERGRAST, AND ELLEN R. PLANE

Faculty Editor: Ryan G. Calsbeek

Abstract: Heterogeneous environments and resource limitation in tropical cloud forests result in highly diverse plant communities. Plants have evolved different physiological mechanisms to maximize fitness under these conditions. A plant’s leaf inclination angle affects both its ability to photosynthesize and to shed water, the latter of which is necessary to prevent epiphyte colonization and facilitate transpiration. Plants with steeper leaf inclination angles sacrifice photosynthetic capacity to improve water shedding. We predicted that the leaf inclination angle of understory plants would vary in response to local light conditions. We used experiments and field observations to investigate the tradeoff between photosynthetic activity and water shedding in the understory plants Piper sp., Razicea spicata, and Begonia involucrata. Our results showed that on a species-wide level, understory plants demonstrate leaf inclination angles suited to the light and moisture conditions of their preferred habitats. This suggests a role for environmental heterogeneity as a driver of species diversity in tropical forest environments. Key words: Begonia involucrata, Piper sp., Razisea spicata, leaf inclination angle, canopy cover, water shedding INTRODUCTION In the tropical cloud forest, low light availability inhibits photosynthesis (Rozendaal et al. 2006), while high rainfall and humidity threaten plant fitness by limiting transpiration and encouraging epiphyte growth on leaves (Peabody 1985). Understory plants maximize photosynthetic capacity through a variety of physiological strategies (Rozendaal et al. 2006). However, prioritizing photosynthesis often requires plants to sacrifice leaf water shedding capacity. This trade-off is central to many aspects of leaf morphology, including angle of inclination (Meng et al. 2014).

Buildup of water on plant leaves reduces plant fitness. Water film reduces leaf surface temperature, slowing transpiration and thereby impairing nutrient transport (Dean and Smith 2014). Standing water on leaves also promotes growth of epiphytes, which intercept light and reduce photosynthetic capacity (Lücking and Bernecker-Lücking 2005). In addition, the added weight of water and epiphytes requires the plant to invest more energy in leaf structure (Dean and Smith 2014).

Tropical plants exhibit adaptations that increase water shedding, including drip tips, waxy leaves, and steep leaf inclination angle (Dean and Smith 2014, Meng et al. 2014). Greater leaf surface area, height from the ground, and chlorophyll density increase photosynthetic capacity, as does a more horizontal leaf inclination

angle (Meng et al. 2014). Leaf inclination angle is influenced by factors such as light availability and moisture conditions (Lightbody 1985). Photosynthetic capacity reaches a maximum as leaf inclination approaches zero, regardless of plant species (Falster and Westoby 2003).

Optimal leaf inclination angle requires a compromise between photosynthetic capacity and water shedding ability (Meng et al. 2014). We predicted that plants in open canopy environments would prioritize water shedding through steeper leaf inclination angles because light is abundant. We expected plants in denser canopy environments to have near-horizontal leaf inclination angles to maximize photosynthesis in light-limited conditions.

METHODS Observational study We selected the tropical cloud forest understory plants Piper sp., Razicea spicata and Begonia involucrata as our focal species. Piper sp. and R. spicata have oval shaped leaves with a single drip tip, while B. involucrata leaves are asymmetrical and have two to seven drip tips (Zuchowski 2005). We haphazardly selected ten plants of each species in the Monteverde Cloud Forest Biological Reserve, under a variety of canopy cover conditions. We photographed the canopy directly above each plant. We then randomly selected five leaves on the plant and took a photograph of each


37

leaf, holding a plumb line at the point where the leaf met the petiole to use as a vertical reference. We also measured the height of each leaf above the ground.

We used ImageJ software to determine percent canopy cover and the inclination angle for each leaf. We determined the angle of inclination with respect to the horizontal by drawing a line connecting the leaf’s apex to the point where the petiole met the leaf, using the plumb line as a vertical reference. Experiment We haphazardly selected two or three undamaged leaves of each species to determine theoretical optimum water shedding angles. We positioned leaves at ten angles between -60° and +60°, randomizing the order in which these angles were tested. Before securing the leaf at each angle, we patted the leaf dry and weighed it. We pipetted a known mass of water directly onto the leaf from base to apex, fully coating the leaf’s surface. After one minute, we weighed the leaf again and used the difference in mass between dry and wet measurements, divided by the total water mass, to determine the ratio of water retained. This was subtracted from one and converted to a percentage to determine percent water shed at each angle. Statistical analyses We used linear regressions to compare both percent canopy cover and leaf height with leaf inclination angle for each species. We conducted an ANOVA with a Tukey’s HSD post-hoc

analysis to test for a difference in mean leaf inclination angle between species.

To interpret the experimental data, we plotted angle of inclination versus percent water shed and used polynomial regression to model leaf water shedding efficiency. Statistical analyses were conducted using JMP 11.0 (SAS Institute, Cary, NC) and polynomial fits using Microsoft Excel. RESULTS There was no correlation between canopy cover and leaf inclination angle for any species (Table 1, Fig. 1). There was also no correlation between leaf height and leaf inclination angle for any species (Table 2, Fig. 2).

Mean leaf inclination angle was -18.0° ± 3.9 (± 1SE) for Piper sp., -23.6° ± 3.5 (±1SE) for R. spicata, and -39.6° ± 2.2 (± 1SE) for B. involucrata (Figure 3). Mean leaf inclination angle was significantly greater for B. involucrata compared to the other two species (ANOVA: F2,149 = 11.7, P < 0.0001; Tukey’s HSD: Piper sp.: P < 0.0001, R. spicata: P = 0.002). The relationship between leaf-water shedding efficiency and inclination angle was best accounted for by fitting a quadratic regression for Piper sp. and R. spicata (Piper sp: P = 0.02, r2 =

Table 1. Linear regression showed no correlation between percent canopy cover and leaf inclination angle for any species.

Species slope P r2 Piper sp. 0.106 ± 18 0.82 0.001 R. spicata -0.248 ± 19 0.57 0.007 B. involucrata -0.166 ± 15 0.71 0.003

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Figure 1: Leaf inclination angle was not correlated with percent canopy cover in any species (A: Piper sp.; B: R. spicata; C: B. involucrata).

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0.68; R. spicata: P = 0.01, r2 = 0.72) and a cubic regression for B. involucrata (P=0.01, r2 = 0.86) (Fig. 3). DISCUSSION Variability in light availability did not cause Piper sp., R. spicata, or B. involucrata to modify leaf angle to maximize exposure to solar radiation (Fig. 1). Similarly, there was no relationship between leaf angle and leaf height in any species (Fig. 2), suggesting that individual leaves within a plant do not respond to variation in light availability. The between-species difference in mean leaf

angle suggests that Piper sp., R. spicata, and B. involucrata have leaf inclination angles suited to the specific light and moisture conditions of their preferred environments. As leaf inclination does not vary in response to light for individual leaves

Table 2. Linear regression showed no correlation between leaf height (cm) and leaf inclination angle (°) for any species.

Species Slope P r2 Piper sp. -0.040 ± 18 0.70 0.003 R. spicata -0.175 ± 19 0.23 0.03 B. involucrata 0.099 ± 15 0.42 0.01

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Figure 2: Leaf inclination angle was not correlated with leaf height in any species (A: Piper sp.; B: R. spicata; C: B. involucrata).

Piper sp.

Figure 3. Polynomial regressions of leaf water shedding efficiency by leaf inclination angle. Black shapes are mean leaf inclination angle observed in the field for each species. Mean (±SE) leaf inclination angle was -18.0° ± 3.9° for Piper sp., -23.6° ± 3.5 for R. spicata, and -39.6° ± 2.2 for B. involucrata.

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or plants of any species, our results suggest that the optimal inclination angle is a species-wide adaptation allowing for specialization in a specific microenvironment.

We found that B. involucrata, an early colonizer species specialized to high-light environments (McMillan et al. 2006), displays an average leaf inclination angle near the experimentally determined optimal range of water shedding (Fig. 3). Piper sp. and R. spicata are not primarily located in light-abundant disturbed areas (Haber 2000) and therefore display morphologies more suited to maximizing photosynthesis than shedding water. Both species displayed mean leaf angles far closer to horizontal than the near-vertical angles that are optimal for water shedding (Fig. 3). However, it is possible that factors besides leaf inclination angle, such as surface area and drip tip length, also affect these species’ ability to tolerate or shed water (Meng et al. 2014).

Limited light availability and high moisture levels in the tropical cloud forest strongly shape the morphology and life history of understory

plant life (Lightbody 1985). Nutrient, light, and moisture conditions vary spatially and temporally due to disturbance, topography, aspect, and soil substrate (Bazzaz and Pickett 1980). Tropical understory plants have evolved diverse strategies suited to highly specific environmental conditions instead of adapting to tolerate and respond to a broader range of conditions. The physiological mechanisms through which these plants maximize fitness in a range of microhabitats sheds light on the underlying drivers of plant diversity in tropical cloud forests. ACKNOWLEDGEMENTS We would like to thank J. Trout-Haney for her guidance in developing methods, as well as the Dartmouth Biology FSP 2014 for their assistance in the peer review process. AUTHOR CONTRIBUTIONS C. Pendergrast and E. Plane collected field data and M. DeGroote conducted all image analysis. All authors contributed equally.

LITERATURE CITED Ackerly, D.D. and E.A. Bazzaz. 1995. Leaf

dynamics, self-shading and carbon gain in seedlings of a tropical pioneer tree. Oecologia 101: 289-298.

Bazzaz, F. and S. Pickett. 1980. Physiological ecology of tropical succession: a comparative review. Annual Review of Ecology and Systematics 11: 287-310.

Dean, J.M. and A.P. Smith. 1978. Behaviour and morphological adaptations of a tropical plant to high rainfall. Biotropica 10: 3441-47.

Falster, D.S. and M. Westoby. 2003. Leaf size and angle vary widely across species: what consequences for light interception? New Phytologist 158: 509–525.

Haber, W.A. 2000. Vascular plants of Monteverde. Pages 457-518 in N.M. Nadkarni and N.T. Wheelwright, editors. Monteverde: ecology and conservation of a tropical cloud forest. Oxford University Press, New York.

Lightbody, J.P. 1985. Distribution of leaf shapes of Piper sp. in tropical cloud forest:

evidence for the role of drip-tips. Biotropica 17: 339-342.

Lücking, R. and A. Bernecker-Lücking. 2005. Drip-tips do not impair the development of epiphyllous rain-forest lichen communities. Journal of Tropical Ecology 21: 171-177.

McMillan, P.D., G. Wyatt, and R. Morris. 2006. The Begonia of Veracruz: additions and revisions. Acta Botánica Mexicana 75: 77-99.

Meng, F., R. Cao, D. Yang, K.J. Niklas, and S. Sun. 2014. Trade‑ offs between light interception and leaf water shedding: a comparison of shade‑ and sun‑ adapted species in a subtropical rainforest. Oecologia 174: 13–22.

Poorter, L. and D.M.A. Rozendaal. 2008. Leaf size and leaf display of thirty-eight tropical tree species. Oecologia 158: 35–46.

Rozendaal, D.M.A., V.H. Hurtado, and L. Poorter. 2006. Plasticity in leaf traits of 38 tropical tree species in response to light; relationships with light demand and adult stature. Ecology 20: 207-216.

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FOREST STRUCTURE AND EPIPHYTE RICHNESS

NATHANAEL J. FRIDAY, LARS-OLAF HÖGER, ZACHARY T. WOOD

Faculty Editor: Ryan Calsbeek Abstract: Trees play an important role in structuring forest habitats and climate. In cloud forests, many species of epiphyte colonize trees. These epiphytes alter water and nutrient cycling. We examined the effects of forest stand age on trunk epiphyte richness in the cloud forest of Monteverde, Costa Rica. We found that epiphyte richness per trunk was positively influenced by tree diameter, and negatively influenced by stand age. Younger stands have higher epiphyte richness due to higher densities of epiphytes per square meter of bark and greater overall stem surface area. We demonstrate that as stands age, they may limit the richness of their co-occurring communities, thereby decreasing the ecosystem processes these communities provide. Key Words: Tropical forests, epiphytes, species richness, ecosystem functions INTRODUCTION The physical structures of trees can alter the abiotic environment of a forest. Trees alter available light, moisture, wind, and nutrient regimes. Because forests undergo large changes over the course of their succession, the way these factors are influenced may change over time.

In cloud forests, trees also provide substrates for epiphytic communities, which often occupy a large portion of tree surfaces and facilitate a variety of ecosystem processes. Because epiphytes rely on trees as a substrate, the structural characteristics of a tree stand can play a major role in structuring epiphyte communities. Factors such as elevation, light availability, and humidity have all been shown to affect epiphyte richness (Holz et al., 2002). Therefore succession can alter epiphyte richness through tree morphology and micro-climactic change. (Flores-Placios and Garcia-Franco, 2006).

Because epiphytes are sensitive to the size, shape, and local assemblage of forest trees, epiphyte communities should change as a forest develops. We therefore hypothesized that epiphyte richness would be affected by successional age and tree diameter.

In this paper, we will use the terms “young” and “old” to refer to forests that are at an earlier or later successional state. We do not intend to make judgments on individual tree age. METHODS We haphazardly established 14 plots, each around a focal tree of random size in the cloud forest of Monteverde, Costa Rica. Within an eight-meter

radius of the focal tree, we measured diameter at breast height (DBH, H = 1.37m) for all trees with a DBH 5cm. For all trees with a DBH 5cm within a five-meter radius of the focal tree, we tallied the number of trunk epiphyte species within a one meter vertical range centered at breast height. We counted all woody stems with a DBH < 5cm and height 1.37 meters within the eight-meter radius.

We predicted epiphyte richness using a regression including DBH, stem ratio (SR), and an interaction between DBH and SR. SR is defined as the ratio of woody plants with a DBH 5cm to woody plants with a DBH < 5cm and H > 1.37m. This ratio of large to small stems allows the approximation of age structure in a forest stand, with the ratio increasing as succession progresses (Harper 601-602). We also fit a log-log regression of epiphyte species density per square meter of bark to DBH. Finally, we used a linear regression to compare stand basal area, calculated as the sum of the cross-sectional areas of all trees in a plot, to SR. RESULTS The model we tested, in which all terms were significant, included DBH, SR, and the interaction between SR and DBH as the predictors of trunk epiphyte richness. Trunk epiphyte richness increased with DBH, but decreased with increasing SR (Figure 1, Table 1). Greater SR also decreased the slope of the richness ~ DBH line. We found a negative linear relationship between log-transformed epiphyte richness per square


41

meter of bark and log-transformed DBH (Figure 2; F3,170 = 68.54, P < 0.0001). Basal area did not change with SR (F1,12 = 0.0485, P = 0.83).

Figure 1. Epiphyte richness per trunk increases with diameter at breast height (DBH). The lower dashed line is the richness~DBH relationship when the stem ratio (SR; large/small) is at mean + one standard deviation, and the upper is when the SR is at mean – one standard deviation (R2 = 0.54).

Figure 2. Species density per square meter bark decreases with tree diameter (R2 = 0.69.). Both axes are on a log-scale.

Table 1. Regression results for epiphyte richness. Effect Est. Std. Err. t p Intercept 4.83 0.289 16.7 <0.00001 DBH 0.067 0.006 12.1 <0.00001 SR -2.32 0.555 -4.19 <0.00001 DBH*SR -0.107 0.031 -3.44 0.00007 DISCUSSION Our results supported the hypothesis that stand age, as well as individual stem diameter, affects epiphyte richness. Although the number of epiphyte species on a tree increased with the diameter of a tree (Figure 1), epiphyte richness per square meter declined with increasing diameter and total surface area (Figure 2).

Our results show that younger stands have a greater richness of trunk epiphyte species. Stand basal area in our study forest remains relatively constant as stands age and average tree size increases. Younger stands therefore have a low SR and high overall stem surface area. The combination of greater species richness per square

meter (Figure 2) and greater total surface area cause younger stands to have a greater epiphyte species richness overall.

We conclude that as stands age, epiphyte species richness will decrease. Because a high richness of plants is necessary to maintain ecosystem processes (Isbell et al., 2011), decreasing epiphyte richness in older forests may thereby reduce the ecosystem processes performed. These processes include atmospheric nitrogen retention, arboreal sedimentation (Clark et al., 2005), rainfall interception (Hölscher et al., 2004), and habitat provision for inquiline communities (Richardson et al., 2000, Armbruster et al., 2002).

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Our results indicate that trees play an important role in shaping epiphytic communities and altering the ecosystem processes made available by such communities. We demonstrate that as forests age, they may limit the richness of their surrounding communities and thereby the quality of the ecosystem processes these communities provide.

ACKNOWLEDGEMENTS We would like to thank the Dartmouth 2014 FSP group for their feedback and reviewer comments. We would also like to thank the FSP faculty for their assistance in editing this manuscript.

AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Armbruster, P., Hutchinson, R. A. & Cotgreave, P.

2002. Factors influencing community structure in a South American tank bromeliad fauna. Oikos, 96: 225–234.

Cardelus, C. L., Colwell, R. K. and Watkins, J. E., 2006. Vascular epiphyte distribution patterns: explaining the mid-elevation richness peak. Journal of Ecology, 94: 144–156.

Clark, K.L., Nadkarni, N.M., Gholz H.L., 2005. Retention of inorganic nitrogen by epiphytic bryophytes in a tropical montane forest. Biotropica. 37: 328-336.

Flores-Palacios, A. and García-Franco, J. G., 2006. The relationship between tree size and epiphyte species richness: testing four different hypotheses. Journal of Biogeography, 33: 323–330.

Harper, J.L., 1977. Population Biology of Plants. Academic Press.

Holz, I., Robbert-Gradstein, S., Heinrichs, J., Kappelle, M., 2002. Bryophyte diversity,

microhabitat differentiation, and distribution of life forms in Costa Rican upper montane quercus forest. The Bryologist. 105: 334-348.

Hölscher, D., Köhler, L., Van Dijk, A.I.J.M., Bruijnzeel, L.A., 2004. The importance of epiphytes to total rainfall interception by a tropical montane rain forest in Costa Rica. Journal of Hydrology. 292: 308-322.

Isbell, F., Calcagno, V., Hector, A., Connolly, J., Sanley-Harpole, W., Reich, P.B., Scherer-Lorenzen, M., Schmid, B., Tilman, D., Van Ruijven, J., Weigelt, A., Wilsey, B.J., Zavaleta, E.S., Loreau, M., 2011. High plant diversity is needed to maintain ecosystem services. Nature.477: 199-202.

Richardson, B.A., Richardson, M.J., Scatena, F.N., McDowell, W.H., 2000. Effects of nutrient availability and other elevational changes on bromeliad populations and their invertebrate communities in a humid tropical forest in Puerto Rico. Journal of Tropical Ecology. 16:167-188.


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DIFFERENTIAL MOBBING IN TROPICAL HIGHLAND BIRDS

ANN M. FAGAN, EMILY R. GOODWIN, AND AARON J. MONDSHINE

Faculty Editor: Matthew P. Ayres

Abstract: Some animal species display context-specific defensive behaviors, which presumably provide potential benefits while minimizing energy expenditure. Predator mobbing is an action in which groups of animals such as birds incur high energetic costs and assume predation risk by flying directly at a predator to deter it. Therefore, birds should selectively mob predators that pose a large risk. We tested the hypotheses that bird response level would vary with both predator type and size class of bird. In the tropical highlands at Cuerici Biological Reserve in Costa Rica, we played the calls of three local owls - the spectacled owl (Pulsatrix perspicillata), the mottled owl (Strix virgata), and the Andean pygmy owl (Glaucidum jardinii) - and counted the number of extra-small, small, and medium sized birds that responded along our transect. The Andean pygmy owl call induced the highest response level by birds, and smaller birds displayed stronger responses than larger birds. This variation in response level suggests that the energetic costs of mobbing are not equal for birds of different sizes. Our results indicate that mobbing response is influenced by bird size and predator type. The behavior could involve a combination of innate predator recognition and learning. Keywords: avian behavior, cooperative defense, mobbing

INTRODUCTION Defense against predators is a necessary energetic cost for many species (Lima and Dill 1990). Whether organisms use crypsis, toxicity, mimicry, or other defensive strategies, energy spent on defense cannot be used for other vital functions such as reproduction or foraging. Selection pressure to complete these other tasks drives organisms to reduce energy spent on defense while minimizing fitness costs. This pressure could drive the evolution of context-specific defensive behaviors.

Mobbing is a defensive behavior exhibited by many bird species. Detection of a nearby predator sometimes prompts groups of birds to harass the predator with vocalizations and physical attacks. This behavior benefits all members of the group, even those not who are not being directly attacked, by driving the predator away from the area and possibly reducing the probability that the predator will return. However, mobbing requires costly, rapid flight as well as increased predation risk in flying directly at the predator. Because of the high cost and risk involved, birds are expected to only exhibit this behavior in response to a perceived threat.

In addition, different bird species have evolved to recognize and respond to unique sets of threatening stimuli. In some bird species, juvenile birds learn which predators to mob by observing

adult birds responding to predator threat. Thus, an adult bird’s failure to respond to a predator call may be due to one of two things: first, that the bird has never heard the call before and does not associate it with predator threat, or second, that the bird has heard the call, but that the predator is not sufficiently threatening to prompt a mobbing response. Because birds only mob certain predators, we expected that birds of Cuerici Biological Reserve would vary in the degree of mobbing depending on the species of predator.

Bird species within the same range may also respond differently to the same predator call. This disparity may be due to the fact that the costs and benefits of mobbing are not shared equally by birds across different size classes. A small bird may be more easily depredated by an aerial predator and thus have more to gain from engaging in cooperative defense. In this case, smaller birds would respond more strongly to a predator call. Alternatively, larger birds may be able to more easily assume the high energetic cost required to sustain the costly flight patterns of mobbing. If this is true, larger birds should respond most strongly to hearing a predator call. METHODS Study System Our study focused on the mobbing behavior of tropical highland birds at Cuerici Biological

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Reserve, Costa Rica (elevation 2500 meters) in response to three avian predators found within Costa Rica: the spectacled owl Pulsatrix perspicillata, the mottled owl Strix virgata, and the Costa Rican pygmy owl Glaucidium costaricanum. Although the Costa Rican pygmy owl was taxonomically separated from the Andean pygmy owl (G. jardinii) in 2000, both owls have the same vocal repertoire and therefore we substituted the call of the Andean pygmy owl for the call of the Costa Rican pygmy owl. Of the three owls used, P. perspicillata and S. virgata are not likely to be found at this high elevation. All three owls occasionally feed on small birds, although P. perspicillata and especially S. virgata depend more on small mammals, insects, lizards, and frogs for food. G. jardinii is the smallest of the three owls (average body length 15 cm), followed by the mottled (36 cm) and spectacled (48 cm) owls (Stills & Skutch 1989). All three species have been reported to be mobbed in the daytime by small birds (Stills & Skutch 1989, Cornell Neotropical Birds database). Data Collection

We conducted 19 trials between the hours of 05:30 and 18:00 on 29-31 January 2014 in Cuerici Biological Reserve. Our two study sites were a partially covered brush field and a small stretch of secondary forest trail. We began each trial with a control period of three minutes, during which we recorded number and size class of all birds seen (stationary or moving) within a 3-meter radius of our transect area. We classified birds with a body length of <10 cm as extra-small, 10-15 cm as small, and >15 cm as medium. After completing the control period and waiting one minute, we began the three-minute treatment period by playing a randomly selected 90-second recording of either a Spectacled owl, Mottled owl, or an Andean pygmy owl followed by 90 seconds of silence. During the treatment period, we again recorded all birds seen within the specified radius of the trial area.


We evaluated a generalized linear model with predator type and bird size as independent variables and overall bird response as the response variable. Bird response was defined as the difference in number and size of birds present

during treatment and control trials. We found no effect of time of day (morning vs. afternoon) (t = 0.30, P = 0.76, df = 38) or location (secondary forest vs. brush field) (t = 0.72, P = 0.48, df = 38) on bird response. All analyses were performed using JMP® 11.0 statistical software (SAS Institute, Cary, NC).

RESULTS We observed 253 birds of at least eight different species during our study. The extra-small species included volcano hummingbirds (Selasphorus flammula) and green violet-ear hummingbirds (Colibri thalassinus). The small species included black-and-white becards (Pachyramphus albogriseus), black-cheeked warblers (Basileuterus melanogenys), collared redstarts (Myioborus torquatus), rufous-naped wrens (Campylorhynchus rufinucha), Wilson’s warblers (Cardellina pusilla), and some unidentified species. The medium species were large-footed finches (Pezopetes capitalis) and various unidentified flycatcher species (Ptiliogonatidae and Tyrannidae).

The Andean pygmy call induced significantly higher total bird response than the mottled or spectacled owl calls (F2,111 = 8.37, P = 0.0004; Fig 1., Table 1) There was a significant effect of the interaction between predator and bird size (F4,111 = 2.56, P = 0.043) with smaller birds exhibiting higher levels of response than larger birds.

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Figure 1. Total bird response to the Andean pygmy owl call was higher than to the other two owl calls. Within the Andean pygmy owl trials, medium birds were less responsive than smaller birds. Mean response was defined as the difference in mean number of birds of each size class between treatment and controls.


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DISCUSSION Cooperative defense can be costly and dangerous. Thus, prudent birds are expected to mob only in situations where the predator represents a potential threat. In our study, birds responded only to the Andean pygmy owl call, indicating bird specificity of the mobbing response. Of the three predators, only the Andean pygmy owl occupies the geographic and elevational range in which we performed our experiment. Since the mottled and spectacled owls are not found at the high elevation of our study area, the birds may not have recognized the calls of P. perspicillata or S. virgata. Alternatively, the increased individual risk associated with mobbing the two larger owl species could outweigh the potential benefits of group defense.

Birds of different size classes also responded differentially to the Andean pygmy owl call. Smaller birds were more responsive than larger birds, which was consistent with the hypothesis that small birds have more to gain from mobbing behaviors. It seems likely that larger birds are less likely to be depredated by a small owl like the Andean pygmy owl, in which case the small predation risk would not justify the energetic costs of mobbing.

The results of our study suggest that avian defensive behaviors in the tropics are context-specific. Not only do birds respond to certain threats and not others, but their level of response may vary by size.

We were unable to resolve the extent to which responses were learned versus innate. Indeed,

innate recognition of predators and learned anti-predatory behaviors are not mutually exclusive. Additional studies would be required to distinguish among the possibilities. The outcome would have particular relevance to newly-introduced or invasive avian predators and their effects on local bird communities. For example, birds that only have innate predator recognition may be at a disadvantage compared to those that learn from a wide variety of stimuli, as the former would not be able to recognize the call of a newly-introduced avian predator. Additionally, introduced predators may have significant effects on community structure, as juvenile birds may have an advantage over adult birds in learning to recognize new predator calls. Beyond differences in size class, relative dependence on innate predator recognition versus learned anti-predator defenses may determine the outcome of bird-predator interactions. Regardless of the mechanism by which birds recognize a predator call, the benefits of group defense presumably outweigh the costs of mobbing or the behavior would not be so well-developed. ACKNOWLEDGEMENTS We thank Carlos Solano for the use of the grounds at Cuerici Biological Reserve. We are also indebted to Jessica Trout-Haney for her assistance in conducting trials and identifying birds. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Caro, T. M. Antipredator Defenses in Birds and

Mammals. Chicago: University of Chicago, 2005.

Sandoval, L., and D.R. Wilson. 2012. Local

predation pressure predicts the strength of mobbing responses in tropical birds. Current Zoology 58: 781–790.

Stiles, F.G., and A.F. Skutch. A Guide to the Birds of Costa Rica. Ithaca, NY: Comstock, 1989.

Lima, S.L. and L.M. Dill. 1990. Behavioral

Table 1. ANOVA of the effects of predator and bird size on level of bird response.

DF Mean Squares F P

Predator 2 26.43 8.37 0.0004

Bird Size 2 2.785 0.88 0.42

Predator*Bird Size 4 8.065 2.56 0.043

Error 111 3.157

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decisions made under the risk of predation: a review and prospectus. Canadian Journal of Zoology 68: 619-640.

The Cornell Lab of Ornithology: Neotropical Birds. Cornell Lab of Ornithology. Web. Accessed 09 Feb. 2014. http://neotropical .birds.cornell.edu.


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DIVERSITY OR DOMINANCE: SYNCHRONOUS MASTING AS A REPRODUCTIVE STRATEGY IN QUERCUS COSTARICENSIS MONTANE FORESTS

MAYA H. DEGROOTE AND CLAIRE B. PENDERGRAST


Abstract: Montane oak forests represent a unique, biologically rich ecotype in the American Tropics. These forests support distinctive plant and animal communities suited to the biotic and abiotic conditions of an oak-dominated forest, a rare occurrence in a region otherwise characterized by exceptionally high tree species richness. Several theories offer possible explanations for the high diversity and heterogeneity of tropical forests. The Janzen-Connell model identifies specialist predation near parent trees as the primary driver of seedling distribution in tropical forests, while the predator-swamping model asserts that generalist predators are primarily responsible for controlling seedling establishment. We assessed the relative capacity of these two forces to explain patterns of seed distribution and viability in the montane oak forest of Cuerici, Costa Rica. We surveyed acorn abundance, predation type, and viability at high- and low-density plots in the interior and at the edge of a masting Quercus costaricensis stand. We found that a higher percentage of acorns were viable at low-density plots, with greater specialist insect predation in interior areas and greater generalist vertebrate predation at the edge of the masting stand. Acorn viability per plot was consistent across locations and densities, suggesting that the combined impact of specialist and generalist predation across a masting area allows for evenly distributed establishment of Q. costaricensis seedlings and a continuation of oak-dominated canopy cover. These findings are relevant to understanding how disturbance affects seedling establishment in tropical oak forests and how deforestation in these unique environments alters ecosystem structure and species diversity. Key words: acorns, Janzen-Connell hypothesis, predator-swamping, Quercus costaricensis, synchronous masting INTRODUCTION Tropical forests harbor hyper-diverse plant communities, far exceeding diversity levels found outside the tropics (Wright 2002). The Janzen-Connell hypothesis attributes high species richness of tropical trees to the combination of specialist seed predators and seed dispersal limitations. Under this model, specialist predators limit seed success in high seed density areas close to the parent tree, resulting in greater likelihood of seedling establishment in areas farther from the parent tree at the outer edges of the dispersal range where specialist predation is less prevalent (Janzen 1970). Thus, conspecific neighboring trees are rare, resulting in high species richness.

The Janzen-Connell model suggests that seedling survival would be chronically low in low diversity forests due to high specialist seed predator abundance (Janzen 1970). However, some trees employ synchronous masting, a reproductive strategy in which neighboring conspecific trees simultaneously produce a high quantity of seeds, while in other years few or no seeds are produced. Tree species that mast synchronously may be able to establish seedlings by overwhelming the capacities of predators to eat

all the seeds produced, a phenomenon known as predator-swamping (Ims 1990).

While the Janzen-Connell hypothesis predicts lower seed survivorship at higher seed density due to the pressure of specialist predators, predator-swamping theory predicts higher survivorship when an area is inundated with a high density of seeds. Near the Cuerici Biological Reserve in Costa Rica, a single tree species, Quercus costaricensis, dominates the high altitude mountain tops. Q. costaricensis mast synchronously every 3-4 years and are able to sustain an apparently stable forest type in which nearly 100% of the canopy trees are a single species (Solano pers comm, Guariguata and Sáenz 2002). Q. costaricensis timber is valuable and many of Costa Rica’s oak-dominated forests have experienced increased pressure as a result of expanded logging activity. The resilience of these forests depends largely on Q. Costaricensis’ ability to successfully establish seedlings and maintain dominance. Loss of oak forest would affect ecosystem structure in Cuerici, with broad consequences for the many organisms dependent on oaks for food and habitat.

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In this oak-dominated forest, insects are host-specific predators while vertebrates are typically generalist seed predators (Kellner et al. 2009). We tested whether the Janzen-Connell model or predator-swamping more strongly determined Q. costaricensis seed survival and subsequent establishment. We surveyed low- and high-density acorn plots within and on the edge of the masting area for Q. costaricensis in 2013-2014. The Janzen-Connell model predicted highest seed mortality in high-density interior plots, where specialist predators would be most abundant, while the predator-swamping model predicted higher seed mortality in low-density edge areas where predators are less likely to be satiated.

METHODS On 31 January, 2014, we surveyed one square meter plots along a cleared 0.5 km trail through a masting stand of Q. costaricensis centered at N 9.56443° and W 83.66628° at 2750 meters elevation (Fig. 1). We identified the edge of the masting area and sampled twelve plots at this edge, in the seed shadow (Janzen 1970) of the last masting tree. Six of these plots were deliberately located where there was a high density of acorns and six where there was a low density of acorns in the plot. We similarly sampled 14 plots within the

interior of the masting area (approximately 100 meters from the edge plots). Seven of these interior plots were deliberately located where acorn density would be high (beneath the crowns of three masting trees) and seven were located where density would be low (below inter-tree gaps in the canopy). All acorns within a plot were collected and categorized as either viable or killed, with the causes of mortality being noted as insect, vertebrate, or other (typically fungi). RESULTS Acorn density varied dramatically between plots, ranging from 5 to 114 acorns per square meter. Dominant predation type differed significantly between interior and edge plots. Percent mortality due to vertebrate predation was higher in edge plots (Fig. 2A). High-density edge plots displayed the greatest percent mortality from vertebrate predation (72.3 ± 3.8%). Interior plots suffered significantly higher percent mortality from insect predation than edge plots (Fig. 2B). Insect predation was greatest in interior high-density plots (46.3 ± 8.3%), while edge low-density plots experienced the lowest mortality from insect predation (10.7 ± 6.1%).

Figure 1. Location of high-density and low-density plots along a cleared trail in the interior and edge of the masting stand. Dotted lines indicate tree canopy.

≈ 50m

≈ 10m

≈ 60m

Low-density plot

High-density plot

Q. costaricensis

Edge Interior

≈ 2.5m


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Insect-depredated acorns all featured a round hole approximately one millimeter in diameter. When cut open, these seeds displayed darker regions extending towards the center of the seed. We assumed these seeds to be non-viable due to predation by specialist acorn-feeding insects, perhaps weevils (Kellner et al. 2009). Vertebrate-depredated acorns displayed ragged holes roughly two to six millimeters wide and endosperm was often entirely absent. During our sampling period we observed acorn woodpeckers and variegated squirrels in the canopy of masting trees. These same species were observed in non-masting stands as well, though they appeared to be most abundant in the masting area.

We observed consistently higher acorn densities in the interior than in the edge for both low-density and high-density plots (Fig. 3A). Percentage of viable acorns was significantly higher in low-density plots in both edge and interior plot types (Fig. 3B). We found no significant relationship between viable acorn frequency and either plot density or plot location (t = 1.82, P = 0.087, df = 17.0). However, high-density interior plots contained more than twice as many viable acorns (9.14 ± 2.9 acorns/m2) as any other plot type (Fig. 3C). DISCUSSION Distribution and abundance of viable Q. costaricensis acorns were determined by seed dispersal from the parent tree as well as insect and vertebrate seed predation. Patches of varying seed densities existed within and at the edge of the masting area due to the overlapping seed shadows of neighboring masting trees (Fig. 2A). The heterogeneous distribution of acorns generated an array of microhabitats suited to different predation strategies (Pérez-Ramos 2008). Vertebrate predators with larger ranges may have a greater impact on low-density areas on the edge of the masting stand while less mobile insects dominate high-density patches directly beneath masting trees. Predators consumed the vast majority of acorns produced by masting oaks. The mechanisms allowing for continued oak dominance in the Cuerici forest depend largely on the success and distribution of different predator types.

We found a significantly higher percentage of viable acorns in low-density than high-density plots (Fig. 3B). That is, seed predation impacted high-density areas more strongly than low-density areas, presumably because mobile predators tend to forage preferentially in places with high prey concentrations (Ims 1990). Our observations of high predation in high-density areas due to the abundance of seeds support both the Janzen-Connell and the predator-swamping models of seed predation. However, the two models assume different predator groups as primarily responsible for controlling seed viability and long-term seedling establishment. The Janzen-Connell model identifies specialist predators found beneath parent trees as the primary threat to seed viability (Janzen 1970), while the predator-swamping model tends

0 10 20 30 40 50 60 70 80

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Figure 2. (A) Percent mortality from vertebrate predation was significantly higher in edge plots than in interior plots (t = 3.23, P = 0.004, df = 19.3). There was no difference in percent mortality from vertebrate predation between high- and low-density plots (t = 0.78, P = 0.45, df = 16.1). (B) Percent mortality from insect predation was significantly higher in interior plots than in edge plots (t = 2.67, P = 0.01, df = 20.5). Percent mortality from insect predation was significantly higher in high-density than low-density plots (t = 2.5, P = 0.02, df = 17)

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to emphasize the importance of larger, more generalized and more mobile seed predators (Ims 1990).

Vertebrate predation contributed most strongly to acorn mortality in edge plots and was less pronounced in interior plots (Fig. 2A). This is in accordance with the predator-swamping model, as generalist predators tend to consume prey resources until they reach the satiation point determined by their population size and prey handling efficiency. Generalist predators can be unable to consume all available seeds in high prey-density areas such as the interior of a synchronous masting stand, resulting in an increased likelihood of seedling establishment. Masting events are too brief to allow vertebrate predator populations to increase their satiety thresholds through population growth or improved prey capture strategies. Therefore, generalist predation favors seedling establishment in interior areas where seeds are most abundant.

Insect predators contributed more strongly to acorn mortality in interior than edge plots (Fig. 2B). Such specialist predators display limited dispersal capability and are therefore dominant in high-density interior areas (Fig. 2B). Our findings supported the prediction of the Janzen-Connell model of high insect predation near the parent tree. Percent mortality due to insects was nearly four times greater in interior high-density plots than in any other plot type (Fig. 2B).

How do the competing Janzen-Connell and predator-swamping models of seed predation explain the sustained dominance of Q. costaricensis in the upper montane forests of Costa Rica? Seedling establishment is dependent on density of viable acorns, which we found did not differ significantly with plot location or acorn density (Fig. 3C). No single plot type offered superior conditions for acorn survival due to the complex impacts of both generalist and specialist seed predators and dispersal limitations. While the vast majority of acorns produced during a masting

event fail to become established (Fig. 3B), Q. costaricensis’ synchronous masting strategy evidently allows enough seedling establishment to maintain oak canopy dominance. This is possible because synchronous reproduction creates a range of densities within the masting oak stand, allowing acorns to evade specialist predation in high-density areas. Synchronous masting also facilitates generalist predator satiation by concentrating seed production in time and space such that predator population size is limited.

Once widely distributed throughout the highland Neotropics, few intact blocks of montane oak forest remain (Kappelle 2006). Patches of primary oak forest as found in Cuerici, Costa Rica face increasing fragmentation and disturbance due to logging for timber and charcoal, as well as clearcutting for agriculture or road construction. The clearing of Neotropical montane forests has led to land degradation and biodiversity depletion. Interest in the protection and recovery of these valuable forests has prompted many questions regarding how forests should be managed and monitored to reestablish stable, healthy forest ecosystems (Guariguata and Sáenz 2002). Recovery of tropical montane oak forests following disturbance depends strongly on acorn availability and activity of seed predators and dispersers (Kappelle 2006). Our findings may inform future monitoring and management efforts to protect the biodiversity and ecosystem services offered by tropical montane oak forests. ACKNOWLEDGEMENTS We would like to thank Vivek V. Venkataraman and Amelia K. Ritger for their discovery of the masting stand of Q. costaricensis and Matthew P. Ayres for sparking our interest in the masting system. Thank you also to Carlos Solano for his hospitality and knowledge of natural history. AUTHOR CONTRIBUTIONS Both authors contributed equally

LITERATURE CITED Guariguata, M. and G. Sáenz. 2002. Post-logging

acorn production and oak regeneration a tropical montane forest, Costa Rica. Forest Ecology and Management 167: 285-293.

Ims, R. 1990. Reproductive synchrony as a

predator-swamping strategy. The American Naturalist 136: 485-498.

Janzen, D. 1970. Herbivores and the number of tree species in tropical forests. The American Naturalist 104: 501-528.

Kappelle, M. Ecolo gy and Conservation of


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Neotropical Montane Oak Forests. Springer, 2006.

Kellner, K., J. Riegel, N. Lichti, and R. Swihart. 2009. Oak mast production and animal impacts on acorn survival in the central hardwoods. Gen. Tech Rep. 108: 176-187.

Pérez-Ramos, I. 2008. Factors affecting post- dispersal seed predation in two coexisting oak

species: Microhabitat, burial and exclusion of large herbivores. Forest Ecology and Management 255: 3506-3516.

Solano, Carlos. Personal communication, January 2014.

Wright, J. 2002. Plant diversity in tropical forests: a review of mechanisms of species coexistence. Oecologia 130: 1-13.

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INSECTIVOROUS BATS ALTER FORAGING DECISIONS BASED ON RISK OF VISUAL PREDATION

ALEXA E. KRUPP, REBECCA C. NOVELLO, AND ELLEN R. PLANE


Abstract: Prudent animals may adjust foraging behavior to limit predation threat. Nocturnal foragers are under greater threat from visual predators when ambient light is relatively high. We tested whether insectivorous bats would adjust their behavior in response to cues of visual predators in combination with the addition of light. We recorded bat activity with an ultrasonic microphone under factorial combinations of light and simulated owl calls. Bat passes were less frequent in trials with simulated predator calls, and with the experimental addition of light without predator calls. These results indicate that an increase in risk of predation from auditory and abiotic cues, including exposure to light, may induce predator avoidance behavior. Over time, selective pressures can reward nuanced predator detection and avoidance behaviors, even at the cost of decreased energy intake from foraging. Keywords: bats, optimal foraging theory, visual predators INTRODUCTION Optimal foraging theory predicts that animals should adopt foraging strategies that maximize their fitness via high resource capture (Pyke et al. 1977). However, risk of predation forces prey animals to balance foraging behaviors with predator detection and avoidance (Kavaliers and Choleris 2001). Bats, the only true mammalian fliers, have extremely high metabolic rates during active flight, 3-5 times higher than the maximum of equivalent-sized terrestrial mammals (Shen et al 2010). When foraging at night, insectivorous bats can fly continuously for up to seven hours, locating and capturing prey using high-frequency echolocation. This high energetic expenditure is fueled through rapid metabolism of ingested nutrients (Voigt et al. 2010).

As nocturnal foragers with visual predators, bats are more vulnerable to predation in higher ambient light (Lima and Dill 1990). Many insectivorous bats remain in their roosts during the high insect activity at dusk, presumably to avoid higher predation risk before full darkness (Rydell 1996). Similarly, some frugivorous bats avoid foraging during the full moon (Clarke 1983), and nocturnal rodents reduce foraging under experimentally manipulated high light conditions (Kotler 1984). On the other hand, light-induced reductions in foraging would needlessly limit resource capture if predators are absent.

We tested whether insectivorous bats in the montane forests of Costa Rica adjusted their foraging behavior in response to different

combinations of light and simulated predator risk. Specifically, we evaluated effects of predator calls and manipulation of light on the frequency and duration of bat passes, which are both proxies for foraging activity. If bats are responsive to variable predation risks, they should reduce foraging in response to signals of predators, especially with increased light levels. Alternatively, if predation has been only a minor factor in bat fitness, or at least less important than meeting metabolic needs, bats may be nonresponsive to light or cues of predator presence. METHODS We recorded bat activity at Cuericí Biological Station on the nights of 29-30 January, 2014. We used Avisoft Bioacoustics hardware and software (Avisoft Bioacoustics, Berlin, Germany) and an ultrasonic microphone to record bat calls between the hours of 18:30 and 21:00. In 40-minute periods, we conducted four ten-minute trials, randomly assigning treatments to time slots within this period. We manipulated the presence and absence of light and the presence and absence of predator calls to create four treatments. We turned on two lights over the lower Cuericí trout pond for light trials. To simulate predator calls, we played recordings of an Andean Pygmy Owl and a Striped Owl for 30 seconds every 90 seconds during the trial. Both the Andean Pygmy Owl and Striped Owl are found at high elevation sites in and around Cuericí and are known to prey on bats and other small mammals (Stiles and Skutch 1989).


53

We used SASLab Pro (Avisoft Bioacoustics, Berlin, Germany) to analyze recordings for bat passes. We recorded passes as independent if there was at least one second of silence between calls. We also randomly sampled ten bat passes from each trial to assess bat pass duration. We used bat passes and pass durations to capture changes in two potentially important measures of foraging behavior: the frequency with which a bat visits a foraging area (pass) and the time it spends in the area on each visit (pass duration).


We performed a chi-square analysis to test for effects of light and predator calls on bat passes for each night. We also tested for effects of light, predator calls, period, and interactions of the three factors on bat passes for each night using a three way log-linear model (Quinn and Keough 2002). We performed an ANOVA to compare bat pass durations across treatments. We conducted all analyses in JMP 10 (SAS Institute, Cary, NC). RESULTS We recorded 222 bat passes in 73 sound files (Electronic supplementary materials). We observed three different bat call sonotypes (Fig.

1), including a Myotis-type call (Fig. 1a) and a Phyllostomidae-type call (Fig. 1c). We saw evidence of search calls, approach calls, and feeding buzzes (Fig. 2). Frequency of bat passes was greater on the first night (N=178) than the second night (N = 44). On the first night, we found a significant difference in frequency of bat passes across treatment types (chi-square = 57.96, P < 0.0001, df = 1; Fig. 3). We observed more bat passes in the absence of predator calls, and number of bat passes varied with both period and the interaction between light and predator calls (Table 1). The most passes were observed during dark treatments without predator calls (Fig. 3).

Table 1. A log-linear model showed significant effects of time of night (period) and predation on number of bat calls, as well as an interaction between predation and light.

Chi-square P

Period 19.75 <0.0001*

Predation 12.45 0.0004*

Light 0.03 0.87

Predation*Light 35.63 <0.0001*

Period*Light 0.61 0.43

Period*Predation 0.20 0.65

Freq

uenc

y (k

Hz)

C B A

Figure 1. Sonograms of three bat searching calls recorded over 20 milliseconds. A) Steep frequency-modulated sweep with drop off in frequency at tail end of call (Myotis-type) B) Relatively narrow bandwidth at end of call C) Relatively short call with three harmonic frequencies (Phyllostomidae-type).

Figure 2. Bat feeding buzz with steep frequency-modulated sweep, recorded over 260 milliseconds.

Freq

uenc

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Hz)

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No factor had a significant effect on bat passes on the second night, but when we combined data from both nights the effects of period (chi-square = 15.59, P < 0.0001 df = 1), predation (chi-square = 8.52, P = 0.004, df = 1), and interaction between light and predator calls (chi-square =19.54, P < 0.001, df = 1) remained significant.

Duration of calls ranged from 0.5 to 16.7 seconds (mean ± SE = 5.25 ± 3.6 sec), and neither predator calls, light, nor the interaction between light and predator calls had a significant effect on call duration (F1,36 < 1, P > 0.65 for all factors).

DISCUSSION We found that bats adjusted their foraging behavior in response to increased risk from visual predators. Bat passes were markedly less frequent in the presence of predator calls on the first night of our study (Table 1, Fig. 3), indicating that bats reduce foraging efforts when risk of predation is high. Light alone did not have a significant effect on bat passes on the first night, but the effect from interaction of light and predator calls was significant (Table

1). In the absence of predator calls, bat passes were less frequent in the light, where risk of visual predation is greater (Fig.3). Thus, bats seemed to exhibit predator avoidance in response to both light levels and auditory cues. This supports the hypothesis that bats can adjust their behavior in nuanced ways according to details of predation threat (e.g., the combination of light with the call of a predator that hunts by sight).

Neither light nor predator calls had an effect on bat passes on the second night, which could indicate bat habituation to our experimental stimuli. Further studies could investigate whether bats respond differentially to predation threat based on the relative abundance of predators in their environment, which would suggest the extent to which predator avoidance behavior in bats is innate versus learned. Our findings provide evidence for a tradeoff between foraging and predator avoidance. Animals subject to predation pressure may commonly evolve avoidance behaviors in response to environmental conditions that indicate increased risk of predation. Despite a sacrifice in resource capture from foraging, long-term selective pressures can reward predator detection and avoidance. ACKNOWLEDGEMENTS We would like to thank Hannah ter Hofstede for teaching us everything we know about bats and trusting us with her valuable recording equipment. We would also like to thank Don Carlos for hanging out with us “mujeres nocturnas.” AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Clarke, J.A. 1983. Moonlight’s influence on

predator/prey interactions between short-eared owls (Asio flammeus) and deermice (Peromyscus maniculatus). Behavioral Ecology and Sociobiology 13: 205-209.

Kavaliers, M., and E. Choleris. 2001. Antipredator responses and defensive behavior: ecological and ethological approaches for the neurosci-

ences. Neuroscience & Biobehavioral Reviews

25: 577-586. Kotler, B.P. 1984. Effects of illumination on the rate of resource harvesting in a community of desert rodents. The American Midland Naturalist 111: 383-389. Lima, S.L., and L.M. Dill. 1990. Behavioral

decisions made under the risk of predation: a review and prospectus. Canadian Journal of Zoology 68: 619-640.

Figure 3. Bat passes on the first night were most frequent in the dark without predator calls. See Table 1 for corresponding analyses.


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Pyke, G.H., H. R. Pulliam, and E.L. Charnov. 1977. Optimal foraging: a selective review of theory and tests. Quarterly Review of Biology 52: 137-154. Quinn, G.P., and M.J. Keough. 2002.

Experimental Design and Data Analysis for Biologists. Cambridge, UK: Cambridge UP.

Rydell, J., Entwistle, A., and Racey, P.A. Timing of foraging flights of three species of bats in relation to insect activity and predation risk. Oikos 1996: 243-252.

Shen, Y., Liang, L., Zhu, Z., Zhou, W., Irwin,

D.M., Zhang, Y., and D.M. Hillis. 2010. Adaptive evolution of energy metabolism genes and the origin of flight in bats. Proceedings of the National Academy of Sciences 107: 8666-8671.

Stiles, G., and A.F. Skutch. 1989. A Guide to the Birds of Costa Rica. Comstock Publishing Associates, Ithaca, NY. Voigt, C.C., Sorgel, K., and D.K.N. Dechmann.

2010. Refueling while flying: Foraging bats combust food rapidly and directly to power flight. Ecology 91: 2908-2917.

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SPATIAL PATTERNING IN THE ABUNDANCE OF FLORAL MITES PHORETIC ON HUMMINGBIRDS

LARS-OLAF HӦGER AND ZACHARY T. WOOD


Abstract: When resources are patchy and dispersal is limited, population stability in a landscape can become sensitive to dispersal and extinction events in the metapopulation. Spatial patterns in abundance can reveal the extent of patchiness and therefore the potential for metapopulation dynamics. We investigated how the distribution of flowers across a range of scales affected the abundance of mesostigmatid mites that feed in hummingbird-pollinated flowers and are phoretic on the hummingbirds. Abundance was relatively uniform among flowers within inflorescences and negatively associated with the occurrence of other inflorescences within one meter. However, higher numbers of inflorescences within five meters were associated with higher abundance of mites per flower, perhaps because mite populations are patchy and exhibit localized aggregations. Organisms with relatively limited dispersal capabilities depend highly on patches in close proximity for colonization and therefore tend to exist in fragmented, spatially patchy metapopulations. Key words: Bomerea alstroemeriaceae, patch structure, phoresy, metapopulations, mites INTRODUCTION Populations occur in space across a landscape. Dispersal limitations (e.g., geographic barriers) or physiological constraints (e.g., survival and reproduction limits) can sometimes confine subpopulations to patches within landscapes. When organisms can disperse among patches, even infrequently, the population of patches or metapopulation can be stable, even though not all patches are occupied and occupancy of some is intermittent. As patches become distant beyond reasonable dispersal, a tipping point may be reached in which a metapopulation collapses and most patches become unoccupied (Hanski, 1998). Therefore, the stability of populations that exist as metapopulations can be highly sensitive to the scale of patches relative to the dispersal capability of individuals.

Organisms that rely on patchily distributed resources are obvious candidates for being structured as metapopulations. We studied phoretic mesostigmatid mites that live within hummingbird-pollinated flowers of Bomerea alstroemeriaceae at the Cuerici Biological Station in Costa Rica. B. alstroemeriaceae flowers occur in large inflorescences, 20 or more flowers on the same stem. The mites move among plants in the nares of feeding hummingbirds (Colwell and Naeem, 1994). Inflorescences can be many meters away from the nearest other inflorescence.

The patchy dispersion of B. alstroemeriaceae inflorescences and the dependence of mites on facilitated transportation suggest that the dispersion of mites should be even patchier than the floral resources that support them. We sought to understand how spatial arrangements of B. alstroemeriaceae affected mite abundance. We tested three hypotheses: (1) mites are patchy (aggregated) at the scale of individual flowers or inflorescences due to the limitations of mite movement, (2) mites are patchy at the coarser scale of groups of inflorescences, or (3) hummingbird facilitated dispersal is so efficient that mite abundance is relatively uniform even at coarse scales, implying that they are not subject to metapopulation dynamics. METHODS We surveyed inflorescences of B. alstroemeriaceae along a research trail at the Cuereci Biological Field Station at approximately 2,600 meters above sea level on Cerro de la Muerte, Costa Rica. We haphazardly selected inflorescences and counted the number of flowers in each inflorescence. We randomly sampled two flowers per selected inflorescence and counted the number of mites in each. We recorded the number of additional B. alstroemeriaceae inflorescences located within a 1, 5, and 10 meter radius of the


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focal inflorescence (for data distributions, see Figure 1).

We employed a nested, random effects analysis of variance (ANOVA) to estimate how much of the variation in mite abundance per flower could be explained by the inflorescence to which it belonged. That is, did flowers within an inflorescence tend to have similar numbers of mites?

We tested for the effects of nearby flowers and inflorescences on the number of mites per flower using a generalized linear model with a Poisson distributed link function. We examined several models with number of flowers per focal inflorescence and number of additional inflorescences within 0-1, 1-5, and 5-10 meters as potential independent variables. We used AICc, Log-likelihoods, and corresponding model weight to compare the five competing models (Anderson, 2008). We used model averaging based on our model weights to arrive at a final predictive model (Anderson, 2008).

To evaluate whether there was clumping of mite abundance at a coarser scale than 10 m, we created and examined a map of mites per flower throughout or study area. RESULTS Mite abundance was strongly structured by individual inflorescences: 70% of the variation in per-flower mite abundance was explained by the inflorescence to which a flower belonged, and only 30% of the variance was attributable to the flowers within inflorescences.

In evaluating the predictors of mite abundance in inflorescences, we found that two models were substantially better than the other three competing models (Table 1; ΔAICc= 54 between Models 1 and 2, and Model 3). These top two models had identical AICc values and model weights (760 and 0.5, respectively); one model containing all the predictors had higher goodness of fit (lower log-likelihood) but had two more parameters than the alternative plausible model which only included inflorescences between meters and 0-5 meters. Because both models were equally plausible we adopted a model-averaging approach (Anderson, 2008) to yield weighted parameter estimates for a single predictive model of mite abundance (Table 1): 𝑁𝑁𝑁𝑁𝑁𝑁 = 6.67−0.15𝑁𝑁 − 1.74𝑁0−1𝑁

+

0.61𝑁1−5𝑁 − 0.11𝑁5 −10𝑁 In this model, Fi is the number of flowers per

inflorescence, and Ix-ym is the number of inflorescences between x and y meters from the focal flower. Inflorescences with more flowers had fewer mites (Figure 2A). Within a 1 meter radius the number of mites per flower decreased with the number of inflorescences (Figure 2B). Within a 1-5 meter band the number of mites per flower increased with number of inflorescences (Figure 2C). Between 5 and 10 meters, increased inflorescence density decreased mite abundance in the focal flower (Figure 2D). Overall, there was a positive trend in mites per flower with number of inflorescences within 10 meter (chi-square = 11.62, df=1, p=0.0006, Figure 3). We saw no obvious large-scale trends in mite abundance (Figure 4).

Table 1. Models predicting number of mites per flower. Model # Parameter estimate ± standard error

Intercept

Flowers per inflorescence

Inflorescences between 0 &

1m


5m


10m

Δ

AICc LogL Wi

1 6.92 ± 0.70 -0.03 ± 0.01 -1.52 ± 0.32 0.65 ± 0.14 -0.21 ± 0.10 0 374.65 0.5

2 6.42 ± 0.65 -1.96 ± 0.26 0.60 ± 0.09 0 377.21 0.5

3 7.95 ± 0.69 -1.88 ± 0.29 54 405.13 0

4 2.75 ± 0.28 0.54 ± 0.09 46 401.28 0

5 4.28 ± 0.23 91 424.4 0

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DISCUSSION Our results indicate that these mesostigmatid mites exist in patchy subpopulations. Our finding that flowers within the same inflorescence have similar numbers of mites suggests that mites are able to move readily about an inflorescence, possibly without hummingbird facilitation. The result that an increased number of flowers per inflorescence or inflorescences within one meter decreases the abundance of mites in each flower suggests that mites are dispersing and spreading out within a one meter patch. As this range seems beyond that of an individual mite’s normal dispersal ability, the observed spreading out is probably hummingbird facilitated. Collectively, this implies the existence of meaningful patches for the mites at a scale of about a one meter radius.

The pattern changed between a one and five meter radius. Mite abundance per flower increased with inflorescence abundance between one and five meters from the focal flower. This positive effect on mite abundance suggests a population subsidization effect from nearby patches. Having more neighbor inflorescences increases the frequency of colonization and immigration events. This could be due to the relatively local feeding patterns of hummingbirds.

The small negative effect on mite abundance per flower from inflorescences outside a five meter radius could be due to a different feature of hummingbird behavior. Hummingbirds may pick areas with higher inflorescence densities to feed, so having a high density of inflorescences far from the focal inflorescence could create a more preferred patch for hummingbirds, which would therefore decrease the frequency of visits to the

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focal patch, decreasing the chance of patch colonization. This remains consistent with the interpretation that most colonization and immigration events in our mite metapopulation are among flowers within five meters. If so, the maintenance of the mite metapopulation would require other flowers within a relatively small distance. Flowers within an inflorescence of B. alstroemeriaceae open simultaneously and appear to be relatively short-lived, so frequent colonization events must be necessary to maintain an extant metapopulation. Without regular movement, the sub-populations of mites would likely perish with their inflorescences. Thus our study population of mesostigmatid mites could be quite sensitive to landscape alterations that increase the nearest neighbor distances of B. alstroemeriaceae inflorescences to greater than five meters.

Our study demonstrates the potential for populations to be structured at multiple scales. Hierarchies of demographic spatial structure might be a common feature of populations that have multiple modes of dispersal (e.g., mites that walk among flowers vs. catching rides among inflorescences). Understanding the scale of patches in spatially heterogeneous populations can be important to understanding population dynamics and stability. ACKNOWLEDGEMENTS We would like to thank Matthew Ayres, Jessica Trout-Haney and Vivek Venkataraman for their help and guidance in this project. AUTHOR CONTRIBUTIONS Both authors contributed equally

LITERATURE CITED Anderson, D.R. 2008. Model based inference in

the life sciences: a primer on evidence (New York; London: Springer).

Colwell, R.K., and Naeem, S. 1994. Life-History Patterns of Hummingbird Flower Mites in

Relation to Host Phenology and Morphology. In Mites, M.A. Houck, ed. (Springer US), pp. 23–44.

Hanski, I. 1998. Metapopulation dynamics. Nature 396, 41–49


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FEMALE MATE CHOICE AND MALE LEKKING BEHAVIOR IN DROSOPHILA

AMELIA L. RITGER

Faculty Editor: Matthew P. Ayres Abstract: The effects of female mate choice on males can be displayed in three major ways: ornamental displays, resource-based mating systems, and non-resource-based mating systems such as leks. Drosophila fruit flies are among the organisms where female mate choice, and its consequences, have been demonstrated in the laboratory. Field studies of sexual selection in Drosophila are rare by comparison. Drosophila males in the Cuericí Biological Reserve in Costa Rica were found apparently competing for females at a mushroom patch. Given that Drosophila larvae consume decaying fruit and fungi, I hypothesized that the Drosophila mating system was resource-based and that fly preferences within the patch would be structured by mushroom size. Results were contrary in that there was no relationship between mushroom size and the intensity of male competition or the number of female visits to mushrooms. However, Drosophila were attracted to some mushrooms more than others and male competition was highest on the same mushrooms that females were most likely to visit. Thus, the Drosophila were using the mushrooms as leks. Additionally, dominant male flies were much larger than non-dominant male flies, suggesting that larger males had higher mating success than smaller males. Keywords: Drosophila, lekking, mating systems, female mate choice INTRODUCTION Female mate choice drives sexual selection on males across many taxa. Females may choose males by their physical attributes (Brown 1978), the quality of territory they control (Pleszczynska 1978), or their behavior (Janetos 1980). One common sexual selection model suggests that female mate choice drives ornamental displays in males. For example, male peacocks often have exaggerated ornamental displays preferred by females even though they come at the cost of higher predation risk (Andersson 2008). Mate choice can also create resource-based mating systems, which exert pressure on males to defend a territory that provides the female with a resource. For example, male Mediterranean pollen wasps defend water collection sites and flowers, both of which are important for female brood care (Groddeck et al 2004). In non-resource-based mating systems such as leks, males display in an area lacking a resource where females aggregate solely for the purpose of mate selection, presumably because male success in leks tends to indicate good genes. For example, lekking male cichlids dig and defend spawning pits where females congregate and mate with the defending male (Nelson 1995).

Many Drosophila species breed in decaying fruit or fungi (Borror et al 1989) and are frequently used for laboratory studies on genetics and evolution due to their short generation time

(10-30 days) and relatively short adult life span (Spieth 1974). Female Drosophila have been found to discriminate between males based on mutant type (Rendel 1945), ability to successfully defend food resources (Hoffmann 1987), and lekking display size (Aspi and Hoffmann 1998).

In the Cuericí Biological Reserve, Costa Rica, I encountered aggregations of Drosophila males aggressively fighting each other in a mushroom patch. There appeared to be a dominant male on each mushroom and a number of floater males and females flying around the mushrooms. I observed females exploring the undersides of the mushrooms as though they were searching for oviposition sites, implying that the mushrooms were a larval feeding resource. I tested the hypothesis that this Drosophila mating system was resource-based and that female choice would be based on mushroom size. If the mushrooms were being used as a resource for feeding larvae, then it would be logical that both sexes would prefer larger mushrooms. If the Drosophila were not using the mushrooms as a resource but merely as a lek, there could be some mushrooms that were more favored than others but this would not necessarily be related to mushroom size. I also tested the theoretical expectation that male dominance would be related to body size (Tokarz 1985).

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METHODS I studied the Drosophila aggregated at a patch of unidentified fruiting basidiomycetes (Fig. 1) along the Cuericí canyon trail approximately 50 meters before the stream on the uphill slope. Observations were made between 10:00 and 15:00 on 30-31 January 2014. I used a compound microscope at 10-50x magnification and identified the flies as Drosophila by examining their body size (typically 2-4 mm), abdominal and sternoplural bristles, and wing venation (Borror et al 1989). I identified the dominant male as the only male on each mushroom that was aggressive towards and won fights against floater males that occasionally landed. I identified floater males as the males without control over a mushroom or as competitors who lost a fight against the dominant male. I measured ambient air temperature around the mushroom patch every 5 minutes using a Hoboware Data Logger JKST thermocouple.

Observational Study I assigned numbers to 43 haphazardly selected mushrooms in the patch (Fig. 1) and recorded observations of mushrooms with dominant males for 28 five-minute periods. During each trial period, I counted the number of male competitors that landed on the focal mushroom, whether or not the dominant male was replaced by a male competitor and the number of females that landed

on the mushroom. I also noted whether or not the dominant male mated with a female and looked for the presence of eggs underneath and inside the mushroom. I collected 16 dominant males and 21 floater males and measured body length, body width, and thorax length. After the trial period, I also measured the length, width, and height of each mushroom. Mushroom Desirability Experiment To test how desirable each mushroom was for males, I removed (by aspiration) the dominant male on each mushroom and measured the amount of time it took for a new dominant male to take over the mushroom. I removed the new dominant male and continued removing new dominant males for one minute, counting the total number of new dominant males that landed on each mushroom. Male Dominance Experiment To test if the dominant males were able to re-establish their dominance after being temporarily removed from their mushroom, I aspirated 10 dominant males and 10 floater males and respectively dyed them with orange or green fluorescent dye. I then waited 24 hours and counted the number of previously dominant males that were dominant on mushrooms and the number of floater males that became dominant on mushrooms. Statistical Analyses To analyze if the mushrooms were being used as a resource and if male and female flies were attracted to the same mushroom, I regressed the total number of females against the number of male competitors, the number of females against mushroom size, and the number of male competitors against mushroom size. I incorporated body length, body width, and abdomen length into a single metric of body size using a principal components analysis (PCA). The first principal component axis explained 80% of the variation with loadings of 0.903, 0.899, and 0.877 for body length, body width, and abdomen length, respectively. To examine the relationship between male dominance and body size, I compared the first principal component axis with fly social status using a two-tailed t-test (Jolliffe 2002). All analyses were performed using JMP® 10 statistical software (SAS Institute, Cary, NC).


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RESULTS Drosophila were only active at the mushroom patch between 10:00 and 15:00 on both observation days. Shaded ambient temperatures within those time periods were 10.8-11.8 °C on 30 January and 10.8-13.9 °C on 31 January. Observational Study Mushroom size was not related to the number of females (Fig. 2A) nor to the number of male competitors (Fig. 2B). However, the total number of female visits and the number of male competitors were positively correlated (Fig. 3). The smallest dominant male was larger than the largest floater male (Fig. 4). During the course of the 28 five-minute periods, I observed one mating by a dominant male and 4 cases where the dominant male was replaced during the trial. No eggs were found under or inside any of the

mushrooms with dominant males.

Mushroom Desirability Experiment There was no relationship between mushroom size and the time until arrival of a new dominant male on the mushroom (P = 0.63, r2 = 0.01). There was also no relationship between time of arrival of a new dominant male and number of male competitors (P = 0.66, N =28) or number of females (P = 0.25, N = 28). Male Dominance Experiment 18 hours after experimental dusting with fluorescent powder, only one orange dyed (previously dominant) male was found in the area. It was not on a mushroom and appeared moribund.

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DISCUSSION Neither female nor male flies showed any discrimination based on mushroom size (Fig. 2), yet both sexes were attracted to the same mushrooms (Fig. 3). Although Drosophila larvae have been known to consume mushrooms during development, I did not find any eggs present under the mushroom. However, I did see a mating occur on one of the mushrooms. Evidently, the mushrooms served as a mating territory but did not provide the flies with food. That is, the Drosophila males were lekking on the mushrooms and the females were aggregating to select mates based on the results of dominance contests among males.

My observations suggested that there might be a “home-field” advantage within this lek system because the dominant fly tended to remain dominant even if the competing male was larger. This could be because ownership status compels territorial residents to value a contested area more than intruders, and thus residents have the potential to gain more from winning and/or risk more by defeat (Enquist and Leimar 1987). Future studies could test for a home field advantage by experimental removal and later release of dominant males.

The strong relationship between body size and male social status suggests intense selection for large body size in Drosophila, especially because the one observed mating was by a dominant male (Fig. 4). The results of the male dominance

experiment would have been informative if there had been re-sightings the next day. I think that the flies were excessively dusted with the fluorescent powder and died as a result. However, it could also be that the typical male fly only competes in the lek for a single day. This would be consistent with the limited seasonal variation in this tropical environment and the typically short generation time of Drosophila. Additional marking studies with less aggressive dusting could indicate whether or not flies are able to retain status as dominant male over successive days.

Mating systems such as leks arise when female mate choice places strong pressure on males to compete or make themselves more attractive to the females by providing direct benefits (such as a high quality territory) or indirect benefits (such as good genes). Studying lekking behavior and other behaviors by males in response to female mate choice allows us to better understand the evolutionary basis for intersexual selection. ACKNOWLEDGEMENTS I thank Matthew Ayres for assisting me with the principal components analysis. Thanks also to Annie Fagan, Emily Goodwin, and Aaron Mondshine for peer-reviewing my paper and Jessica Trout-Haney and Vivek Venkataraman for reviewing my paper.

LITERATURE CITED Andersson, M. 2008. Sexual selection, natural

selection, and quality advertisement. Biological Journal of the Linnean Society 17: 375-393.

Aspi, J., and Hoffmann, A. 1998. Female encounter rates and fighting costs of males are associated with lek size in Drosophila mycetophaga. Behavioral Ecology and Sociobiology 42: 163-169.

Bateman, A.J. 1948. Intra-sexual selection in Drosophila. Heredity 2: 349-368. Borror, D.J., and White, R.E. 1970. A Field Guide to Insects: America North of Mexico. Houghton Mifflin Co., Boston, MA.

Borror, D.J., Triplehorn, C.A., and Johnson, N.F. 1989. An introduction to the study of insects, 6th edition. Saunders College Publishing, Philadelphia, PA.

Brown L.P. 1978. Polygamy, female choice, and

the mottled sculpin, Cottus bairdi. PhD dissertation, Ohio State University.

Enquist, M., and Leimar, O. 1987. Evolution of fighting behaviour: the effect of variation in resource value. Journal of Theoretical Biology 127: 187-205.

Groddeck, J., Mauss, V., and Reinhold, K. 2004. The resource-based mating system of the Mediterranean Pollen Wasp Ceramius fonscolombei Latreille 1810 (Hymenoptera, Vespidae, Masarinae). Journal of Insect Behavior 17: 397-418.

Hoffmann, A. 1987. A laboratory study of male territoriality in the sibling species Drosophila melanogaster and D. simulans. Animal Behaviour 35: 807-818.

Janetos, A. 1980. Strategies of female mate choice: a theoretical analysis. Behavioral


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Ecology and Sociobiology 7: 107-112. Jolliffe, I.T. 2002. Graphical representation of data

using principal components. Principal Components Analysis, 78-110. Springer, New York, NY.

Nelson, C. 1995. Male size, spawning pit size, and female mate choice in a lekking cichlid fish. Animal Behaviour 50: 1587-1599.

Pleszczynska, W.K. 1978. Microgeographic prediction of polygyny in the lark bunting. Science 201: 935–937.

Rendel, J.M. 1945. Genetics and cytology of

Drosophila subobscura. Journal of Genetics 46: 287-302. Spieth, H. 1974. Courtship behavior in

Drosophila. Annual Review of Entomoloy 19: 385-405.

Taylor, C.E., and Kekic, V. 1988. Sexual selection in a natural population of Drosophila melanogaster. Evolution 42: 197-199.

Tokarz, R. 1985. Body size as a factor determining dominance in staged agonistic encounters between male brown anoles (Anolis sangrei). Animal Behaviour 33: 746-775.

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SIZE DISTRIBUTION AND AGGRESSION IN RAINBOW TROUT, ONCORHYNCHUS MYKISS

ALLEGRA M. CONDIOTTE, NATHANAEL J. FRIDAY, AND ADAM N. SCHNEIDER


Abstract: Many organisms use aggression to increase access to limited resources. However, aggressive behaviors are energetically costly, and carry a risk of injury that could reduce an individual’s fitness. These risks are higher when aggression is among similarly-sized individuals, which may be capable of inflicting equal damage. Thus, the distribution of body sizes could have an effect on the frequency of aggressive behaviors. We tested whether agression in aquacultured Oncorhynchus mykiss (rainbow trout) varied with the size distribution of interacting fish. We surveyed O. mykiss in two enclosures, one with mostly large trout and some small trout and another with mostly small trout and some large trout. We counted the number of fin tears resulting from bites. The trout showed more signs of aggression in the smaller-size population than in the larger population, but neither sex nor fish length was related to frequency of fin tears. Larger fish in the small-size population had more tears on their dorsal, anal and caudal fins, suggesting that the majority of attacks on larger competitors was from behind, which is a significantly less risky approach, while smaller fish in this population had more tears in their pectoral and pelvic fins, suggesting that they are typically attacked from above or ahead, a strategy that is riskier for the attacker but may yield greater damage. The size distribution of interacting individuals can influence aggressive behaviors among organisms that live in groups. Key words: aggression, intraspecific competition, size distribution, rainbow trout, Oncorhynchus mykiss INTRODUCTION Individuals within a population compete for access to limited resources, from prey items to mating opportunities. Organisms can sometimes increase their success in intraspecific competition by asserting dominance over other conspecifics through aggressive behaviors. Aggression can increase an individual’s access to a limited resource, but it comes at a cost through loss of feeding opportunities and the risk of damage (Jaeger 1981).

The extent to which aggression is favored or disfavored is likely to be influenced by the social context. Animals can limit injury risks by tending to direct aggression toward smaller, less threatening individuals. In this case, larger animals should be more aggressive on average because they have more safe targets. On the other hand, animals of similar size are more likely to compete for the same set of resources; therefore, aggression against individuals of the same size yields greater rewards than against smaller individuals.

We examined the influence of differing size distributions of rainbow trout (Oncorhynchus mykiss) on the extent and nature of conspecific aggression. Specifically, we compared the aggression preferences of two groups of O. mykiss,

one with mostly large fish but some small fish and one with mostly small fish but some large fish, by examining the number of fin tears resulting from bites by conspecifics.

If fish direct their aggression to maximize the likelihood of successfully exerting dominance while simultaneously minimizing the risk of injury, there should be an inverse relationship between fish size and fin damage (more fin tears in small fish). However, if fish direct their aggression primarily to maximize competitive gain, fin damage should tend to be greatest in the most abundant size class. METHODS Data Collection We surveyed the length of fish in two enclosures in a small-scale trout farm in Cuerici, Costa Rica on 29 and 30 January 2014. One enclosure contained trout primarily of large body sizes (Interquartile Range = 37 cm to 44 cm), while the second enclosure contained trout of mostly small body sizes (IQR = 25 cm to 32.5 cm). Population densities were not known precisely between enclosures, but it seemed evident that density was much higher in the small fish enclosure. A mesh


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barrier separated the enclosures. Fish had been in the present enclosures for about six months.

In each enclosure, we attracted trout with food and captured them with a net. We put the captured trout in a submerged holding cage, then transferred them one at a time to a measuring tank, where we assessed them for fin damage. We counted the number of tears between the spines on each fin (Figure 1). We did not include bite marks and severed spines, which scar and usually do not regrow. We used fin tears as a measure because they heal relatively quickly and are thus an indicator of recent aggression, as opposed to other wounds that could have been inflicted much earlier, including when the fish were in different enclosures.

We also recorded the length and sex (based on jaw structure) of each trout before tagging and releasing it. We did not include fish under 20 cm in length because they could not be sexed.

Statistical Analysis We used a Kolmogorov-Smirnov test to compare the distributions of sizes in the two enclosures. We compared the number and distribution of fin tears between the two enclosure treatments. We employed a general linear model that included enclosure, sex, length, enclosure x sex, and enclosure x length as predictors of total number of fin tears. We used the same model to evaluate patterns in the first and second axes from a principal components analysis of number of tears on each of the seven individual fins (Table 1). RESULTS The variance was very similar between enclosures (SD = 6 vs. 7; F24,49=1.36), so the coefficient of variation was greater in the enclosure with smaller fish (CV = 100 x SD / mean = 23.0% vs. 14.0%). The Kolmogorov-Smirnov test showed clear differences between the enclosures in the size distributions of fish (P<0.0001). This reflected differences in mean body lengths and a tendency for the enclosure with large fish to be skewed to the left while the enclosure with smaller fish was skewed to the right (Figure 2).

Enclosure was the only factor that had a significant effect on total number of fin tears, with more tears in the enclsoure with smaller fish (F1,69 = 5.48, P = 0.022; Table 1). The first axis of the PCA was interpretable in about the same way as total tears (all positive loadings; Table 2). The second axis captured variation in the distribution of damage (high scores reflect more tears in pectoral fins and fewer in dorsal fins). The interaction between length and treatment was significantly related to the second principal component (F1,69 = 6.48, P = 0.013; Table 1). In the enclosure with small fish, but not the enclosure with large fish, PC-2 scores decreased with fish

Table 2: Loading values for the first two principal components of tears per fin.

Source Fin PC1 PC2 Dorsal 0.42 -0.44 Pectoral Left 0.23 0.35 Pectoral Right 0.10 0.68 Anal 0.42 -0.29 Caudal 0.44 -0.13 Pelvic Left 0.41 0.24 Pelvic Right 0.47 0.25 Percent Variation Explained 25.2 18.8

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size (Figure 3); this indicated that relatively small fish in this enclosure tended to show more damage

to the dorsal fins, while relatively larger fish showed more damage to the pectoral fins.

Table 1: Results of tests for enclosure, sex, and length on the number and distribution of fin tears. Total tears Principal component 1 Principal component 2 Source DF F Ratio p F Ratio P F Ratio p Enclsoure 1 5.48 0.022 3.97 0.05 1.23 0.27 Sex 1 1.74 0.19 2.38 0.13 2.20 0.14 Sex*Enclosure 1 0.18 0.68 0.24 0.62 0.03 0.87 Length 1 0.79 0.38 2.14 0.15 1.84 0.18 Length*Enclosure 1 0.62 0.43 0.22 0.64 6.48 0.013 Error 69

Figure 2: Distribution of trout size classes in the (A) large-size enclosure (mean ± SD = 41 ± 6 cm), and (B) small-size enclosure (mean ± SD = 29 ± 7 cm). The small-size enclosure also included some fish under 20 cm in length, which were not measured.

Figure 3: Number of fin tears in the (A) large-size enclosure and (B) small-size enclosure. Dashed lines show the non-significant lines of best fit. Note that axes are scaled differently.

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Figure 4: The second principal component of fin tears plotted against body length in the (A) large-size enclosure, and (B) small-size enclosure. DISCUSSION Intraspecific aggression among fish could theoretically be driven by either fish maximizing success and minimizing injury, which would involve mostly attacking smaller fish, or maximizing competitive gain by attacking the competitors closest in size. In our study, Oncorhynchus mykiss showed more signs of aggression in the population with smaller fish that were relatively more varied than in the population with larger fish that tended to be closer in relative size.

Neither sex nor individual-size was related to trout aggression, and the size of the most damaged fish did not differ between the large-size and small-size fish populations.This suggests that aggression was undirected among larger fish, rather than being strategically directed to minimize risk or maximize reward, while in the population of smaller fish, where relative size variation was greater, aggression was more evident (more tears in general) and the pattern of tears was structured by size. Larger fish in the small-size population had relatively fewer tears on their pectoral and pelv fins and more tears on their dorsal, anal and caudal fins, suggesting that the majority of attacks on larger competitors were from behind, which is a significantly less risky approach. In contrast, smaller fish in this

population had more tears in their pectoral and pelvic fins, suggesting that they are typically attacked from above or ahead, a strategy that is riskier for the attacker but may yield greater damage. This implies that length, or relative length within a population, may have an effect on aggressive strategy, perhaps based on reprisal avoidance.

Our study was unable to separate effects that might have been due to fish density. The trout population with smaller fish had more fish per cubic meter than the other enclosure (Carlos Solano, personal communication), but we were unable to estimate relative biomass per cubic meter. High population density can cause higher levels of competition and increase encounter rates, which can lead to greater frequency of aggressive behaviors (Skogland 1983). However prior studies at Cuerici Biological Station have shown that population density had no effect on fish body condition (Costello et al. 2012). This is consistent with the increased aggression in the small size population being more related to the size distribution than to the population density. However, there would be value in further studies that more effectively isolate effects of size distributions vs. density.

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other. In a population skewed toward smaller fish, where the few large trout may be more likely to attack and seriously injure smaller competitors, this could lead to a higher level of aggression overall.

Our results indicate that the size distribution of a population may affect the distribution of damage on a fish’s body. A possible explanation for the damage on smaller fish being located more on the front and top of the body relative to larger indivials is because larger fish are likely to be more willing to risk attacking a fish from the front. Future studies could examine this pattern further. Also, focused observational studies of trout aggression might shed light on the costs and benefits of aggression vs. non-aggression.

When resources are limited, many organisms direct aggression toward conspecifics to expand their access to resources and presumably increase their

fitness. However, the costs of aggressive behaviors, including the risk of injury, may put limits on the extent of aggression. Our results indicate that size distribution of interacting individuals can influence aggressive behaviors, and that aggression can change with age, size, or position in group social hierarchy. ACKNOWLEDGEMENTS We thank Carlos Solano for the use of his aquaculture pools and his assistance in data collection. We also thank Jessica Trout-Haney and Vivek Venkataraman for their advice on the writing process, and Matthew Ayres for his direction. AUTHOR CONTRIBUTIONS All authors contributed equally in the data collection and writing process.

LITERATURE CITED Costello, R.A., Deffebach, A.L, Gamble,

M.M., and Kreher, M.K. 2012. Effects of density on farmed Oncorhynchus mykiss body condition and implications for economic sustainability of small-scale fisheries. Dartmouth Studies in Tropical Ecology 22: 62-68.

Skogland, T. 1983.The effects of density

dependent resource limitation on size of wild reindeer. Oecologia 60:156-168.

Jaeger, R.G. 1981. Dear enemy recognition and the costs of aggression between salamanders. The American Naturalist 117: 962-974.


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AQUATIC COMMUNITY COMPOSITION OF THE RÍO CLARO ESTUARY

ALLEGRA M. CONDIOTTE, NATHANAEL J. FRIDAY, LARS-OLAF HÖGER, AARON J. MONDSHINE, ELLEN R. PLANE, AND AMELIA L. RITGER


Abstract: Species distributions are determined by environmental factors, which may create abrupt or gradual ecotones between habitats. Estuaries mark a transition zone between freshwater and marine systems, and create a unique overlap of conditions that change temporally with tidal flow. Ecotones like estuaries create edge habitat that may increase species richness by providing additional niches; however, temporal salinity shifts create an osmoregulation challenge that may reduce species diversity. Species assemblages could change suddenly near the edge of high tide incursions, or individualistic responses in tolerance could create a gradual gradient in community structure along the estuary. In fact, we found a gradual transition in species composition from the mouth of the river upstream, with an intermediate zone of high species richness at about 1100 meters that could not be explained by a salinity gradient. Our results demonstrate the importance in defining niches of factors more diverse than salinity, including substrate and flow rate. Key words: community dynamics, ecotone, estuary, Río Claro INTRODUCTION Species distributions are limited by abiotic and biotic factors that vary over geographic distances. Such ecostones can be graded or abrupt. Resource availability, tolerance for environmental conditions, species interactions, and niche partitioning all influence the degree of overlap of species assemblages at ecotones. Estuaries form an ecotone between freshwater and marine ecosystems. Tidal flow alters these ecosystems on a temporal scale as the high tide shifts marine and brackish conditions up river. Changing water depth and direction of flow can also alter the riverbed substrate, moving cobbles and organic particulates of varying sizes, affecting river characteristics even far above the high tide line. Estuary systems can contain generalist species that are quite tolerant of changes in water chemistry, as well as relatively specialized species that are more restricted in where they occur. Many species in the Río Claro spend part of their life cycle in the Pacific Ocean (Lyons and Schneider 1990, Schneider and Lyons 1993), and some marine species use estuaries as temporary nursery sites (Claridge et al. 1986). The Osa Peninsula is unusual in a way that might matter to the estuary ecosystem because most freshwater fish species in the area are derived from marine species (Constantz et al. 1981). We studied the Río Claro for estuary species richness, species assemblages, and the nature of

shifts in the community structure. Ecotones such as the estuary boundary between fresh and saltwater provide a type of edge habitat that can facilitate species richness by encouraging colonization by edge-adapted species (Naiman et al. 1988, Ward et al. 1999). However, the temporal variation in water chemistry in estuaries poses a physiological challenge to aquatic species. Tidal swings in osmolarity and pH, for example, might result in reduced species richness at the ecotone. The ecotone associated with tidal fluxes might produce edges in the community structure as entire groups of organisms meet a threshold, or species might respond quite individualistically to the many ecological factors correlated with tidal fluxes, in which case there would be a gradual shift in species assemblages along the estuary. METHODS Data Collection We assessed species composition along the Río Claro in Corcovado National Park, Costa Rica on 4-5 January, 2014. We sampled a site every 100 meters between 286 and 2050 meters upstream of the river mouth (Fig. 1). At each site we recorded water temperature, pH, and salinity using a YSI Model 63 handheld system. We also noted light condition, water clarity, presence of algae, approximate water depth, substrate type, and water flow.

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We spent 10 minutes at each site categorizing aquatic vertebrate and invertebrate species and assessing their relative abundances. When possible, we recorded total counts of individuals of a species. We estimated abundances of high-density organisms using quadrats of approximately 1 m2. We classified fish species using the FishBase.org database. Statistical Analysis We square root transformed the species richness data and then employed a Principal Components Analysis to reduce the dimensionality of community structure (Table 1). We described pH and salinity along the study transect with a 4-parameter logistic function (Equation 1):

f (x) = 11+ eB´(xmid-x) ´ (Ymax -Ymin )+Ymin

Equation 1

where x is the measured value of salinity or pH, B is the slope, xmid is the inflection point, and Ymax

and Ymin are the maximum and minimum for the function. Table 2. Estimated parameters for logistic fits of salinity and pH as a function of distance from river mouth (Equation 1).

Salinity pH

B ± SE -0.003 ± 0.02 -0.002 ± 0.003

Ymax ± SE 35.0 ppt 8.14 ± 0.17

Ymin ± SE 0.09 ± 0.02 ppt 7.78 ± 0.06

Xmid ± SE 1409 ± 335 meters

We employed a 3-parameter unimodal

function to describe species richness along the transect (Equation 2):

f (x) = a´(dist -h)2 +k Equation 2

Table 1. First and second principal components axis comparing community structure across the river.

Taxa Principal Component 1 Principal Component 2 Coenobita sp. 0.17 -0.54 Uca sp. 0.07 -0.52 Neritina latissima -0.67 0.46 Cochliopina tryoniana -0.45 0.32 Atyidae -0.86 0.36 Bufo marinus -0.39 0.39 Actinopterygii morphotype 1 0.37 -0.71 Sphoeroides annulatus 0.68 0.48 Actinopterygii morphotype 2 0.38 -0.57 Agonostomus monticola -0.45 0.29 Actinopterygii morphotype 3 0.37 0.36 Unique Algal Morphotype 0.18 0.49 Apogon pacificus 0.68 0.33 Apogon dovii 0.53 0.41 Cryptoheros myrnae 0.32 0.10 Gobidae 0.41 0.33 Bathygobius soporator 0.39 0.55 Bathygobius ramosus -0.22 -0.04 Lutjanus inermis 0.32 0.10 Lutjanus guttatus 0.62 0.39 Centropomus undecimalis 0.38 0.29 Mugil curema 0.18 -0.09 Pseudophallus starksi -0.6 0.16 Percent variation: 21.50% 16.00%


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where a is the concavity of the curve, dist is the distance from the mouth of the river, h is the x-value of the vertex, and k is the y-value of the vertex. We constructed a floating bar chart to examine the differences in range among species. RESULTS Salinity and pH both decreased with distance from the mouth of the river (Table 2, Figs. 2-3). Each aquatic species had a different spatial distribution along the Río Claro (Fig. 4). Species richness peaked (k = 7.3 ± 0.7 species) at an intermediate

distance of 1132 ± 98 meters from the mouth (Fig. 5).

The first axis from a PCA of community structure indicated a gradual, approximately linear, change in the community along the length of the river (Table 1, Fig. 6). The second axis of community structure indicated a relative rapid change in structure in the lower river with relative stasis above 800 meters. Patterns in PC-2 reflected that two species of Actinoptergii fish were abundant near the river mouth and rare higher in the river (Table 1, Fig. 7).

Figure 1. The sites at which we measured stream characteristics and community structures. Waypoints are overlaid on a GoogleTM Earth map of the first 2.5 kilometers of the Río Claro. ‘RioCMouth’ marks the defined mouth of the river.

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Figure 2. Water salinity (measured 0 to 2 hours after low tide on 05 February 2014) decreased with distance from the mouth of the river.

Figure 3. pH (measured 0 to 2 hours after low tide on 05 February 2014) decreased with distance from the mouth of the river.

Figure 4. The observed range of aquatic species found along the Río Claro for each of 23 taxa. Species such as Coenobita sp. and Actinopterygii morphotype 2 were found closer to the ocean whereas species such as Pseudophallus starski and Neritina latissima were found furthest upstream.


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Figure 5. Species richness peaked at 1132 ± 98 meters from the mouth of the river, with 7.3 ± 0.7 species.

Figure 6. The first axis of community structure showed a gradual and approximately linear transition in structure along the river from the river mouth to 2000 meters above the mouth (P<0.0001, r2 = 0.58). DISCUSSION Our results indicated a gradual transition in species composition and abundance upstream from the mouth of the river (Fig. 6, Fig. 7). The ecotone did not create a congruent threshold for multiple species. Apparently, the niches of our 23 study taxa were defined by a diversity of different environmental features (probably a combination of biotic and abiotic factors). The result was that species distributions were quite individualistic. Species richness peaked at about 1100 meters

Figure 7. The second axis of community structure vs. distance from the river mouth, P = 0.0043, r2 = 0.49 from the mouth of the river. This marked a transition zone in which marine and freshwater species overlapped.

Additionally there were four fish species that occurred only in the transition zone. Evidently, this area represents a unique set of conditions not available at any other point along the stream system. The low salinity (0.0-0.1 ppt) in this area suggests that factors other than salinity, possibly including mixed sediment size, varying water flow/depth, and intermittent shading of the river are responsible for the high community richness.

River width likely affects community composition and species abundance as well. As stream or river width changes, so does substrate size, flow rate, and solar input. For example, the flat snail Neritina latissima began to appear only once cobble size and water flow increased, which seemed to be where their body shape gave them an advantage in clinging to large cobbles in a fast-moving water column. We also saw that average fish size tended to decrease with distance upstream for those fish species with broad ranges. This suggests that life stage dictates a portion of community structure in the Río Claro.

The principal components analysis suggested the importance of certain species as drivers of community composition. In sites on the upstream side of 1100 meters, Pseudophallus starksi, Neritina lattissima, and Atyidae sp. became important elements of the community (Table. 1).

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In sites downstream of 1100 meters, Sphoeroides annulatus and Actinopterygii became more important members of the community. This change in species likely indicates changes in trophic structure. The species becoming more abundant upstream were frequently at low trophic levels (e.g., algal consumers), while at least one of the notable downstream species (Sphoeroides annulatus) is a known predator at multiple trophic levels (Chávez Sánchez et al., 2008). This may reflect there being relatively greater energy availability in communities near the ocean, allowing for more species at higher trophic levels.

The shift in community structure and species richness along the Río Claro lends insight into the factors affecting transitional zones. The

importance of factors other than salinity, including substrate and flow rate, in determining community composition, merits further study. These physical changes in the environment can influence optimal organism morphology, influence the range of niches that are available, and thereby shape community structure. ACKNOWLEDGEMENTS We thank Matthew Ayres, Jessica Trout-Haney, and Vivek Venkataraman for their assistance. AUTHOR CONTRIBUTIONS All authors contributed equally in the data collection and writing process.

LITERATURE CITED Bell, J.D., Pollard, D.A., Burchmore, J.J.,

Pease, B.C., and Middleton, M.J. 1984. Structure of a fish community in a temperate tidal mangrove creek in Botany Bay, New South Wales. Australian Journal of Marine and Freshwater Research 35: 33-46.

Chávez Sánchez, M.C., Álvarez-Lajonchère, L., Abdo de la Parra M.I., and García Aguilar, N. 2008 Advances in the culture of the Mexican bullseye puffer fish Sphoeroides annulatus, Jenyns. Aquaculture Research. 39: 718–730.

Claridge, P.N., Potter, I.C., and Hardisty, M.W. 1986. Seasonal changes in movements, abundance, size composition and diversity of the fish fauna of the Severn Estuary. Journal of the Marine Biological Association of the United Kingdom 1: 229-258.

Constantz, G.D., Bussing, W.A., Saul, W.G. 1981. Freshwater fishes of Corcovado National Park, Costa Rica. Proceedings of the Academy of Natural Sciences of Philadelphia 133: 15-19.

Gore, P. 2000. Cluster Analysis. Pages 297-

321 in H.E.A. Tinsley and S.D. Brown, editors. Handbook of Applied Multivariate Statistics and Mathematical Modeling. Academic Press, Waltham, MA.

Lyons, J. and Schneider, D.W. 1990. Factors influencing fish distribution and community structure in a small coastal river in southwestern Costa Rica. Hydrobiologia 1: 1-14.

Martino, E.J., and Able, K.W. 2003. Fish assemblages across the marine to low salinity transition zone of a temperate estuary. Estuarine, Coastal and Shelf Science 56: 969-987.

Quinn, G., and Keough, M. 2002. Multiple and complex regression. Pages 111-154. Experimental Design and Data Analysis for Biologists. Cambridge University Press, Cambridge, UK.

Schneider, D.W., Lyons, J. 1993. Dynamics of upstream migration of two species of tropical freshwater snails. Journal ofthe North American Benthological Society 121: 3-16.


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CONSTRAINTS ON RESOURCE TRANSPORT IN ATTA COLOMBICA

MAYA H. DEGROOTE, ANN M. FAGAN, EMILY R. GOODWIN, ADAM N. SCHNEIDER, AND ZACHARY T. WOOD


Abstract: Organisms that transport and store resources can be constrained by their carrying ability. Atta colombica, leaf-cutter ants, are colonial central place foragers that transport massive quantities of leaf fragments to their mutualistic fungal partner (Agariceaea spp.) Unlike many organisms that transport resources, A. colombica alter the physical form of their resource by cutting it into smaller pieces. We studied the factors that influence A. colombica transport of leaf fragments. When harvesting leaves of variable mass per surface area, ants standardized leaf fragment surface area relative to body mass. Therefore, average fragment mass varied by about two-fold. Atta columbica may be constrained by leaf fragment surface area, which could be a function of handling ability. Ants also carried loads that were only half as heavy as the apparent optimum, possibly because of physiological constraints or selection acting on colonies rather than individuals. Key words: Atta colombica, central-place foraging, leaf-cutter ants, load size determination INTRODUCTION Organisms that transport and store resources can be constrained by their ability to carry those resources. Transport strategies are varied and often dependent on details of the specific resource. Morphological structures used to transport resources can vary from beaks to pincers, claws, and pouches. Despite these derived morphologies, organisms often have little to no control over the physical form of the resource they carry. The size or mass of the resource may limit transportation efficiency and compromise other biological imperatives such as mating and avoiding predation.

Atta colombica, leaf-cutter ants, are somewhat unusual in that they cut their own resource fragments and therefore have control over the size and mass of the resources they carry. As colonial central place foragers, leaf-cutter ants transport plant matter to a communal nest (Orians and Pearson 1979). Leaf-cutter ants have a caste system within which ants of different body sizes perform different tasks. Mediae ants cut fragments from the leaves of a variety of tropical plants and carry them to the colony. These leaf fragments are delivered to considerably smaller minim ants, which cut the leaf fragments into smaller pieces and feed them to Agaricaceae fungi. The fungi are farmed as food for the leaf-cutter ants (Weber 1972). The rate of transportation of plant material to the Agaricaceae fungi determines the amount of

fungus that can be farmed and influences the growth and reproductive success of the colony.

We sought to understand the factors that constrain A. colombica transport of leaf fragments. Since leaf-cutter ants transport leaves of different densities, leaf fragment surface area (henceforth: leaf area) and leaf fragment mass (henceforth: leaf mass) may vary. Both characteristics could affect ease of leaf handling by ants. While heavier leaf fragments may require more energy to carry, leaf fragments with larger surface area may be difficult to handle or have poor aerodynamics (Rudolph and Loudon 1986). Standardization of a characteristic (e.g., always carrying the ideal mass of leaf) can optimize transport efficiency or handling ability. If two characteristics are related (e.g., fragment size and mass across different densities), only one of these characteristics can be standardized at once, and whichever characteristic is the most constraining should be constant. We tested whether ants standardize leaf area or mass when harvesting from plants with variable leaf densities. We also examined whether ants maximize the rate of leaf mass transport. METHODS Data Collection

We conducted our study at the Sirena Biological Station, Corcovado National Park, Costa Rica, on 4-5 February, 2014. We followed eight different leaf-cutter ant trails to their source plants, where we randomly collected twenty ants and their

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respective leaf fragments. We selected single worker ants carrying leaves, ignoring leaves shared by multiple workers or those with minim ants riding on top. We measured ant mass, leaf mass, and leaf area. We calculated leaf density using leaf mass and leaf area.

We also conducted speed trials on one trail of leaf-cutter ants. We timed a single ant carrying a leaf fragment one meter and calculated its velocity. We then weighed each ant and its leaf fragment.


We employed a general linear model to evaluate effects on leaf mass and leaf area of ant mass, plant type, and their interaction. Visualizations of leaf area and leaf mass across plant types used the full model to adjust for effects of ant mass. We also examined how adjusted leaf size and leaf mass were related to mean leaf density (mass / surface area).

From the speed trials, we calculated ant load as mg of leaf carried per mg of ant body mass. We fit a regression of ant speed to load, leaf mass, and ant mass, then used the regression equation to compare carrying efficiency (milligrams of leaf carried per milligram of ant per second) to load carried.

All analyses were performed using JMP® 11.0 statistical software (SAS Institute, Cary, NC). RESULTS Ant mass ranged by about 10-fold (≈ 3 to 30 mg) and did not significantly vary among the ant-groups harvesting different plant types (F7,147 = 1.95, P = 0.07). Larger ants carried leaves with more surface area and greater mass (Equations 1-2, Figure 1B):

(1)

(2) The mean area of leaf fragments ranged from about 2 to 3 cm2, and the mean leaf density ranged from about 6 to 10 mg/cm2. Across trails, leaf area was unrelated to leaf density, but the mass of leaf fragments increased with increasing leaf density (Table 1, Figs. 2-3).

The load of leaf-carrying ants varied by about 8-fold (≈ 1 to 8 mg leaf/mg ant). The velocity of leaf-carrying ants decreased over this range of loads from about 4 cm/sec to about 2 cm/sec (Equation 3, P = 0.0016, r2 = 0.34, Fig. 4):

(3) The residuals of ant velocity vs. mg leaf/mg ant (Fig. 4) showed no pattern with either leaf mass or ant mass. Equation 4 is a rearrangement of Equation 3 to give a measure of leaf transport efficiency: (4)

This equation has a maximum when Mleaf / Mant, or load, is 7.53. This indicates that ants maximize the mass of leaf per mass of ant that can be moved a given distance per unit time when the fragment has a mass 7.53 times that of the ant. Actual ants carried much smaller loads than this (mean ± SD = 2.16 ± 1.31 mg/mg).

Table 1. Linear regression ANOVA results for the relationship between (1) ant mass

and leaf area and (2) ant mass and leaf mass across the eight plants that were sampled. MS is the mean squares.

Surface area of leaf fragment Mass of leaf fragment Source df MS F P MS F P Plant 7 1.72 2.85 0.009 297 5.77 < 0.0001 Ant mass 1 19.87 32.73 < 0.0001 1016 19.75 < 0.0001 Plant*Ant mass 7 0.89 1.47 0.18 102 1.98 0.06 Error 139 0.61 51.4


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Figure 1. Larger ants carried leaf cuttings with (A) greater surface area and (B) greater mass. See text for equations.

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DISCUSSION We tested whether leaf-cutter ants standardized leaf mass or area when harvesting leaves of variable mass per surface area. Leaf area remained fairly constant across A. colombica loads (Fig. 2), while leaf mass did not (Fig. 3). This indicates that ants are more constrained by leaf area than leaf mass. However, ants that carried heavier loads traveled more slowly (Fig. 4). Furthermore, ants carried much smaller leaf fragments than would be expected if they were maximizing leaf transport efficiency. A similar study on a different species of leaf-cutter ants also found that they carried pieces smaller than would be expected to maximize efficiency (Frankel and Johnson 2012).

Conservation of leaf area by ants may be due to several factors. For example, ant body size may constrain leaf area if leaf cutting is a fixed action pattern (Weber 1972). In fact, it seems that ants of the same size tend to cut leaf fragments of the same surface area because they position themselves on the edge of a leaf and rotate around a stationary point while cutting with their mandibles. This does not preclude the possibility that conservation of leaf area is optimal in terms of handling and transport efficiency. Analyses of ant velocity vs. loading indicated that ants carried lighter than optimal loads. Ants may be simply incapable of optimizing load. Alternatively, their behavior may be optimal in a way that our study did not capture. Lewis et al. (2008) reported that ants adjust load sizes depending on trail gradient, apparently to maintain transportation efficiency under variable trail conditions. It is possible that the ants in our study were optimizing load for a different trail gradient than where we recorded velocities. Another set of possible explanations for the apparently suboptimal loading of foraging ants follows from leaf-cutter ants being eusocial insects. For Atta columbica, fitness is an attribute of the colony, not of the individual workers. It would not be surprising if optimization for the colony has a different solution than optimization for individual workers (Burd 1996). For example, single ants burdened by heavy loads can slow the rate of all ants traveling in the same column, thereby reducing overall colony efficiency. Thus, apparently suboptimal behaviors on the individual level may be due to physiological constraints or to

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selection acting on colonies rather than individuals. ACKNOWLEDGEMENTS We would like to thank the staff of Corcovado National Park for their guidance as well as M.

Ayres for his assistance with the statistical analyses. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Burd, M. 1996. Foraging performance by Atta

colombica, a leaf-cutting ant. American Naturalist 148: 597-612.

Franco, N.B., N.S. Johnson. 2012. Load—velocity tradeoffs and optimal foraging behavior in Atta cephaloides, Dartmouth Studies in Tropical Biology 124-131.

Lewis, O.T., M. Martin, T.J. Czaczkes. 2008. Effects of trail gradient on leaf tissue transport and load size selection in leaf-cutter ants. Behavioral Ecology 19: 805-809.

Orians, G.H., and N.E. Pearson. 1979. On the

theory of central place foraging. D.J. Horn, R.D.

Mitchell, G.R. Stairs (Eds.), Analysis of Ecological Systems, Ohio State University Press, Columbus pp 154-177.

Rudolph, S.G., and C. Loudon. 1986. Load size selection by foraging leaf-cutter ants (Atta cephalotes). Ecological Entomology 11: 401-410.

Weber, Neal Albert. Gardening Ants: The Attines. Vol. 92. Philadelphia: American Philos. Soc., 1972.

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THE SPATIAL DISPERSION OF IMMATURE AND ADULT TROPICAL CICADAS

ALEXA E. KRUPP, REBECCA C. NOVELLO, AND CLAIRE B. PENDERGRAST

Faculty Editor: Matthew Ayres

Abstract: Many organisms experience different selection pressures between different life stages, with consequences for their physiology and behavior. Disparate selection pressures on juvenile and adult life forms may influence the spatial distribution of individuals at different life stages. Like many insects, cicadas have very different ecology as juveniles and adults. Nymphs feed underground on xylem fluids from tree roots and emerge as adults to feed, attract mates, and reproduce in the forest canopy. We examined the spatial distribution patterns of nymphal and adult cicadas in Corcavado National Park, Costa Rica. Mapping of nymphal exoskeleton abundance and adult calling intensity in the forest revealed a strong tendency for aggregation in both life stages, but nymph abundance was not related to adult abundance. Immatures and adults may rely on different resources even though both stages feed on xylem fluids and show high spatial aggregation. Key words: aggregation, cicadas, life stages, spatial distribution INTRODUCTION Selection pressures on organisms differ across life stages, which may influence their spatial distribution through time (Schluter et al. 1991, Soler et al. 2013). In many insects, growth is the measure of fitness relevant to immatures, while adult life stages are focused on reproduction. Saturinidae moths, for example, feed on leaves as caterpillars, storing lipids that they will use throughout their life. Adults, however, lack the capacity to feed, instead devoting their energy primarily to reproduction (van Nieukerken et al. 2011).

Cicadas also pass through distinct life stages associated with varying selection pressures. After females oviposit in trees, nymphs drop to the ground and burrow into the soil, where they feed on xylem fluids from tree roots. When they are fully developed, flightless nymphs emerge from the ground and molt to become adults capable of flight. Adult males then aggregate in the canopy, feeding on xylem fluids of above-ground branches and chorusing to attract females for mating (Young 1980). This basic life cycle is well established, but the spatial distribution patterns of cicadas and their dispersal abilities at different life stages, particularly in the tropics, are not well understood (Karban 1981, Williams and Simon 1995).

We examined the spatial distributions of juvenile and adult male tropical cicadas, considering possible drivers of these distributions. If nymph development is driven by competition

for limited food resources, survival should be higher in low-density areas, which would tend to a uniform spatial distribution of nymphs. The effects of competition could also cause nymphs in high-density areas to be smaller than those in low-density areas. Alternatively, if nymph fitness is driven by habitat quality, rather than limited by food resources, nymphs should be most likely to survive in high-quality habitats. In this case, we would expect nymphs to be spatially aggregated and larger nymphs to be found where densities are high.

We also tested hypotheses of spatial relationships between juvenile and adult cicada populations. Since nymphs and adults both feed on tree xylem, adults may have little incentive to disperse from their site of emergence. Adult males would then remain near the site of their nymphal development to aggregate and chorus, perhaps resulting in females oviposition near mating sites, creating self-perpetuating assemblages around high-quality resources (Oberdorster and Grant 2006). We could also see a spatial correlation between sites of male chorusing and nymph emergence if female dispersal is constrained. Alternatively, if adults are mobile and their resource needs are different, there might be no correlation between juvenile and adult spatial distribution. METHODS We conducted an observational study on 4-5 February 2014 at the Sirena Biological Station in


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Corcovado National Park. We measured the amplitude in decibels (dB) of cicada chorusing (a putative measure of adult male cicada abundance) every 200 paces along the Los Patos, Espaveles, and Guanacaste trails between 08:00 and 15:00 on 5 February (N = 28 sites). We recorded elevation and latitude/longitude of each site using Garmin GPSmap 76CSx. We used the Real Time Analyzer (RTA) iPhone application to estimate sound energy at each site to the nearest half decibel. We repeated decibel measurements on the Espaveles trail three times (N = 11 sites) – once on 4 February (14:30 - 15:30) and twice on 5 February (9:30 - 10:30 and 12:30 - 13:00). We also used the RTA application to estimate the peak frequency of cicada calls.

We divided each sample site into three sections, and each investigator searched one section for cicada exoskeletons for three minutes. We took haphazard subsamples of exoskeleton length at sites with fewer than 10 exoskeletons (low-density) and more than 40 exoskeletons (high-density). We measured 5-8 exoskeletons at each of four low-density plots (total of 18 exoskeletons) and 8-10 exoskeletons at each of three high-density plots (total of 19 exoskeletons). Statistical Analyses We converted decibels to sound intensity in milliwatts/m2.

(1)

dB =10 ´ log10(I1

I0),

I0 =10-9mW/ m2

The intensity of a sound wave represents the amount of energy transported through an area per unit time. This range of sound intensities detectable to the human ear is so large that a logarithmic scale is often used to represent sound intensities. The lower threshold of human hearing (10-9 mW/m2) is assigned an intensity of 0 decibels. A sound 10x times more intense than another sound is 10x the decibels (The Physics Classroom 2014). We chose to represent cicada calling in mW/m2 rather than decibels to accurately represent the differences in energy expended by adult male cicada communities rather than the sound landscape detected by the human ear.

We tested the repeatability of sound intensity across sites with a random effects ANOVA of measurements on the Espavales Trail (11 sites x 3

measurement times). We compared the variation among sites in the abundance of nymphs and adult males in terms of the relative difference between the 10th and 90th percentiles of exoskeleton abundance and sound intensity. We also evaluated the dispersion of exoskeletons using the ratio of variance to mean for the sample plots (variation/mean = 1 for random dispersion, < 1 for uniform dispersion, > 1 for clumped dispersion).

We used regression analyses to test for a relationship between sound intensity and exoskeleton frequency. We used a two-tailed t-test to compare exoskeleton sizes between randomly selected low-density and high-density sites. Finally, we used a Monte Carlo randomization test to evaluate whether sound intensity or exoskeletons were more similar between adjacent sites (separated by 200 paces) than among sites in general. For the randomization tests, we created a frequency distribution of the sum of squared differences between each pair of adjacent sites over 1000 randomizations with the vector of observed values randomly distributed among sites. We compared the actual sum of squared differences among adjacent sites to this spatially randomized distribution. We conducted all analyses in JMP 10 (SAS Institute, Cary, NC), Microsoft Excel 2008, and R.

Figure 1. Spatial patterns in sound intensity (chiefly from cicadas) were repeatable through multiple observation periods.

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RESULTS Points in space had repeatable patterns in noise. 60% of variation in sound intensity was attributable to location (Fig.1, random effects ANOVA). Peak frequency recorded at each site was 3.15 kHz. Hundreds of adult cicadas were observed feeding in Cedrela odorata trees at multiple sites between approximately 35 and 65 m from the ground. Average exoskeleton length was 24.78 ± 2.45 mm and did not vary with exoskeleton density (t = 0.16, P = 0.87, df = 33.26; Fig. 2). Sound intensity increased approximately 70-fold between 10th and 90th percentiles, and abundance of exoskeletons ranged from 0 to 48.8

between 10th and 90th percentiles. Based on a variation-to-mean ratio (VMR) of 25, exoskeleton spatial distribution was aggregated at the site level (Cox and Lewis 1966). Neither sound intensity nor exoskeletons were more similar between adjacent sites than expected by chance (Fig. 3). Local exoskeleton abundance did not predict sound intensity from cicadas (P = 0.18: Fig. 4, 5). DISCUSSION Male cicada calling displayed consistent trends of sound intensity over time; while average sound intensity varied somewhat between observation periods, the locations of relative high and low intensity remained consistent (Fig. 1). This suggests that groups of male cicadas do not modify their position or chorusing intensity relative to others in response to short-term environmental variations. These findings justify our choice of sound intensity as a metric for adult male cicada abundance. As cicadas of both sexes are attracted to congregational male chorusing (Karban 1981), it is also likely that cicada song indicates the presence of both male and female adults.

We frequently found adult cicadas, high sound intensity, and high exoskeleton abundance concurrently in areas containing Cedrelo odorata, a species of canopy tree. C. odorata is

Figure 2. Exoskeleton size (± SE) was almost identical between sites of low and high local abundance.

Figure 3. Results of randomization tests for adjacent sites being more similar than by chance in sound intensity (left) and exoskeleton. In neither case was there evidence of adjacent sites being similar. Arrows represent actual values relative to random distribution.


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Figure 4. Exoskeleton abundance and sound intensity were not correlated.

found in well-drained soils with moderate to high light availability, and therefore may indicate soil and canopy conditions conducive to both adult and nymph growth and survival (ISSG 2006).

Exoskeleton length did not vary between high- and low-density areas (Fig. 2). We observed little variation in exoskeleton size and a consistent peak frequency of cicada song across all sites, suggesting that our study system primarily contained a single species (Sueur 2002, Janzen 1983). If exoskeleton size indicates nymph habitat quality, stable exoskeleton length across plots with

variable local densities suggests that nymphs are not resource-limited at any density. However, nymph abundance may be a more appropriate indicator of habitat quality than individual size.

Exoskeleton abundance and sound intensity varied greatly through space, suggesting aggregations in both nymphs and adults (Fig. 5). Adult aggregation has been established as an essential component of mating behavior while nymph aggregation may be explained by patchy resource availability for nymphs.

The locations of high abundance for adults and nymphs were not obviously correlated (Fig. 5). This was surprising because cicadas are weak fliers and energy requirements and predation associated with dispersal may be high (Karban 1981). Additionally, habitat quality for nymphs and adults may be driven by similar factors due to the shared xylem resource (Yang 2006). The lack of correlation may indicate that adult males are re-aggregating in the canopy following emergence. Favorable canopy areas for adult male cicadas may be located in different areas than high quality nymphal environments. Cicada chorusing behavior has been linked to environmental conditions such

Figure 5. Spatial patterns in cicada sound intensity (light gray) and exoskeleton abundance (dark gray). Circe size represents low to high values.

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as light availability (Yang 2006), while the drivers of oviposition site selection by female cicadas are largely unknown. Further investigation of the biotic and abiotic factors affecting adult cicada distribution may shed light on the relationship between nymphal and adult populations.

Nymph and adult male cicadas both display highly aggregated spatial dispersion in the forest at the site level. As neither exoskeletons nor sound intensity were aggregated across sites (Fig. 3), aggregation appears to occur only at a finer scale. Location and spacing of aggregations may reflect habitat quality, mating pressure, or an interaction of factors at these sites. However, ultimate drivers of cicada distribution cannot be fully understood without further analysis of the scale at which these

aggregations occur and the environmental features associated with them. A deeper understanding of what factors influence organisms’ spatial distributions over the course their lifetimes could provide better predictions for how a changing environment will affect organisms’ distributions. ACKNOWLEDGEMENTS We would like to thank Matt for his wisdom and guidance in the forest and out and Hannah ter Hofstede for sharing her extensive knowledge of tropical insects and sound measurements. AUTHOR CONTRIBUTIONS All authors contributed equally. Becca ran analysis in R.

LITERATURE CITED Aanen, D. K., H. H. D. Licht, A. J. M. Debets, N.

A. G. Kerstes, R. F. Hoekstra, and J. J. Boomsma. 2009. High symbiont relatedness stabilizes mutualistic cooperation in fungus-growing termites. Science 326:1103-1106.

Calsbeek, R., L. Bonvini, and R. M. Cox. 2010. Geographic variation, frequency-dependent selection, and the maintenance of a female-limited polymorphism. Evolution 64:116-125.

Chazdon, R. L., R. W. Pearcy, D. W. Lee and N. Fetcher. 1996. Photosynthetic responses of tropical plants to contrasting light environments. Pages 5-55 in Mulkey, S., R. L. Chazdon and A. P. Smith, editors. Tropical

forest plant ecophysiology. Chapman and Hall, New York.

Kozlowski, T. T., P. J. Kramer, and S. G. Pallardy. 1991. The physiological ecology of woody plants. Academic Press, New York.

Darwin, C., and A.R. Wallace. 2012. An hypothesis to explain the diversity of understory birds across Costa Rican landscapes. Dartmouth Studies in Tropical Ecology 2012, in press.

Wilkinson, B. L., J. H. M. Chan, M. N. Dashevsky, D. L. Susman, and S. E. Wengert. 2009. Maternal foraging behavior and diet composition of squirrel monkeys (Saimiri oerstedii). Dartmouth Studies in Topical Ecology 2009, pp. 141-144.


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SPATIAL DISTRIBUTION OF LYCOSIDAE SPIDERS WITHIN AND ACROSS HABITATS

ALEXA E. KRUPP, AARON J. MONDSHINE, CLAIRE B. PENDERGRAST, AMELIA L. RITGER, AND ADAM N. SCHNEIDER


Abstract: Animals should preferentially select habitats where their fitness is maximized. Good habitats may offer high quality food resources, favorable abiotic conditions, or reduced vulnerability to predation. Ecological theories such as ideal free distribution, ideal despotic distribution and optimal foraging theory can predict common patterns of animal distribution. We considered the relative capacity of these three theoretical models to explain Lycosidae spider spatial distribution patterns at the micro and macrohabitat scale at the La Selva Biological Station, Costa Rica. We observed smaller spiders at higher densities in forest areas, and bigger spiders at lower densities in open fields. We found no relationship between body size and spider density at any spatial scale. Our findings suggest that optimal foraging determines community structure within a macrohabitat. Prey availability within a plot influenced spider density, providing evidence for ideal free distribution theory on the microhabitat level. Key words: Ideal despotic distribution, ideal free distribution, Lycosidae, macrohabitat, microhabitat, optimal foraging theory INTRODUCTION Organisms achieve maximal fitness by preferentially selecting habitats best suited to their morphology, life history, and resource requirements. Habitat selection should maximize opportunity for resource acquisition while minimizing the risks of predation. As environmental conditions vary with location, time, and scale, individuals seek out habitats best suited to their physiology and needs.

Various ecological models seek to identify and predict distribution patterns observed throughout the natural world. Under the ideal free distribution individuals distribute themselves among habitats in proportion to the quality of each habitat (Sinervo 2006). The ideal free distribution theory assumes that habitat quality decreases with increasing population density due to reduced per capita resource availability, and that all individuals are competitively equal (Sinervo 2006).

Alternatively, under the ideal despotic distribution, the best competitors dominate and control access to highest quality habitats, resulting in the exclusion of less competitive individuals from areas of high resource availability (Sinervo 2006). Finally, optimal foraging theory posits that different foraging strategies allow individuals to maximize energy acquisition while minimizing risk of predation in response to environmental conditions (Pyke et al. 1977). Optimal balance of foraging strategy and predator avoidance often

varies with organism size or life stage. For example, many reef fishes occupy protected mangroves as juveniles before moving onto the open reef as adults with higher energy demands and reduced vulnerability to predation (Nagelkerken et al. 2000).

We compared the population densities and body sizes of wolf spiders (family Lycosidae) within and across two habitat types, which allowed us to assess the spider population’s spatial distribution on both the macro and microhabitat level. Lycosidae are generalist predators that consume prey of variable size and type, from ants to small vertebrates (Marshall 1996; McCormick and Polis 2008). Body size varies widely among individuals and species, resulting in variable energetic requirements and prey capture efficiency (Birkhofer et al. 2006).

If Lycosidae distribute themselves accordaning to ideal free distribution, spider density would increase with microhabitat quality, while mean body size would remain constant across microhabitats. If instead, spiders are distributed accordaning to the ideal despotic distribution, higher quality microhabitats would be occupied by larger spiders capable of excluding smaller spiders (Birkhofer et al. 2006), while spider density would remain constant. Finally, if spider populations displayed dynamic optimization between size classes, mean body size would vary between macrohabitats.

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METHODS Data Collection We assessed Lycosidae density and habitat suitability in La Selva National Park, Costa Rica between 17:00 and 21:00 on 15-16 February 2014. We selected three sites in open fields and three sites in secondary forest. For each site, we randomly sampled eight 1 x 3 meter plots along a 60-meter transect and measured spider abundance and spider body length from head to thorax. We found spiders by scanning a plot using a headlamp to locate eye-shine, an established method for locating Lycosidae spiders (Kaston 1978). We also spent 60 seconds counting the number of insects, which are potential prey items, inside each plot. Statistical Analysis We performed an Analysis of Variance (ANOVA) for spider body size and density with respect to habitat type. We regressed spider density with respect to mean body size within each habitat type. We also performed an ANOVA to compare prey density across habitat types and for average body size with respect to prey density and habitat. Finally, we evaluated spider density with respect to prey density, habitat type and their interaction using a generalized linear model. All analyses were performed using JMP® 11.0 statistical software (SAS Institute, Cary, NC). RESULTS We found Lycosidae spiders with head to thorax lengths ranging from 1.4 to 21.0 mm from a number of different genera (N = 168), including

Figure 2. Spider density was higher in the forest than in the field.

Figure 3. Spider body size did not predict density in either habitat type.

Figure 1. Spiders were smaller on average in forest sites than field sites but field sites also varied in body size.


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Sosippus sp. (adults vary between 2.5 and 18.5 mm), Pardosa sp. (adults vary between 4.5 and 9.5 mm), and Lycoa sp. (adults range from 9.0 to 35.0 mm; Kaston 1978).

Spider size range was consistent across habitat type, but spiders were smaller on average in the forest than in the field (F5, 157 = 4.51, P = 0.004). Mean body size also varied significantly within each habitat (F5, 157 = 4.51, P = 0.002: Fig. 1).

Spider density was significantly higher in the forest than in the field (F5, 42 = 4. 22, P = 0.001: Fig. 2). Although there was a higher density of smaller spiders in the field and a lower density of larger spiders in the forest, there was no relationship between body size and spider density within each habitat (slope = -0.09 ± 1.5, P = 0.59, r2 = 0.01 and slope = 0.09 ± 2.2, P = 0.67, r2 = 0.01 respectively: Fig. 3). Therefore, body size did not predict spider density within or across habitats.

Prey density was over three times higher in the field than in the in forest (F5, 38 = 2.97, P = 0.003). Spider density was positively correlated with prey density (F3, 40 = 4.53, P = 0.03) and was higher in the field than the forest (F3, 40 = 4.53, P = 0.001: Fig. 4). The slope of spider density versus prey density did not differ between habitats (F3,38 = 1.84, P = 0.23). DISCUSSION We tested whether the spatial distribution of Lycosidae spiders at the micro and macrohabitat

scale was a better match with the ideal free distribution, ideal despotic distribution theory, or optimal foraging theory.

Within both forest and field sites, we observed greater spider density in plots with more prey items (Fig. 4), indicating that resource availability influences spider distribution on the microhabitat scale. No relationship between spider body size and prey density was observed, suggesting that competitive exclusion by size was not occurring in areas of high resource availability. These results jointly support the ideal free distribution theory as an explanation for spider distribution on a microhabitat scale.

On a macrohabitat scale, patterns of spider size and density did not fit the assumptions of ideal free distribution observed on the microhabitat level. Larger, less densely distributed spiders were found in the field sites, while forests contained greater densities of smaller spiders (Fig. 1, Fig. 2), despite higher prey availability in the field sites. This pattern indicates that optimal foraging theory may drive spider spatial distribution across habitat types, as spiders in different size classes likely perceive the value of the two habitats differently and therefore do not compete over the same areas.

Prey abundance was not the sole driver of spider distribution. Risk of predation, among other factors, may outweigh potential fitness gains offered by higher prey abundance in an environment. Although field sites had more prey available than did forest sites, capture success may be lower or the cost of increased vulnerability to predation may be higher for smaller spiders in the open environment. Thus, environmental conditions in the forest might be more optimal for smaller spiders. Our methodology did not fully capture variations between forest environments with regards to predator and prey presence, as well as with substrate types, and it is likely that these and other differing macrohabitat conditions also strongly affect spider fitness.

Organisms distribute themselves in response to environmental conditions according to different patterns depending on scale. Recognizing how these patterns interact with one another is important to understanding interactions between populations and their environment.

Figure 4. Spider density was positively correlated with prey density and was higher in the field than the forest.

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ACKNOWLEDGEMENTS We thank Matthew Ayres, Jessica Trout-Haney, and Vivek Venkataraman for their assistance.

AUTHOR CONTRIBUTIONS All authors contributed equally in the data collection and writing process.

LITERATURE CITED Birkhofer, K., Henschel, J.R., and S. Scheu. 2006.

Spatial-pattern analysis in a territorial spider: evidence for multiscale effects. Ecography 29: 641-648.

Kaston, B.J. 1978. How to know the spiders. Brown Company Publishers, Dubuque, IA.

Kacelnik, A., Krebs, J.R., and C. Bernstein. 1992. The ideal free distribution and predator-prey populations. Tree 7: 50-55.

Marshall, S.D. 1997. The ecological determinants of space use by a burrowing wolf spider in xeric shrubland ecosystem. Journal of Arid Environments 37: 379-393.

McCormick, S., and G.A. Polis. 2008. Arthropods that prey on vertebrates. Biological Reviews 57: 29-58.

Nagelkerken, I., van der Velde, G., Gorissen,

M.W., Meijer, G.J., Van Hof, T., and C. den Hartog. 2000. Importance of Mangroves, Seagrass Beds and the Shallow Coral Reef as a nursery for Important Coral Reef Fishes, Using a Visual Census Technique. Estuarine, Coastal and Shelf Science. 51: 31-44.

Pyke, G.H., H. R. Pulliam, and E.L. Charnov. 1977. Optimal foraging: a selective review of theory and tests. Quarterly Review of Biology 52: 137-154.

Samu, F., Sziranyi, A., and B. Kiss. 2002. Foraging in agricultural fields: local ‘sit-and-move’ strategy up to risk averse habitat use in a wolf spider. Animal Behavior 66: 939-947.

Sinervo, B. 2006. The Ecology of Foraging. University of California, Santa Cruz. Santa Cruz, CA.


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THE RICH GET REDDER: DELAYED GREENING IN WELFIA REGIA PALMS

ALLEGRA M. CONDIOTTE, ANN M. FAGAN, NATHANAEL J. FRIDAY, ELLEN R. PLANE, AND ZACHARY T. WOOD


Abstract: Plants must allocate resources efficiently to maximize growth, defense, and reproduction. Herbivory has selected for a variety of antiherbivore defenses. One of these defenses is delayed greening in new leaves, which reduces attractiveness to herbivores until a leaf has matured and become less palatable. However, leaves that delay greening have reduced photosynthetic capacity, making this form of defense energetically costly. We examined whether the duration of delayed greening in Welfia regia palms is fixed or is plastic with respect to previous hervivory, light availability, or plant size. The duration of leaf redness increased with plant size, but was not related to light availability or herbivory. Evidently W. regia does not adjust the developmental program for young leaves based on previous herbivory or light availability. An increase in metabolic capacity with size appears to be the major factor driving duration of delayed greening. Probably large palms can afford the cost of reduced photosynthetic capacity in their youngest leaf in order to avoid herbivory, while small palms require more photosynthetic area earlier in order to develop. Our results suggest that large individuals of W. regia have enough resources to simultaneously grow, reproduce, and defend their young leaves against herbivory.

Keywords: delayed greening, herbivore defense, resource allocation, Welfia regia

INTRODUCTION Plants must manage limited resources for growth, reproduction, and herbivory defense, all of which are essential to fitness. Plants should allocate the highest proportion of resources to whatever particular attribute will maximize long-term fitness. In environments with high rates of herbivory, plants may forgo some amount of growth and reproduction to protect tissue from herbivory. As energy and nutrients are limiting, plants must maintain a balance between using energy to support expensive defenses and growing energetically but losing productivity to herbivores.

Rates of herbivory are higher in tropical environments than in the temperate zone, and most damage to plant tissue is caused by herbivory (Coley and Barone 1996). Young leaves, which are less tough than mature leaves during the rapid expansion of early growth, are particularly vulnerable to herbivory (Dominy et al. 2002). Plants in the tropics have evolved a variety of antiherbivore defenses to protect these younger leaves. For example, delayed greening postpones the input of chlorophyll, nitrogen, energy, and rubisco into young leaves, keeping leaves red and keeping them low in nutritive value to potential herbivores until leaves become tougher and less palatable at maturity (Kursar and Coley 1992).

Delayed greening reduces photosynthetic capacity (Kursar and Coley 1992). Leaves with

delayed development of chloroplasts absorb only 18-25% of maximum possible light, as opposed to normally developed leaves, which absorb 80% (Kursar and Coley 1996). This loss of photosynthetic ability is costly, and may limit the number of new leaves that exhibit delayed greening.

We studied the factors affecting delayed greening in the tropical palm Welfia regia (Arecaceae). We tested whether delayed greening in W. regia is inducible (activated in response to an external stimulus) by examining whether plants experiencing increased herbivory had more delayed greening. We also tested whether light availability or metabolic capacity (by size) limited delayed greening. If photosynthetic needs limit delayed greening, W. regia with high amounts of epiphyte cover on leaves and low canopy openness should be less likely to delay leaf greening because they have higher need to photosynthesize. If plant metabolic capacity (e.g., total pools of carbohydrates and nutrients) is limiting, small palms should exhibit less delayed greening. METHODS We sampled 36 W. regia palms along the Camino Experimental Sur and Sendero Tres Rios trails at La Selva Biological Station, Costa Rica on 15-16 February, 2014. We measured plant height at the tallest point using a hypsometer and measured

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width at the longest horizontal axis and the perpendicular horizontal axis using a measuring tape.

On each palm, we ranked the five newest leaves from youngest (A) to oldest (E) based on their proximity to the center of the plant. We then randomly selected one leaflet on each of these leaves and inspected it for herbivore damage and epiphyte cover. Two observers independently ranked herbivore damage on a scale from 0 to 5 (0 = no damage, 1 = small hole, 2 = small holes, 3 = large hole, 4 = large holes, 5 = most of leaf damaged). We also ranked epiphyte cover on a 0-5 scale (0 = no epiphytes, 1 ≈ 10% epiphyte cover, 2 ≈ 25% epiphyte cover, 3 ≈ 50% epiphyte cover, 4 ≈ 75% epiphyte cover, 5 ≈ 100% epiphyte cover). We evaluated the correlation of scores between observers and then averaged the rankings of both observers for subsequent analyses of herbivore damage and epiphyte cover.

We dried each of the randomly selected leaflets from the three youngest leaves for 24 hours in a drying oven. We subtracted the final dry mass from the initial wet mass to determine percent water content of each leaf. We took four canopy photographs above each plant using a Canon PowerShot SX130IS and analyzed percent canopy cover in ImageJ 764 software. We also took flash photographs of a random leaflet on the plant’s youngest leaf and analyzed its redness using Photoshop 5.0 (Adobe Systems Incorporated, San Jose, CA). We compressed a cropped section of each leaf photograph into one pixel, recorded its red, blue, and green saturation, and used Equation 1 to determine leaf redness (Barber et al. 2000; Freschknecht 1993).

Equation 1 Statistical Analyses

We used principal component analyses (PCA) to derive a single measure for each plant of size, herbivory, and epiphyte coverage. For our size measure, we took the first principal component of a PCA of plant height, maximum width, and orthogonal width (see loadings in Table 1; 90% of variation explained). For herbivory, we took the first axis of a PCA on herbivory scores for the five

leaf ages (loadings for leaves A-E = -0.49, -0.48, -0.49, -0.29, and -0.45, respectively; 33% of variance explained). We followed the same procedure for epiphyte coverage (loadings for leaves A-E = -0.33, -0.45, -0.52, -0.46, and -0.46, respectively; 40% of variance explained). Henceforth, the terms “size,” “herbivory,” and “epiphyte coverage” all refer to the first axis of the corresponding PCA. We performed analysis of variance (ANOVA) to compare water content of leaves of different ages. We conducted a regression comparing leaf water content with leaf redness of the youngest leaf. To assess effects other than age, we used the residuals from this regression for further analysis.

We fit regressions of residual redness to size, herbivore damage, epiphyte cover, and canopy openness and used ANOVAs to test whether each regression was significant. RESULTS Water content varied as a function of leaf age (F2,108 = 34.9, P < 0.0001). The youngest leaf contained more water than the second-youngest and third-youngest leaves (Fig. 1). Thus, water

content served as a proxy for leaf age. leaf redness increased with water content (Fig. 2), indicating that W. regia leaves become less red with age (slope = 0.029 ± 0.019, F1,33 = 19.7 P < 0.001, r2 = 0.37).

Canopy cover, herbivory, and epiphyte cover were all unrelated to residual leaf redness. However, residual redness increased with palm size, indicating that larger palms retain the redness in their leaves for longer into leaf maturation than smaller palms (slope= -0.050 ± 0.175, F1,33=8.1, P= 0.008, r2= 0.20; Fig. 3).

Table 1. PCA results for palm size

Metric Mean ± S.D. PC-1 loadings

Maximum Height 3.3 ± 1.4 0.55

Maximum Width 4.5 ± 2.7 0.59

Orthogonal Width 3.6 ± 2.3 0.59

R

(R +G + B)3


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The youngest leaves (leaf A) had the lowest herbivory rankings and usually had less than 10% herbivory. Older leaves tended to have more surface area loss due to herbivory, often with multiple large holes (Fig. 4). DISCUSSION Delayed greening reduces herbivory on young leaves. As new leaves age, they transition from red to green, although not necessarily at the same rate.

We found that greening was delayed longer in larger plants but was not related to light availability, previous herbivory, or epiphyte cover.

The greater metabolic capacity resource pools in larger W. regia could explain why new leaves on larger plants stayed redder longer than those on smaller plants (Figure 3). Larger plants have greater photosynthetic surface area, more access to light in the understory, and greater energy and nutrient reserves. Therefore, they can invest more energy in creation of a red leaf and afford the cost of reduced photosynthetic efficiency in their youngest leaf for longer. This pattern suggests a shift in W. regia resource allocation priorities as plants develop. Plants at earlier life stages prioritize quick growth and establishment despite increased risk of herbivory, while older, established palms minimize herbivory at the cost of diminished growth rate. Mortality in young tropical palms that do not engage in delayed greening is significantly higher than in older palms and in young palms that delay greening (Numata et al. 2004), indicating that young plants, which have limited resources to allocate to defense, are more vulnerable in general than their older counterparts. Young plants face high mortality rates due to their location in the understory. There may be an age structure bottleneck, in which mortality is high in young plants due to herbivory, low light availability, and disease, from which the few surviving plants are released upon reaching a certain age.

There was no relationship between duration of redness and herbivory, epiphyte cover or canopy cover. Because delayed greening did not change based on any of these factors, it appears that this defense is not a plastic response to environmental conditions, but is instead a fixed ontogenetic trajectory. Keeping the youngest leaf red as long as possible may be necessary for survival in an environment with high rates of herbivory. Plants face tradeoffs in resource allocation for growth, defense and reproduction. Plants in high resource systems have their highest realized growth with minimal defense allocations because they can afford to sacrifice some resources (Coley et al. 1985). Young plants that devote all their resources to defense will not grow big enough to establish themselves. However, new growth fails to benefit a plant if it is all consumed by herbivores. Our finding that younger W. regia

Figure 3. The young leaves of larger plants were redder than the young leaves of smaller plants.

Figure 2. Redness (Equation 1) increased with water content, indicating that younger leaves were redder than older leaves.

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invest fewer resources in delayed greening suggests that growth to a certain size threshold is the limiting factor for long-term plant success, after which the plant has enough resources to continue growth and reproduction while simultaneously investing in greater herbivore defense. ACKNOWLEDGEMENTS We thank Matthew Ayres, Jessica Trout-Haney and Vivek Venkataraman for their guidance and

direction in the experimental design and writing process. AUTHOR CONTRIBUTIONS All authors contributed equally. A. Fagan, N. Friday, E. Plane, and Z. Wood collected field data and A. Condiotte conducted water content analysis.

LITERATURE CITED Barber, I., Arnott, S.A., Braithwaite, V.A.,

Andrew, J., Mullen, W., and Huntingford, F.A. 2000. Carotenoid based sexual coloration and body condition in nesting male sticklebacks. Journal of Fish Biology 57: 777-790.

Coley, P.D. and Barone, J.A. 1996. Herbivory and plant defenses in tropical forests. Annual Review of Ecology and Systematics 27: 305-335.

Coley, P.D., Bryant, J., and Stuart Chapin III, F. 1985. Resource availability and plant antiherbivore defense. Science 230: 895-899.

Dominy, N.J., Lucas, P.W., Ramsden, L.W., Riba- Hernandez, R., Stoner, K.E., and Turner, I.M. 2002. Why are young leaves red? Oikos 98:163-176.

Frischknecht, M. 1993. The breeding colouration

of male three-spined sticklebacks (Gasterosteus aculeatus) as an indicator of energy investment in vigour. Evolutionary Ecology 7:439- 450.

Kursar, T.A., and Coley, P.D. 1992. Delayed greening in tropical leaves: an antiherbivore defense? Biotropica 24:256-262.

Kursar, T.A. and Coley, P.D. 2006. The consequences of delayed greening during leaf development for light absorption and light use efficiency. Plant, Cell & Environment 15:901-909.

Numata, S., Kachi, N., Okuda, T., and Manokaran, N. 2004. Delayed greening, leaf expansion, and damage to sympatric Shorea species in a lowland rain forest. Journal of Plant Research 117:19-25.

Figure 4. The youngest leaves had little to no herbivory, while older leaves had higher levels of herbivory. Leaf A is the youngest, and E the oldest (leaf ages determined by position within the whorl).


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LOG PREFERENCE IN LEPIDOPHYMA FLAVIMACULATUM: SHELTER, HUNTING GROUND, OR BOTH?

MAYA H. DEGROOTE, LARS-OLAF HÖGER, AND REBECCA C. NOVELLO


Abstract: Many animals use structures found in their environments to provide protection from predators, and these structures are sometimes also used as hunting grounds. The secretive yellow-spotted tropical night lizard, Lepidophyma flavimaculatum, is often found under logs, and little is known about its activity. We tested whether tropical night lizards use logs as hunting grounds or solely as shelters. We compared various log and soil attributes that might influence arthropod abundance between occupied and unoccupied logs and tested for relationships between these attributes and lizard condition. We found that tropical night lizards use logs primarily as hiding places. In addition, tropical night lizard condition decreased with log and soil temperature, indicating that tropical night lizards benefit from having a lower metabolic rate as they rest beneath logs during the day. This suggests that tropical night lizards are nocturnal, foraging at night and using the logs as shelters during the day. Understanding the conditions that influence habitat suitability provides insight into how animals position themselves in their environment. Keywords: Lepidophyma flavimaculatum, log, shelter INTRODUCTION Throughout their lives, animals must make decisions about where to spend their time. Many animals use structures in their environments as shelters to reduce risk of predation. Some birds, for example, excavate or co-opt previously excavated chambers in trees for nesting. These cavity-nesting birds are protected from both environmental stressors and predation as they rest at night. Sometimes, an organism’s shelter also acts as a hunting ground. A combined shelter and hunting resource is beneficial as it reduces an organism’s need to travel and risk exposure to predation.

Lizards escape predation using a wide array of mechanisms, including crypsis, speed, tail-dropping, and hiding under environmental shelters. The parthenogenetic yellow-spotted tropical night lizard, Lepidophyma flavimaculatum, tends to hide beneath logs and within rock crevices (Cooper, 1994). As such, herpetologists are unsure if tropical night lizards are nocturnal or diurnal (Savage 1992), raising questions about their foraging strategies and daily activities. Much of what we know about the tropical night lizard is inferred from the ecology of closely related species, many of which are ambush predators (Cooper 1994). Ambush predators hide in plain site or conceal themselves using environmental structures where they wait to strike when prey approach. If night lizards are in fact

ambush predators, a log that can conceal them may serve as both a shelter and a hunting ground, providing them access to arthropod prey.

We investigated whether tropical night lizards use logs solely as shelter or as both shelter and hunting grounds. If lizards use logs as both shelter and hunting grounds, factors improving their hunting success should be greater beneath occupied than unoccupied logs. We used several soil and log attributes that might increase arthropod abundance, including soil moisture, organic horizon depth, and soil and log temperature, as proxies for hunting success. Under this hypothesis, attributes promoting arthropod abundance should also be positively correlated with lizard condition. However, if lizards use logs solely as shelters, there should be no difference in these attributes between occupied and unoccupied logs. Under this hypothesis, there should likewise be no relationship between these attributes and lizard condition. METHODS We surveyed logs in a 3000 m2 area of the arboretum and within 10 m of the sides of 500 m of the Sendero Tres Rios trail at the La Selva Biological station on 15-16 February, 2014 between 10:00 and 16:00. We overturned logs that appeared to be structurally viable for lizard occupation, which we defined as having a space of at least an inch between the log and the soil. We

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noted the presence or absence of lizards under each log. We surveyed a total of 15 logs without lizards (7 in the arboretum, 8 along the trail) and 8 logs with lizards (4 in the arboretum, 4 along the trail). We measured the snout-to-vent length (SVL) and body mass of all lizards found. We also noted the presence or absence of arthropods under each log and took measurements of several log and soil conditions that might affect arthropod success and abundance (i.e., soil moisture, temperature of both the soil and underside of the log, log decomposition, area under the log, log volume, and soil organic horizon depth). We placed soil samples from under each log in a drying oven at 70˚C for 20-25 hours and calculated soil moisture as the percent moisture by mass. Temperature was determined using a Raytek Raynger MX temperature gun. Log decomposition was estimated on a scale of 1-5 with 5 being highly decomposed. Statistical Analyses and Modeling We used two-tailed t-tests to compare log and soil attributes between occupied and unoccupied logs. We used the residuals of a linear regression of log-transformed lizard mass vs. log-transformed SVL as a metric of lizard condition. SVL is primarily an indicator of lizard age, and greater mass to SVL ratio indicates a more robust lizard for its length. We tested for correlations among log, soil, and lizard attributes. RESULTS We found tropical night lizards under fewer than half of all the logs that we overturned. Not all logs that we initially overturned were considered structurally viable. However, we easily found

occupied and unoccupied structurally viable logs. We considered logs in contact with one another to be one log. Tropical night lizards were often found under flat, board-like logs. When we overturned logs, lizards generally remained still and were easily caught, only occasionally biting the handler.

Up to three tropical night lizards were found under a single log. Log occupation was not related to percent soil moisture, organic horizon depth, log decomposition, soil temperature, log temperature, area under log, or log volume (Table 1). We found other organisms under logs including centipedes, millipedes, beetles, grubs, worms, a land crab, and a caecilian (Fig. 2). However, the presence of other organisms was not related to log occupation by lizards (Χ2 = 0.18, P = 0.67, df = 1).

Along the trail versus in the arboretum, organic horizon depth was greater (t = 2.1, P = 0.05, df = 20) and logs were more decomposed (t = 3.21, P = 0.004, df = 24). Lizard condition tended to be more robust in the arboretum than along the trail (t = 2.21, P = 0.058, df = 8), but no

Table 1. Two-tailed t-test statistics showed that occupied and unoccupied logs did not exhibit differences in any of the log and soil attributes.

Table 2. Correlation matrix for relationships between log, soil, and lizard attributes. Lizard condition decreased as both soil and log temperature increased. No other log and soil attributes were significantly correlated with lizard condition.


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other attributes varied between locations. Lizard condition was negatively correlated with both soil temperature and log temperature (Table 1, Fig. 1). No other log attributes predicted the robustness of lizard condition (Table 2), nor did the presence of other organisms (t = 0.06, P = 0.96, df = 2).

DISCUSSION Safety from predators and access to prey can affect the success of ambush predators. Our data suggest that tropical night lizards use logs as shelters but not as hunting grounds. Tropical night lizards were not more likely to be found under logs with attributes promoting arthropod abundance, nor were the lizards in better condition under these logs (Table 1). Tropical night lizards appear to select logs irrespective of the quality of hunting under the log. This suggests that the lizards are choosing logs mainly just for shelter. Since over half of structurally viable logs surveyed were unoccupied and resources did not differ between occupied and unoccupied logs, logs do not seem to be a limiting resource for tropical night lizard populations.

We found near-significant negative correlations between lizard condition and both log temperature and soil temperature (Fig. 1). Since lizards do not appear to be foraging under logs, it is possible that they maintain a lowered metabolic rate by resting under cooler logs. Since higher resting body temperatures will lead to more rapid utilization of fat stores, keeping cool may be

energetically advantageous. Many animals reduce their metabolic rates when they are not active to prevent this unnecessary burning of fat stores (Geiser 2004).

If tropical night lizards reduce their metabolic rate during the day and utilize logs only as hiding places, this may indicate that tropical night lizards are nocturnal, hiding under logs during the day and leaving to forage at night. Tropical night lizards have been known to return to the same log each day for shelter (Brian Folt, pers. comm.), and our observations showed that several of the tropical night lizards found on the first day of study remained under the same log the next day. Log preference may explain why tropical night lizards are returning to the same logs each day to rest. Such preferences may be due to log quality variations influenced by environmental conditions that our study did not account for.

Logs appear to primarily provide shelter for and satisfy the physiological needs of tropical night lizards at rest. The preferences of tropical night lizards might help us understand factors that influence shelter selection among lizards and other ectotherms. Physiological requirements and constraints, along with predator evasion, may drive the spatial choices that animals make throughout their lives. An understanding of conditions that determine habitat suitability may provide insight into how an animal uses that habitat.

-0.100

-0.050

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A B

Figure 1. Lizard condition decreased with (A) soil temperature (slope = -0.032 ± 0.014, P = 0.069, r2

= 0.35) and (B) log temperature (slope = 0.070 ± 0.033, P = 0.055, r2 = 0.39).

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ACKNOWLEDGEMENTS Special thanks to Matthew P. Ayres of Dartmouth College for helping us analyze our data and Bryan Folt and providing information on L. flavimaculatum.


LITERATURE CITED Cooper, W. E. 1994a. Chemical discrimination by

tongue-flicking in lizards: a review with hypotheses on its origin and its ecological and phylogenetic relationships. Journal of Chemical Ecology 20: 439-487.

Folt, Brian. Personal communication, February 2014.

Geiser, F. 2004. Metabolic rate and body

temperature reduction during hibernation and daily torpor. Annual Review of Physiology 66: 239-274.

Savage, J. M. 2002. The amphibians and reptiles of Costa Rica: a herpetofauna between two continents, between two seas. University of Chicago Press, Chicago. 498-499.


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BEHAVIORAL THERMOREGULATION IN FACULTATIVELY POIKILOTHERMIC SLOTHS (BRADYPUS VARIEGATUS AND CHOLOEPUS HOFFMANNI)

EMILY R. GOODWIN


Abstract: Behavioral thermoregulation is critical to maintenance of body temperature for many organisms. Both homeotherms and poikilotherms use this strategy- homeotherms use it to decrease metabolic costs associated with maintenance of a constant body temperatures and poikilotherms use it to alter their body temperature. As facultative poikilotherms, sloths are unusual among mammals in that they allow their body temperature to change with ambient temperature. This strategy minimizes metabolic energy expenditures and is important for sloths as they eat low-quality folivorous diets that are digested slowly by symbiotic microbial fermentation. When sloth body temperature is high, their metabolic rate and fermentation rates increase. I hypothesized that sloths behaviorally thermoregulate to maximize sunlight cover throughout the day through both moving throughout the canopy and sleeping in postures that maximize exposure of their bodies to direct sunlight. I focally observed six sloths of two species – one Bradypus variegatus individual and five Choloepus hoffmanni individuals over three days. Sun coverage was unrelated to sleep posture or canopy height, but sloths occasionally performed energetically expensive movements both vertically and horizontally to canopy gaps where sun exposure was higher. This suggests that the thermoregulatory and digestive benefits of sloth basking behavior outweigh the energetic costs of both horizontal and vertical movements. Key words: behavioral thermoregulation, Bradypus variegatus, Choloepus hoffmanni, poikilothermy, sleep posture

INTRODUCTION Temperature influences many biological and physiological processes in organisms across taxa. Homeotherms maintain a constant body temperature through metabolic processes, while the body temperature of poikilotherms conforms to environmental temperature, resulting in lower energetic requirements. In both homeotherms and poikilotherms, behavioral adjustments such as basking and group huddling can decrease the energetic costs of thermoregulation (Terrien et al. 2011). Homeothermic organisms such as birds and mammals may congregate in close groups to maintain their body temperatures, while poikilothermic organisms such as reptiles may bask in the sun to increase their body temperatures, and move into shaded locations to lower their body temperatures.

While most endotherms are homeotherms and most ectotherms are poikilotherms, the thermoregulatory strategies of organisms can vary. Sloths are endothermic mammals that utilize facultative poikilothermy, allowing their body temperature to conform to ambient temperature, unlike most other endotherms (Britton & Atkinson 1938). Sloths are specialized folivores with multi-compartment stomachs for symbiotic microbial

fermentation of fiber. Due to their extremely low basal metabolic rate (BMR), they must devote a large proportion of their energy budget to digestion (Goffart 1971). Thus, poikilothermy allows sloths to minimize non-digestive energetic expenditures (Costello and Rieb 2012). At high ambient temperatures, sloths are able to decrease energy spent on thermoregulation and increase digestive rates. The evolution of poikilothermy in sloths may be a strategy for aiding in digestion of fiber-rich leaves while minimizing energetic costs. If high temperature increases the efficiency of sloth digestion, sloths would benefit from maximizing their body temperatures through basking. Sunning behavior has also been proposed to enhance the activity of symbiotic gut bacteria (Goffart 1971, Urbani and Bosque 2007). Increased sun exposure can be achieved in two ways: either by physically moving throughout the canopy to sun gaps, or by sleeping in postures that will maximize the surface area of their bodies exposed to the sun. However, low canopy exposure increases predation risk from visual predators such as the harpy eagle (Harpia harpyja), a major sloth predator that plucks sloths directly from the canopy (Touchton et al. 2002). Thus there is a tradeoff between the physiological

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benefits of sunning behavior and increased energetic expenditure to seek light gaps. I investigated whether behavioral thermoregulation occurs in the brown-throated sloth (Bradypus variegatus) and Hoffman’s two-toed sloth (Choloepus hoffmanni) in Heredia, Costa Rica. Given the tradeoff between enhanced digestion and predation risks, I hypothesized that sloths would change their location in the canopy throughout the day with temperature. Sloths could move into sunnier locations in the mornings and evenings, when ambient temperatures are lower and the costs of increased digestive rates outweigh the risk of predation. Conversely, when ambient temperatures are higher during midday, descending from the canopy for protection from visual predators would bear minimal costs to digestion. However, since moving throughout the canopy is energetically expensive for sloths, they may remain in low exposure areas and instead adjust their sleeping posture within a tree throughout the day to maximize sun exposure. METHODS I observed six sloths, five C. hoffmanni and one B. variegatus, at La Selva Biological Station, Costa Rica, between the hours of 0700 and 1745 on 15-17 February 2014. All sloths were found within a 1 km stretch of secondary rainforest. Two C. hoffmanni individuals, one with an infant, were found in close proximity to one another, and were observed resting while cuddling. One B. variegates also had an infant. I used focal sampling (Altmann 1974) on individuals for 10 minute periods approximately every hour, using binoculars (8x magnification) and a spotting scope (20x magnification) to observe sloth posture and behavior.

I recorded proportion of canopy cover, proportion of direct sun cover on body, sloth posture, sloth behavior, and weather conditions for each focal period (Table 1). I assessed sloth height in the canopy using a hypsometer. Ambient air temperature was collected at three locations placed approximately 2 m off the ground under focal sloth trees using a HOBO logger recording every 10 minutes.

Statistical Analyses and Energy Budget Calculations

I calculated the daily activity budget of the sloths by averaging the amount of time spent in any given position, canopy cover, sun cover, sloth behavior, and postures for each focal sample. I used chi-square tests to analyze the effects of sun cover and canopy cover on sloth posture. I calculated the energy expenditure of vertical climbing and horizontal movements using published regression models for mammals

Table 1. Variables assessed during sloth focal sampling. Variable Description Canopy Cover Low 0-30% of sloth’s body covered by

leaves

Medium 30-60% of sloth’s body covered by leaves

High 60-100% of a sloth’s body covered by leaves

Sun Cover Low 0-30% of sloth’s body being hit by direct

sunlight Medium 30-60% of sloth’s body being hit by

direct sunlight High 60-100% of sloth’s body being hit by

direct sunlight Sloth Behavior Resting Stationary, eyes closed

Traveling Horizontal or vertical locomotion, either hanging or upright

Scanning Eyes open, moving head around scanning its environment

Feeding Biting or chewing leaves, reaching for branches

Sloth Posture Huddle sitting Stationary, sitting on at least one

branch, getting support from additional branches.

Huddle hanging

Stationary, holding self up with limbs by holding a branch or tree trunk.

Extended laying

Stationary, reclined with back on a branch, ventral-side up, often supporting itself with its forelimbs.


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(Equation 1 and 3) (Pontzer 2011). All statistical analyses were performed using JMP 11.0 software (SAS Institute, Cary, NC).

I used the following equation to estimate the energetic cost of climbing movements:

(Eq. 1)

In the above equation, mass = average weight for each species, from literature (4.25 kg for B. variegatus, 5.5 kg for C. hoffmani), g= 9.8 m/s2, h = vertical distance moved (in meters). I calculated energetic efficiency from regressions derived from studies of primate energetics (Pontzer 2011):

(2)

It should be noted that efficiency is essentially invariant with body mass.

For horizontal movements, I estimated cost of transport from studies of energetics in a large sample of mammals (Pontzer 2011):

(3)

Here, mass = average weight for each species from literature. I then multiplied cost of transport by horizontal distance traveled to acquire the energy required to make the movements (in Joules).

Although these models derived from other mammals do not reflect the idiosyncrasies of sloth physiology and energetics, they provide useful estimates for ranging costs, which have not been directly measured or estimated in previous studies.

RESULTS Daily temperatures ranged from 22°C to 31°C during the three-day study period. Temperature peaked around noon and was lowest at dawn. The data were consistent across the three HOBO loggers. Weather was consistently sunny throughout the three sampling days.

Sloths were found in a total of seven trees at positions high in the canopy. Sloths spent 90% of their daily activity budget resting, 6.5% traveling to new locations, and 3.2% scanning (Table 1). I never observed sloths feeding during the daytime, which was consistent with the reported nocturnal nature of sloths (Goffart 1971). However, one sloth moved to a Cecropia tree in the late

afternoon, a favorite food tree of many foliviorous mammals due to the high nutrient and low fiber content of leaves (Urbani and Bosque 2007). There was no effect of time of day or temperature on sloth movement on height in the canopy.

Figure 1. (A) C. hoffmanni in the extended laying resting posture. (B) B. variegatus in the huddle-sitting resting posture. (C) B. variegatus in the huddle-hanging resting posture. Note the infant holding on to her ventral side. (D) C. hoffmanni in the hanging traveling posture. All photos courtesy of the author.

When resting, sloths spent 47% of their time in extended laying positions, 29% of their time in huddle-sitting positions, and 22% of their time in huddle-hanging positions (Fig. 1). There was no effect of time of day or temperature on sloth posture. Posture did not vary with sun exposure (chi-square = 7.28, P = 0.12, df = 4; Fig. 2), but did vary with degree of canopy cover (chi-square = 26.3, p <0.0001, df = 4; Fig. 3). On the afternoon of 16 February, it rained (while still sunny) for approximately twenty minutes. During the rainfall, the focal sloth (C. hoffmanni) changed postures from extended laying to huddle-sitting until the rainfall stopped.

When repositioning or moving within the canopy, sloths most often repositioned to another spot with the same sun coverage (85%), followed by repositioning to a spot with higher sun coverage (12%) (chi-sqaure = 65.59, P = 0.000, df = 8; Fig. 4). Sloths were rarely seen moving to a

A B

C D

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location with less sun coverage than the previous position (3%; Fig. 4). When traveling, sloths spent 89% of time hanging (Fig. 1) and 11% of time upright.

Figure 2. Sloth resting posture did not vary with sun exposure (chi-square = 7.28, P = 0.12, df = 4).

Figure 3. Sloth resting posture varied with canopy cover (chi-square = 26.3, p <0.0001, df = 4).

Across observation periods, five large (> 0.5 m) horizontal and vertical movements were observed. Each movement resulted in a new position in a canopy gap with equal or higher sun cover (Table 2). Estimated energy expenditures for these movements were highest for sloths moving from low to high sun exposure (Table 2).

DISCUSSION As facultative poikilotherms, sloths use ambient temperature changes to increase their body temperature and thus digestive rate. However, sloths did not try to get higher into the canopy during the cooler parts of the day, or descend into the canopy at high temperatures to avoid the temperature-predation risk tradeoff. It may be that predation risk varies little with position in canopy. Moreover, postural and morphological mechanisms could be more relevant for avoiding predation in sloths, since they spend most of the day sleeping. For example, sloths have specially adapted fur to encourage algae growth on the pelage, as well as very slow body movements, which may help reduce visual detection from predators. Additionally, I saw that sloths in high-exposure canopy gaps were more likely to adopt huddling postures that minimized body surface area, possibly making them more cryptic than they would be in the extended laying posture.

When a sloth repositioned, it almost always led to a new position in equal or higher sun coverage (Fig. 4). Additionally, all large horizontal and vertical movements ended with sloths in canopy gaps with equivalent or higher sun cover (Table 2). These results suggest that the maximization of sun cover on body surface area from any given resting posture in the canopy is less beneficial than relocating to a new canopy gap with higher exposure, possibly because of changes in the position of the sun throughout the day. Although the range of movements observed in this study result in low energy expenditures per movement, the costs of these movements are more expensive for sloths as compared to other mammals of similar body size due to their low basal metabolic rate (147 kJ/(kg*day); (Nagy and Montgomery 1980). Both of the highest observed sloth energetic expenditures were associated with a move from low to high sun cover (Table 2). Relocations that result in increased sun exposure appears to be worthwhile despite the costs, perhaps because of an increased digestive rate associated with higher body temperature. I therefore suggest that the benefits of increased digestive efficiency outweigh the energetic costs of canopy movements.

0%

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% T

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Table 2. All long distance movements during or between focals (defined >0.5 m in distance). Energy expenditures were calculated using Equation 1 for vertical climbing & Equation 2 for horizontal movement.

Date Time Species Movement Change in Sun Exposure

Observations Energy Expenditure (J)

2/15/14 8:45 B. variegatus vertical descent 0.7 m low to medium Descent partially out of canopy and into bare patch near tree trunk

N/A

2/15/14 10:59 C. hoffmanni vertical ascent 0.9 m high to high Partial emerging from canopy; head above the leaves

31.52

2/16/14 9:01 C. hoffmanni vertical ascent 1.3 m medium to high Movement towards canopy gap 45.53

2/16/14 15:39 C. hoffmanni horizontal movement 22.3 m

low to high Between focals, movement to large canopy gap one tree over

760.6

2/16/14 15:51 B. variegatus vertical descent 4.3 m, ascent 5.0 m

low to high Between focals, left current tree and then moved to the canopy of a mostly bare Cecropia

Descent: N/A

Ascent: 139.16

Sloths spent the highest proportion of their resting in the extended laying posture, followed by the huddle-sitting posture, and the huddle-hanging posture. Both of the huddle postures are predominately upright positions. Thus sloths spent approximately half of their time upright and the other half reclining, despite the reclining posture exposing their ventral sides to the sun and potentially aiding in digestion more so than the other postures. Alternatively, the upright posture of could also be important in digestion. In ruminant herbivorous mammals, upright posture had been hypothesized to be an adaptation to increase stratification of forestomach contents, allowing larger food particles to pass more quickly (Clauss 2004). The strictly foliviorous B. variegatus spent a higher proportion of time in the two upright postures (91%) than C. hoffmanni (22.2%), which consume more readily fermentable fruits, flowers, and buds in its diet (Meritt 1985). C. hoffmanni’s more easily fermentable diet may increase the freedom of individuals to vary resting positions, while B. variegatus must rest upright and therefore ease digestion. This is consistent with the forestomach stratification hypothesis (Clauss 2004).

Sloths did appear to utilize behavioral thermoregulation as a function of their facultative poikilothermy. Sloths relocated throughout the day to get to canopy gaps, despite the high energy costs of those movements. This sunning behavior, in addition to dietary knowledge of both species of sloth and its effects on posturing, indicate that sloths must maximize their use of their resting

time to maintain their body temperatures. Associated increases in body temperature may aid in fermentation and digestion of their folivorous diets by symbiotic microbial communities.

Figure 4. Most sloth repositioning led to maintenance of sun exposure (85%), followed by repositioning or new postures that led to a higher percntage of sun exposure (18%). Rarely did sloths reposition to a posture with less sun exposure (4%) (chi-sqaure = 65.59, P = 0.000, df = 8.). Notation (e.g. ML) represents sun exposure before reposition (M) and sun exposure after reposition (L) where L= low (0-30%), M= medium (30-60%), and H= high (60-100%).

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ACKNOWLEDGEMENTS Thanks to the guides of La Selva Biological Station for their help and humor in locating sloths.

I am also indebted to V. Venkataraman and J. Trout-Haney for all of their assistance and support at all steps of this process.

LITERATURE CITED Altmann, J. 1974. Observational study of

behavior: sampling methods. Behavior 49: 227- 267.

Britton, S.W. and W.E. Atkinson. 1938. Poikilothermism in the sloth. Journal of Mammalogy 19: 48-51.

Clauss, M. 2004. The potential interplay of posture, digestive anatomy, density of ingesta and gravity in mammalian herbivores: why sloths do not rest upside down. Mammal Review 34: 241-245.

Costello, R., and J. Rieb. 2012. How Choloepus hoffmanni’s poikilothermy preserves the sloth niche. Dartmouth Studies in Tropical Ecology 2012, pp. 118-123.

Goffart, M. 1971. Function and form in the sloth. Oxford: Pergamon Press.

Merritt D.A. 1985 The two-toed Hoffman’s sloth, Choloepus hoffmanni. The evolution and ecology of armadillos, sloths and vermilinguas. Washington, D.C.: Smithsonian Institution Press pp 333-41.

Nagy, K.A., and G.G. Montgomery. 1980. Field

metabolic rate, water flux, and food consumption in the three toed sloth (Bradypus variegatus). Journal of Mammalogy 61: 465-472.

Pontzer, H. 2011. From treadmill to tropics: calculating ranging cost in chimpanzees. Primate Locomotion (eds. D'Aout K, Vereecke EE). Springer.

Terrien, J., M. Perret, and F. Aujard. 2011. Behavioral thermoregulation in mammals: a review. Frontiers in Bioscience 16: 1428.

Touchton, J.M., Y. Hsu, and A. Palleroni. 2002. Foraging ecology of reintroduced captive-bred subadult harpy eagles (Harpia harpiya) on Barro Colorado Island, Panama. Ornitologia Neotropical (The Neotropical Orinthological Society) 13.

Urbani, B. and C. Bosque. 2007. Feeding ecology and postural behavior of the three-toed sloth (Bradypus variegatus flaccidus) in northern Venezuela. Mammalian Biology 72: 321-329.


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PATTERNS IN CORAL ASSEMBLAGES OF A PATCH REEF SYSTEM

MAYA H. DEGROOTE, EMILY R. GOODWIN, AARON J. MONDSHINE, AND ELLEN R. PLANE

Faculty Editor: Celia Y. Chen

Abstract: Bleaching, disease, and natural disasters are leading to increased destruction and degradation of coral reef habitats. Coral habitat loss reduces overall reef size and continuity, increasing the prevalence of patchy reef environments. In order to determine what affects recruitment of corals in patchy environments, we conducted a study of coral communities on reef patches in Grape Tree Bay, Little Cayman. Coral recruitment may follow the theory of island biogeography, which predicts that larger islands and islands closer to the source population, in this case the main reef, will have greater species richness. We found that larger reef patches had greater species richness, diversity, and percent coral cover than smaller reef patches. However, we found no significant effect of distance from the main reef on coral assemblages, indicating that distance is not a limiting factor for coral dispersal and recruitment in this system. We conclude that conservation efforts should focus on preserving areas with larger reef patches, since these patches foster greater species diversity and tend to have higher coral cover.

Key words: coral communities, island biogeography, patch reef, SLOSS, species diversity

INTRODUCTION Globally, coral reef health is becoming increasingly threatened due to habitat destruction and degradation (Nyström et al. 2000, Hughes et al. 2003). Many factors can lead to coral reef habitat destruction, including disease, coral bleaching, and natural disasters, all of which are exacerbated by global climate change (Shepard et al. 2009). Reef habitat loss decreases overall reef size and continuity, reducing the resilience of reefs to subsequent disturbance (Nyström et al. 2000). The resulting patch reefs, or small coral reefs that are isolated from a larger, continuous main reef, therefore may become more common. Changes in species richness, diversity, and percent live coral cover caused by habitat loss can impact coral health and resilience (Hughes et al. 2003, Roff and Mumby 2012), so it is important to understand what affects coral recruitment in patchy reef environments.

Recruitment of corals to patch reefs may be comparable to the species colonization of islands, as described by the theory of island biogeography (MacArthur and Wilson 1967). Patch reefs are islands of suitable habitats that are surrounded and separated by areas of unsuitable habitats (MacArthur and Wilson 1967). The theory predicts that species diversity on an island increases with island size because immigrants are more likely to find and migrate to larger islands. Species-area relationships show that larger areas are correlated with increased species diversity

because of a variety of factors including increased habitat heterogeneity and resource availability (Shen et al. 2009). Species diversity also increases with proximity of the island to the mainland, or source population. Since immigrants must travel shorter distances, a larger variety of species with different mobility and dispersal strategies are able to travel to the island. The theory of island biogeography may help to explain patterns of coral diversity, measured by species richness and abundance, in a patch reef system.

Corals have a variety of reproductive strategies that may influence their recruitment to near and far reefs. Coral species can be hermaphroditic (both sexes in one individual), or gonochoristic (two distinct sexes), and many Scleractinian corals can reproduce asexually (Richmond & Hunter 1990). Location of fertilization can also vary, either occurring within the individual (brooding) or externally when gametes are released in the water column (broadcast spawning) (Richmond and Hunter 1990). In both cases, planktonic larvae must travel through the water column and land on a suitable substrate before a colony can form. Because of this broadcast spawning strategy, recruitment may not be limited by the same factors that limit terrestrial organisms, and thus corals may not follow the predictions of island biogeography theory.

We conducted this study to determine whether the distribution of corals in a patch reef system are

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consistent with the predictions of island biogeography. If so, coral species richness should increase with (1) reef patch size, and (2) proximity of the reef patch to the main reef. Larger reef patches should have greater coral species richness since they have greater surface area and greater habitat heterogeneity. While many previous island biogeography studies have measured species diversity using species richness, we also predict that patterns of overall species diversity, and percent coral cover will follow these trends. It is also possible that coral dispersal and recruitment are not limited by distance from the source population, so there will be no effect of proximity to the reef crest on species richness. In this case, other factors, including water depth, tides, current, and dispersal strategy, may determine coral recruitment patterns in patch reefs.

METHODS We surveyed 30 reef patches between the main reef and shore in Grape Tree Bay, Little Cayman, on 26-27 February 2014. A reef patch was defined as a patch of reef separated by at least two meters from the fringing reef crest and from all other reef patches. We haphazardly selected reef patches across a range of both sizes and distances from the main reef, defined as the reef crest. We calculated reef patch surface area as an idealized cylinder, excluding the base, by measuring circumference and height of each patch using measuring tape. In addition, at each reef patch we measured water depth and distance from the reef crest on an axis perpendicular to the shoreline. We performed a visual census of live coral communities at each reef patch, identifying all species and counting the abundance of individual colonies of each species. Three observers estimated the percentage of the reef patch that was covered by live coral. We averaged these estimates to yield a measure of percent coral cover. Statistical Analyses We conducted linear regressions to test whether water depth influenced either coral species richness or percent coral cover. We performed a linear regression of reef patch surface area and distance from the reef crest and found that these factors were not correlated, meaning that our data included reef patches of all sizes across a range of distances from the reef crest. We calculated

species diversity for each reef patch using the Shannon-Weiner Diversity Index. Since species-area relationships are generally described by logarithmic curves, we then fit logarithmic regressions comparing patch surface area to species richness, species diversity, and percent coral cover. We then performed linear regressions comparing distance from the reef crest to species richness, species diversity, and percent coral cover. For coral species that were present on at least five reef patches, we fit regressions of abundance to reef patch surface area and distance from the reef crest. We categorized species as stony (Scleractinia) or soft (Alcyonacea and Gorgonacea) and tested for a relationship between their abundances using a linear regression.

Figure 1. Species richness, species diversity, and percent coral cover increased logarithmically with reef patch surface area.

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RESULTS We identified 20 total coral, hydrozoan, and zoanthid species (Appendix 1). The most common species (stony corals: Agaricia agaricites, Porites astreoides; soft corals: Briareum asbestinum and Eunicea spp.), were found on over half of reef patches surveyed.

Water depth ranged from 1.3 to 2 m and did not have an effect on species richness, species diversity, or percent coral cover. Reef patch surface area ranged from 1.5 to 69.6 m2, and distances between reef patches and the fringe reef crest ranged from 3.8 to 68.4 m.

Reef patches with greater surface area had higher species richness, species diversity, and percent coral cover (Table 1, Fig. 1). Exclusion of the reef patch with a surface area of 69.6 m2 did not have an effect on any of the three logarithmic relationships. Larger patches had more colonies of B. asbestinum (P < 0.0001, r2 = 0.73) and Montastrea annularis (P < 0.0001, r2 = 0.49).

Distance from the reef crest did not have an effect on species richness, species diversity, or percent coral cover (Fig. 2). Isophyllastrea rigida was the only species to demonstrate a distance effect, and was found in patches at least 40 m from the reef crest. There was no difference between stony and soft coral abundance in response to any factor. Patches with more stony corals also had more soft corals (P = 0.01, r2 = 0.20). Table 1. Species richness, species diversity, and percent coral cover were described by logarithmic relationships with reef patch surface area (x).

Equation P r2 Species richness y = 2.42 + 1.55(log(x)) <0.0001 0.47 Species diversity y = 1.33 + 0.28(log(x)) 0.0053 0.25 Percent coral cover y = 9.91 + 11.98(log(x)) <0.0001 0.52

DISCUSSION The patterns of coral species composition in reef patches were consistent with the species-area relationships predicted by island biogeography theory. We found that species richness increased with reef patch surface area (Fig. 1). However, we found no relationship between distance to reef crest and species richness (Fig. 2), indicating that dispersal mechanisms from the source reef population may differ from those operating in

other island scenarios, especially in terrestrial systems. Species diversity and percent coral cover followed similar trends for both surface area of reef patches and proximity to reef crest. Although we counted abundance of each coral species by counting number of individual colonies, whether or not each colony was a distinct colonization event was unclear.

Figure 2. Distance from the reef crest did not have an effect on species richness, species diversity, or percent coral cover on reef patches.

The theory of island biogeography states that

larger islands are likely to contain more niches (MacArthur and Wilson 1967). With increased habitat heterogeneity, immigrating species are more likely to find an available niche to occupy and therefore be more successful. Our findings, which show that reef patches with more surface area had greater species richness, could be the result of variations in physical habitat heterogeneity. However, additional abiotic and

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biotic factors may also be influencing niche availability. Further studies should investigate how island size affects coral recruitment and colonization by investigating the abundance of new coral colonies on both large and small reef patches.

Larger reef patches have previously been associated with higher fish abundance and species richness (Lobben and Cheek 2008, Chowdhury et al. 2012). Increased algal herbivory and coral predation by fish could help explain why we found higher coral species richness on larger reef patches. Algal herbivory reduces algal-coral competition, creating space for coral recruitment and increased abundance, while intermediate levels of coral predation could allow for increased species diversity.

We found no relationship between species richness and distance from the reef, indicating that distance may not be a limiting factor for coral dispersal. Additionally, neighboring reef patches may affect species richness and therefore the reef crest may not be the only source from which coral species emigrate. In this study, patch reefs were no more than 70 m from main reefs. Reef patches located at greater distances from a reef crest may exhibit coral distributions consistent with the

theory of island biogeography because dispersal may be a limiting factor at larger spatial scales.

A better understanding of coral recruitment to patchy reef environments may help determine appropriate sizes and locations for marine protected areas. Our findings also have implications for the SLOSS (Single Large or Several Small) debate in ecology, which explores whether it is preferable to preserve a single large area or several smaller areas. The theory of island biogeography is addressed by the SLOSS debate that uses diversity as an indicator for conservation (Diamond 1975, Simberloff and Abele 1982). Our study suggests that coral reef restoration efforts, including implementation of artificial reefs, should focus on larger reef patches, which promote greater species richness. Conservation efforts should concentrate on preserving areas with larger reef patches, as these patches foster greater species diversity and tend to have more coral cover. ACKNOWLEDGEMENTS We would like to thank Katie Lohr of CCMI for her assistance with coral identification.


LITERATURE CITED Chowdhury, M.H.R., S.F. Flanagan, and N.B.

Frankel. 2012. Factors affecting fish diversity on coral reef “islands.” Dartmouth Studies in Tropical Ecology 2012, pp. 154-158.

Diamond, J.M. 1975. The island dilemma: lessons of modern biogeographic studies for the design of natural reserves. Biological Conservation 7: 129–146.

Hughes, T.P., A.H. Baird, D.R. Bellwood, M. Card, Connolly, S.R., Folke, C., Grosberg, R., Hoegh-Guldberg, O., Jackson, J.B.C., Kleypas, J., Lough, J.M., Marshal, P., Nystrom, M., Palumbi, S.R., Pandolfi, J.M., Rosen, B., and J. Roughgarden. 2003. Climate change, human impacts, and the resilience of coral reefs. Science 301: 930-933.

Lobben, T.J. and L.M. Cheek. 2008. Coral patches on a back reef do not conform to diversity-distance predictions of island biogeography theory. Dartmouth Studies in Tropical Ecology 2008, pp. 161-164.

MacArthur, R. H., and E. O. Wilson. 1967. The

theory of island biogeography. Princeton University Press, Princeton, NJ.

Nyström, M., C. Folke and F. Moberg. 2000. Coral reef disturbance and resilience in a human-dominated environment. Trends in Ecology and Evolution 15: 413-17.

Richmond, R.H., and C.L. Hunter. 1990. Reproduction and recruitment of corals: comparisons among the Caribbean, the Tropical Pacific, and the Red Sea. Marine Ecology Progress Series 60: 185-203.

Roff, G., and P.J. Mumby. 2012. Global disparity in the resilience of coral reefs. Trends in Ecology and Evolution 27: 404-413.

Shen, G., M. Yu, X. Hu, X. Mi, H. Ren, I. Sun, and K. Ma. 2009. Species–area relationships explained by the joint effects of dispersal limitation and habitat heterogeneity. Ecology 90: 3033–3041.

Sheppard, C.R.C, S.K. Davey, and G.M. Pilling.


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2009. The biology of coral reefs. Oxford University Press, Oxford, UK.

Simberloff, D. S. and L. G. Abele. 1982.

Refuge design and island biogeograpic theory - effects of fragmentation. American Naturalist 120: 41-56.

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GRAZING AND EPIBIONTS IN MARINE TURTLE GRASS BEDS

ALLEGRA M. CONDIOTTE, NATHANAEL J. FRIDAY, LARS-OLAF HÖGER, AND ZACHARY T. WOOD


Abstract: Turtle grass serves as habitat for small herbivores and various types of epibionts. We examined the effects of depth and turtle grass density and distribution on herbivory and epibiont cover in the shallow coastal water of the Caribbean. We found that large-scale herbivory (cropped tips) increased with grass density while epibiont cover increased with water depth. These results suggest that herbivores that take large bites (crop tips) selectively graze dense areas of turtle grass, which are more easily visible and may provide protection from large predators, and that epibiont herbivores may be limited to shallow water by predation.

Key words: community structure, epibionts, herbivory, habitat complexity, Thalassia testudinum, turtle grass

INTRODUCTION Plants affect the physical structure of the communities they inhabit. In marine ecosystems, plants are not only a food source for herbivores and decomposers; they provide shelter for organisms, alter local currents, and are often a substrate for other benthic organisms.

Turtle grass, Thalassia testudinum, is a widespread submerged macrophyte in tropical coastal marine systems commonly found bordering the shoreline. As well as providing food for herbivores, turtle grass creates a unique substrate for communities of epibionts: organisms, ranging from encrusting algae to hydrozoans, that live fixed to the surface of the grass blades. Photosynthetic epibionts, such as algae, can serve as important primary producers in oligotrophic systems, fixing carbon for higher trophic levels in local communities (Kitting et al. 1984). Nutrient-rich epibionts can make plants more attractive to herbivores, increasing herbivore grazing on epiphyte covered plants (Hay and Whal 1995). However, as turtle grass provides the growth substrate for these epibionts, its physical location and patterning should impact epibiont distribution and abundance. Herbivore activity should also be affected by turtle grass distribution.

We examined how turtle grass structure influences herbivory and epibiont coverage in shallow coastal water in the Caribbean. If herbivores graze indiscriminately and always consume the same number of blades per area, there should be a negative relationship between herbivory and density, because less dense areas will experience a proportionally greater effect of

herbivory. In turn, if herbivores graze proportionally to the density of grass, there should be no relationship between herbivory and density. Finally, if herbivores favor high density areas of grass, there should be a positive relationship between herbivory and density. Seagrass distribution is primarily limited by light availability, particularly in deep water (Ralph et al. 2007). If photosynthetic epibionts have similar light limitations to their host macrophytes, they should grow throughout turtle grass beds. However, it is possible that photosynthetic epibionts have more stringent light requirements than seagrasses, in which case turtle grass may be able to grow where epibionts cannot. METHODS

Data collection

We studied turtle grass at the Little Cayman Research Centre, Cayman Islands, on 27 February

2014. We surveyed six parallel 18-meter transects perpendicular to the shore, starting two meters from the shoreline. We spaced transects two meters apart. At two-meter intervals along each transect, we haphazardly selected five blades of turtle grass, which we later assessed for herbivory and epibiont cover in the lab. Epibiont cover was assessed visually by estimating and averaging the percent cover from each side of each blade. To assess herbivory we counted the bites taken out of the blades and noted whether the tip of the blade had been cropped off. We defined our tips cropped variable as the proportion of blades sampled at each site that had cropped tips, and we defined our bites variable as the average number of small


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(<0.5 cm diameter) bites per blade at each site. Tips cropped was classified as large scale herbivory, and small bites were classified as small scale herbivory, reflecting the size of animals most likely capable of inflicting such damage. At each site we calculated the density of turtle grass by counting blades within a 25.5*25.5 cm quadrat, and measured the average height of the grass and depth of the seafloor.


To examine how turtle grass bed characteristics influenced herbivory and epibiont cover, we fit several linear models with proportion of tips cropped, bites per leaf, and epibiont cover as dependent variables. For these dependent variables, we separately tested all individual and interaction effects.

RESULTS

Variation in turtle grass density was great, with the ocean floor nearly invisible in many parts of the beds and almost barren in others. Grass height ranged from 6.5 to 28.0 cm (Appendix figures). Variation in epibiont cover was also large, with some blades almost completely covered and others nearly bare.

Deeper areas had more than twice the epibiont cover of shallower areas (Figure 2, Table 3, F1,58 = 5.497, P = 0.023). Although turtle grass height also negatively affected epibiont cover, this effect became insignificant when depth was included in the model. Grazers did not appear to seek epibiont-rich leaves; neither tip cropping or bites per leaf were related to epibiont cover (F1,58 = 1.453, P = 0.23; F1,58 = 1.609, P = 0.21; respectively). Tip cropping was more frequent in denser turtle grass beds (Figure 1, Table 2, F1,58 = 5.478, P = 0.023). Bites per leaf were not related to turtle grass height, seafloor depth, or epibiont cover (Table 1). For all other models, see Table 1.

Figure 1. Tips cropped increased with turtle grass density. R2 = 0.09. Table 2. Regression coefficients for tips cropped on turtle grass density

Effect Est. ± Std. Err. P

Intercept -0.078 ± 0.090 0.031

Density 0.00012 ± 0.00005 0.023

Table 1. Model results. H = bed height, D = density, P = depth.

Response Effects and direction P

Epibiont cover

↓H 0.0081

↓D 0.18

↑P 0.023

↓H, ↓D, ↑H*D 0.073

↓D, ↓P, ↑D*P 0.11

↑P, ↓H, ↑P*H 0.034

Tips cropped

↑H 0.63

↑D 0.039

↑P 0.67

↑H, ↑D, ↑H*D 0.20

↓D, ↓P, ↑D*P 0.12

↓P, ↓H, ↑P*H 0.13

Bites

↑H 0.087

↑D 0.14

↓P 0.12

↑H, ↑D, ↓H*D 0.31

↓D, ↓P, ↑D*P 0.17

↓P, ↓H, ↑P*H 0.20

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Figure 2. Epibiont cover increased with depth. R2 = 0.09. Table 3. Regression coefficients for epibiont cover on water depth

Effect Est. ± Std. Err. P

Intercept 12.78 ± 13.30 0.34

Depth 49.10 ± 20.94 < 0.0001

DISCUSSION

We examined how the abiotic and biotic structure of turtle grass beds affects herbivory and epibiont cover. Turtle grass height, grass density, water depth, epibiont cover, and herbivory varied in different areas of the turtle grass (Appendix figures). Large-scale herbivory (cropped tips) was highest in near-shore areas of dense grass (Figure 1), while epibiont cover increased with depth (Figure 2). Small-scale herbivory (all bites < 0.5 cm in diameter) did not vary with any of our independent variables. Epibiont cover and herbivory were not related, indicating that herbivory and epibiont cover are driven by different factors.

Our results suggest that herbivores that take large bites (crop tips) selectively graze dense areas of turtle grass. Large herbivores, such as buck-tooth parrotfish, are visual grazers (Ogden 1976).

Other large herbivores such as green sea turtles return to the same grazing plots to feed periodically (Thayer et al. 1984). Because of this, dense beds of turtle grass may be more visible or desirable to these herbivores. Large herbivore preference for denser patches of grass may regulate seagrass expansion by limiting further growth in already dense areas. Small-scale herbivory did not vary with depth, grass height, or grass density, suggesting that small herbivores such as small fish and sea urchins feed sporadically and do not show a preference for denser patches.

Increased epibiont coverage in deep water may also be related to predation. Animals that feed on epibionts, including small snails, fish and crabs, may be limited to shallow areas due to increased predation by large fish in deep zones. Deep water provides more space for large predators, making these areas more dangerous for epibiont grazers than shallow water. This could allow the epibiont community to expand more easily in deep water. Epibiont cover could also increase with depth because of a decrease in ultraviolet radiation, which is harmful to algae and can limit its growth in shallow water (Bischof et al. 1998).

In tropical marine systems, seagrass structure and distribution affect herbivory and epibiont coverage, which in turn have implications for the success of the macrophyte community and the overall system. Our results suggest that herbivory in these systems is density-driven, and that the range of epibionts is limited by depth of water. The density and distribution of turtle grass affects organisms at multiple trophic levels, suggesting a large-scale impact on the community.

ACKNOWLEDGEMENTS We thank Celia Y. Chen, Jessica Trout-Haney, and Vivek Venkataraman for their guidance and direction in the experimental design and writing process. AUTHOR CONTRIBUTIONS

All authors contributed equally to the data

collection and writing process.


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LITERATURE CITED

Bischof, K.., Hanlet., D., and Wiencke, C. 1998. UV-radiation can affect depth-zonation of Antarctic macroalgae. Maine Biology 131: 597-605.

Hay, M.E., and Fenical, W. 1988. Marine plant- herbivore interactions: the ecology of chemical defense. Annual Review of Ecology and Systematics 19: 111-145.

Hay, M.E., and Whal, M. 1995. Associational resistance and shared doom: effects of epibiosis on herbivory. Oecologica. 102: 329-340

Jensen, K. 1977. Effect of epiphytes on eelgrass photosynthesis. Aquatic Botany 3: 55-63.

Kitting, L.C., Fry, B., and Morgan, D.M. 1984. Detection of inconspicuous epiphytic algae supporting food webs in seagrass meadows. Oecologica 62: 145-149.

Ogden, J.C. 1976. Some aspects of herbivore-plant relationships on Caribbean reefs and seagrass beds. Aquatic Botany 2: 103-116.

Ralph, P.J., Durako, M.J., Enriquez, S., Collier, C.J., and Doblin, M.A. 2007. Impact of light limitation on seagrasses. Journal of Experimental Marine Biology and Ecology 350: 176-193.

Thayer, G.W., Bjorndal, K.A., Ogden, J.C., Williams, S.L., Zieman, J.C. 1984. Role of larger herbivores in seagrass communities. Estuaries. 7: 351-376.

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BURROW OCCUPATION AND MOVEMENT IN ECHINOMETRA LUCUNTER

ANN M. FAGAN, REBECCA C. NOVELLO, AND CLAIRE B. PENDERGRAST

Faculty Editor: Celia Y. Chen Abstract: Animals often select or create shelters in their environments that reduce predation risk and vulnerability to environmental stressors. However, animals must devote time and energy to retaining these shelters, and they may choose to abandon shelters when protective benefits do not outweigh the costs of maintenance and defense against conspecifics. Echinometra lucunter is a tropical rock-boring sea urchin that creates and occupies burrows that offer protection against predation and wave action. E. lucunter often return to the same burrow after nocturnal foraging and must sometimes fight conspecifics to retain possession of high-quality burrows. However, burrow availability could influence patterns of burrow selection and defense in E. lucunter. We hypothesized that in environments where burrow availability is high, urchins would occupy burrows whose depths are proportional to their body sizes because these burrows offer the greatest protection. We also hypothesized that in such environments, E. lucunter would move freely between burrows and be more likely to leave a burrow to forage if the burrow was not proportional to their body size. We found that in tidal pools with high burrow availability, E. lucunter occupy burrows with depths that are proportional to their body size and fail to display site fidelity. Urchins occupying shallower burrows or burrows that are not proportional to their body size are more likely to leave them to forage at night. Population density and burrow availability appear to influence movement and site fidelity in E. lucunter, suggesting that urchin behavior and habitat choice reflect local environmental conditions. Keywords: body size, Echinometra lucunter, resource availability, site fidelity, urchin INTRODUCTION Many animals construct or select shelters in their environment that facilitate protection, food acquisition, or reproductive success. Since the maintenance and defense of such shelters can be energetically costly, selecting a high quality site is critical. In areas with low shelter availability, animals often prioritize the defense of adequate shelters over the potentially expensive pursuit of higher quality shelters. Animals in such areas often display site fidelity, returning to the same shelter over time and defending it from conspecifics (Switzer 1997). In contrast, when shelter availability is high, competition for shelter may be weaker. Animals in these environments may instead devote energy to foraging or seeking out higher quality shelters instead of defending current sites. They may also be less likely to display site fidelity when competition for shelter is reduced, as the potential benefits of exchanging sites outweigh the low risk of competitive interactions over quality shelters (Newton and Marquis 1982).

Echinometra lucunter is a nocturnal tropical sea urchin found in west Atlantic coastal areas from Florida to Brazil that abrades rock surfaces with its teeth and spines to create protective burrows (Hendler et al. 1995). Burrows offer

protection from predators and wave action, and urchins rarely leave their burrows except to forage on nearby algal patches at night (Shulman 1990). E. lucunter often occur at high densities around food patches ( 240 individuals m2, Metaxas and Young 1998). Many habitats with abundant algal resources and suitable substrate support high-density urchin populations (Davidson and Grupa 2014), increasing competition for burrows. Conspecific aggression over burrows is well documented in E. lucunter (Grunbaum et al. 1978). Previous studies suggest that E. lucunter in these environments of low burrow availability commonly exhibit site fidelity, consistently returning to the same burrow after a night of feeding (McPherson 1969). Experiments have shown that starved urchins are more likely to leave burrows than those that have fed, and many species forage only at night when predation risk is low (Yanagisawa 1991). This suggests that leaving burrows to forage requires risking predation and that urchins may avoid leaving burrows unless the benefit of feeding outweighs this risk.

E. lucunter occupy tidal pools formed in limestone karst – an easily eroded limestone landscape formed by dead coral – on the northern shores of Little Cayman Island. These tidal pools differ from sites of commonly studied urchin


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aggregations in that they contain many empty burrows of varying sizes. This high burrow abundance may be caused by movement of urchins to and from the ocean over time, leaving behind unoccupied burrows that can be used by incoming individuals. While site fidelity is commonly observed in E. lucunter, this behavior could be a product of the high urchin density and low burrow availability where these urchins have been traditionally studied. When the pressures of burrow competition are reduced, E. lucunter populations may display different burrow selection behaviors. The high availability of suitable burrows in Little Cayman tidal pools provides the opportunity to examine these behaviors. We investigated the burrow preferences and between-burrow movements of E. lucunter where burrow availability is high. We hypothesized that urchins would not display site fidelity in environments where suitable unoccupied burrows are abundant. Urchins should also spend more time in burrows that provide better protection from predators and wave action. If deeper burrows provide better protection, then urchin size and burrow depth should not be related. Additionally, urchins in deeper burrows should be less likely to leave those burrows to forage and sacrifice their high quality shelter. However, if burrows that fit closely around an urchin provide more protection by covering its body tightly, urchins should occupy burrows of depths proportional to their body size. In this case, urchins in burrows of depths proportional to their body size should be less likely to leave those burrows and risk predation or wave damage. METHODS We examined patterns of E. lucunter burrow occupation and movement in three limestone karst tidal pools at Barge Dock, Little Cayman Island on 27-29 February 2014. On 27 and 28 February, weather and surf conditions were mild. On 29 February, winds became stronger and wave frequency and energy increased. We conducted all movement observations under moderate tidal conditions immediately preceding a new moon. All three tidal pools were approximately 1 m2. They all contained at least three times as many unoccupied burrows as urchins and ranged in burrow depths and urchin sizes. We defined burrows as deep, cup-shaped indentations in the

limestone substrate of the tidal pools. Two of the pools were approximately 2 m from the ocean, and each was consistently replenished by waves through an opening approximately 0.5 m wide. Algal type and abundance, urchin density, urchin size distribution, and burrow density were roughly equivalent between these two pools. The third pool was approximately 5 m from the ocean and was infrequently replenished during our study period. This pool had considerably lower algal cover, urchin density, urchin size, and burrow density. E. lucunter Burrow Occupation To investigate the relationship between burrow depth and E. lucunter body size, we surveyed 45 urchins in one pool 2 m from the ocean (near-ocean) on 27 February (N = 26) and one pool 5 m from the ocean (far-ocean) on 29 February (N = 19). We exhaustively surveyed urchins in half of the near-ocean pool and all of the far-ocean pool. We did not assume that urchins bored the burrows in which they resided because it is very possible that urchins actively abandon and exchange burrows (Yanagisawa 1991). We removed urchins from their burrows using forceps and measured each urchin’s test diameter and full diameter (including both test and spines, Fig. 1). We also measured burrow depth, defined as the distance from the flat rock surface surrounding the burrow to the deepest point of the burrow (Fig. 2). After we measured urchin size and burrow depth, we returned urchins to their original burrows.

Figure 1. Body size measurements of sea urchin E. lucunter.

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E. lucunter Movement We examined E. lucunter movement and site fidelity in the two pools each located 2 m from the ocean on 27 and 28 February. On 27 February, we marked 10 urchins from each pool (N=20) by tagging a spine with a labeled piece of electrical tape. For each tagged urchin, we measured the depth of its burrow, placed a numbered marker at the burrow, and took a photograph of the urchin and its marker. We again measured urchin test diameter and full diameter.

We returned to the two pools at 2230 and noted whether each urchin had remained in its original burrow or had moved elsewhere in the tidal pool. On 28 February at 1530 we returned and recorded which urchins had remained in their original burrows and which had moved overnight.

Statistical Analyses and Modeling To determine the burrow depth preferences of E. lucunter, we performed a linear regression of burrow depth by full urchin diameter in the two pools sampled for burrow occupation. We then used two-tailed t-tests to analyze patterns of E. lucunter size (test diameter and full diameter) and burrow depth between “movers” (urchins observed outside their burrow at some point during the observation period) and “non-movers” (urchins never observed outside their original burrow).

We chose to measure the fit of an urchin to its burrow depth by calculating the residuals of the linear regression of burrow depth by full diameter. For this analysis, we used only data from the near-ocean pool in the burrow occupation, which was more representative of the habitat type and size distribution of urchins used for the movement study. We assumed that urchins in appropriately sized burrows would fall close to the regression line, while urchins in burrows too large or small for their bodies would show larger residuals. We compared residuals between movers and non-movers to evaluate whether burrow fit influenced likelihood of urchin movement. RESULTS Burrow occupation Urchin body size was positively correlated with burrow depth (Table 1, Fig. 3A, 3B). Urchin test diameter ranged from 0.6 to 3.6 cm, and full diameter ranged from 1.1 to 8.4 cm. Burrow depth ranged from 0 to 7.7 cm.

Figure 2. Burrow depth was measured from lip of burrow to its deepest point after an urchin was removed.

Figure 3. Burrow depth increased with test diameter (A) and full diameter (B).

A B


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Movement At 1530 on 28 February, we located 15 of the 20 urchins that we tagged at 1600 on February 27. We did not locate 5 of the tagged urchins, possibly because they left the pool, their tags fell off, or they were depredated over the course of the night. We identified seven of the 15 located urchins as movers. Of these 7 movers, only one returned to its original burrow the following day. The remaining 8 urchins were non-movers. We acknowledge the possibility that non-movers left and returned to their burrows between observation periods. Urchins in deeper burrows were less likely to

move over the course of the night (Fig. 4, t = -3.37, P = 0.008, df = 9). Urchin body size (test diameter or full diameter) was not related to likelihood of movement from the original burrow (Table 2). Non-mover burrow depth closely fit the regression line of burrow depth to body size found in the tide pool also surveyed for burrow occupation (Fig. 5A).

Non-mover:

burrow depth = 0.60*( full diameter) +0.93 R2 = 0.70 (Equation 1)

Burrow occupation:


However, burrow depth and full diameter were not related in mover urchins (Fig. 5B).

Mover:


Burrow occupation:


DISCUSSION Our results suggest that in environments where burrows are abundant, E. lucunter occupy burrows

Figure 4. Urchins in deeper burrows were less likely to move than those in shallower burrows.

Figure 5. (A) In non-movers, the positive correlation between full urchin diameter and burrow depth was identical to that of all 26 urchins in the pool sampled (predicted correlation). (B) In movers, there was no relationship between full urchin diameter and burrow depth. Solid line represents a linear regression of urchins surveyed for burrow occupation in the refreshed pool. Points represent non-movers (A) or movers (B). Dashed line represents a linear regression of burrow depth and full diameter in movers or non-movers.

A B

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that fit tightly around them. Instead of using the deepest burrows available, urchins occupied burrows that were proportional to their body size (Fig. 3). Urchins in burrows poorly fitted to their body size were more likely to leave burrows to forage (Fig. 5), suggesting that the shelter benefits offered by these burrows did not outweigh the potential benefits of feeding. It is also possible that these urchins moved to actively seek out a better burrow wave action or that wave action dislodged them. Burrows with a tight fit around an urchin’s body may be more protective, allowing the urchin to dig its spines into the substrate (McClanahan 1988). Digging into a burrow could give an urchin the ability to resist wave action and predation more effectively than in a burrow that is either too large or small. Alternatively, small urchins may choose shallower burrows in order to avoid invoking an aggressive response from larger urchins. Deeper burrows may be more important for larger urchins, and larger urchins are capable of forcefully removing smaller urchins from these burrows by initiating aggressive interactions (Morishita 2009). Although this interaction may be less likely in an environment where shelters are abundant, it is possible that urchin burrow choice is not plastic and does not change with shelter availability in their environment.

The majority of movers (86%) did not return to their original burrows. While previous studies have reported site fidelity as common in E. lucunter, these observations suggest that burrow selection and fidelity may vary with environmental conditions. Since these tidal pools have high burrow availability, it may be less important for E. lucunter to find and defend a single burrow. Instead, they may leave a burrow at night to forage and then occupy a similarly sized, closer burrow to

avoid the energy costs and predation risks of returning to and defending their original burrow.

Finally, we found that non-movers occupied deeper burrows and burrows that were more closely proportional to their body size than movers (Fig. 4, 5), but that body size did not differ between movers and non-movers. These results suggest that movers left burrows that were proportionally too shallow for their bodies (and therefore less protected from predators and waves) in order to forage outside the burrow. Non-movers

remained in burrows that were proportional to their body size, possibly because potential energy gains from foraging do not outweigh the costs associated with leaving a highly protective burrow.

These results suggest that E. lucunter balance burrow selection and foraging activities differently depending on burrow availability. When available burrows are abundant, urchins frequently move among sites rather than displaying site fidelity, and movement decisions appear to be influenced by the protective capacity of a burrow. The variation in how urchins choose burrows suggests that studies of site selection, defense, and movement may benefit from looking across a gradient of resource limitation. This may help to better understand the complex energetic trade-offs made by organisms competing for limited resources. ACKNOWLEDGEMENTS We would like to thank CCMI for their hospitality and Jessica Trout-Haney for her assistance with data collection. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Grunbaum, H., G. Bergmann, D. Abbott, and J.

Ogden. 1978. Intraspecific agonistic behavior in the rock boring sea urchin Echinometra lucunter (L.). Bulletin of Marine Science 28:


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181-188. Hann, H.W. 1937. Life history of the Oven-bird in

southern Michigan. The Wilson Bulletin 49: 145-237.

Hendler, G., J.E. Miller, D.L. Pawson and P.M. Kier. 1995. Sea stars, sea urchins, and allies: echinoderms of Florida and the Caribbean. Smithsonian Institution Press. Washington, D.C.

Lewis, J.B. and G.S. Storey. 1984. Differences in morphology and life history traits of the echinoid Echinometra lucunter from different habitats. Marine Ecology Progress Series 15: 207-211.

McClanahan, T. 1988. Coexistence in a sea urchin

guild and its implications to coral reef diversity and degradation. Oecologia 77: 210-218.

McPherson, B.F. 1969. Studies on the biology of the tropical sea urchins Echinometra lucunter and Echinometra viridis. Bulletin of Marine Science 19:194-213.

Shulman, M.J. 1990. Aggression among sea urchins on Caribbean coral reefs. Journal of Experimental Marine Biology and Ecology 140: 197-207.

Switzer, P.V. 1993. Site fidelity in predictable and unpredictable habitats. Evolutionary Ecology 7: 533-555.

Yanagisawa, T. 1991. Biology of Echinodermata. CRC Press.

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A. PALMATA COLONY HEALTH AND SIZE AFFECT FISH ABUNDANCE, NOT SPECIES DIVERSITY

ALEXA E. KRUPP, AMELIA L. RITGER AND ADAM N. SCHNEIDER

Faculty Editor: Celia Chen

Abstract: Reef building corals are integral to coral reef fish communities, providing fish with shelter and benthic fauna upon which to feed. As global coral bleaching events destroy reefs and percent coral cover (health) decreases, damaged benthic communities and degraded habitat structures may negatively impact fish abundance and diversity. We examined how variation in live coral coverage and colony size of individual Acropora palmata colonies influences resident fish assemblages at Bloody Bay Marine Park of Little Cayman Island. We surveyed and compared fish communities at twenty-five A. palmata colonies of varying live coral coverage. We found that at the colony level, coral health and colony size influences fish abundance, but not fish species diversity. Because species did not have a preference for live coral coverage or colony size at the colony level, is important to consider scale when making conservation decisions aimed at preserving species diversity. In coral reef systems, efforts to preserve species diversity may require protection of the entire reef rather than individual colonies. Key words: Acropora palmata, coral health, fish species diversity INTRODUCTION Reef building corals provide the structural complexity and foundation of the benthic community that numerous invertebrates and fish rely on for shelter and food (Bell and Galzin 1984). The coral polyps form the foundation of the benthic community that many fish rely on for food while the calcium carbonate skeleton secreted by corals contributes to the topography of reef environments, providing fish with shelter and hunting grounds (Lirman, 1999).

As the condition of reef-building corals is negatively affected worldwide by anthropogenic inputs and environmental stressors induced by climate change, the health of the entire reef community is also harmed. Fish abundances have been disproportionately negatively impacted by loss of live coral coverage and are sensitive to as little as 5% loss of coral coverage across a reef (Feary et al. 2007, Bell and Galzin 1984). Fish diversity is also highly vulnerable to coral loss. Up to 80% of fish species may be lost in reefs with coral die-offs (Pratchett et al. 2011).

Fish vary in their reliance on coral colonies as a function of their functional feeding group. Herbivores might use corals as shelter, while invertivores and carnivores might use corals as hunting grounds. Such differences suggest that although entire fish communities are affected by changes in coral colony health, these impacts vary by functional group (Pratchett et al. 2011).

Acropora palmata (elkhorn coral) is one of the dominant species comprising overall coral coverage and contributing to the topography of shallow reef environments (Lirman 1999). We examined how fish assemblages residing in elkhorn coral changed with varying degrees of host colony health in a Caribbean fringing reef. First, we tested the hypothesis that fish abundance and diversity are affected by the size and percent live coverage of elkhorn coral. If live coverage alone predicts fish communities, fish may be primarily attracted to the benthic fauna on the coral. If fish abundance and diversity are solely a function of colony surface area, fish may inhabit the coral primarily for shelter. Finally, if fish are dependent on both the benthic community and habitat structure provided by elkhorn coral, we would expect both size and percent live coral coverage to positively influence fish abundance and diversity.

We also examined the effect of live coral coverage on fish abundance and diversity by functional group. Since functional feeding groups have been found to respond differently to disturbance (Pratchett et al. 2011), we predicted that there would be variation among these groups in response to changes in live coral coverage. Differences in fish community composition with live coverage would indicate that particular functional groups or species may be particularly susceptible to coral loss.


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METHODS We sampled along the fore reef of Bloody Bay Marine Park in Little Cayman Island on 27 February from 9:00 to 17:30. We selected twenty-five A. palmata colonies along a transect parallel to shore, excluding colonies less than 1m long and those with extensive macroalgal, octocoral or hydrozoan coverage. We measured the height and length of each colony and calculated two-dimensional colony area. We estimated live tissue coverage within six percentage classes (dead, 1-20%, 21-40%, 41-60%, 61-80%, 81%-100%). We watched each colony of A. palmata for two minutes, during which time we recorded fish abundance and identified species of fish located within ½ meter away of the focal colony and present for the full two minutes. We categorized observed fishes by functional feeding groups as herbivores, invertivores, or carnivores. Statistical analysis We calculated fish species diversity at each colony using Simpson's Diversity Index. We used Analysis of Variance (ANOVA) to examine whether fish abundance or species diversity changed with live coral coverage class. We performed an ANOVA to determine if coral colony size had an effect on live coral coverage. We performed an Analysis of Covariance (ANCOVA) of fish abundance and diversity with respect to coral coverage and coral size. Finally, we used an ANOVA to examine the relationship between reef fish abundance by functional feeding group and live coral coverage. All analyses were

performed using Al Young Biodiversity Calculator and JMP® 10 statistical software (SAS Institute, Cary, NC).

RESULTS We observed a total of twenty fish species from ten families on A. palmata colonies (Appendix 1). On average, we found six resident fish individuals from four different species at each colony. Live coral coverage class affected fish abundance (F4,24= 5.47, P = 0.0073, Fig. 1) but did not affect species diversity (F5, 22 = 0.23, P = 0.942; Fig. 2). A. palmata colony size ranged from 0.48 m2 to 3.38 m2 and showed no relationship with live coral coverage (F5, 42 = 1.34, P = 0.289). Although coral colony size did not affect fish diversity (slope = -

Figure 1. Fish abundance increased as area increased on coral colonies containing some level of live coral coverage. Larger dead corals had less fish than smaller dead corals. No regression line is presented for size class 4 due to low sample size.

Figure 2. Reef fish species diversity did not change with A. palmata percent live coral cover.

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0.047 ± 0.074, P = 0.53, r2 = 0.019), coral colony size was a predictor of fish abundance (slope = 2.13 ± 1.1, P = 0.058, r2 = 0.15). The interaction of live coral coverage and coral size was a significant predictor of fish abundance (F4, 42 = 0.002, P = 0.02; Fig. 1) but not fish diversity (F4, 42 = 1.16, P = 0.37). Fish abundance by functional feeding group did not change with live coral coverage (Carnivores: F5, 24 = 0.41, P = 0.84; Invertivores: F5, 24 = 1.20, P = 0.35; Herbivores: F5, 24 = 2.68, P = 0.053; Fig. 3). DISCUSSION The health of coral reefs may have substantial effects on resident fish communities. We found that health and size individual A. palmata coral colonies influenced fish abundance but not species diversity. Greater numbers of fish were found around healthier, larger corals, but species composition did not change across coral colonies (Fig. 1). This suggests that fish communities are dependent upon corals that provide both habitat structure and a benthic community. We found no meaningful difference between fish abundance within each functional feeding group across live coral coverage. There were, however, slightly higher herbivore abundances on dead corals (Fig. 3), which could be the result of increased grazing opportunities from higher algal coverage on dead coral.

Although more fish congregated around larger, healthier coral colonies, many different fish species from all functional feeding groups could

be found at individual live colonies of varying health and sizes (Fig. 3). It may be that A. palmata live coral coverage influenced fish abundance but not fish diversity because changes in diversity cannot be seen by examining a single coral colony; rather, impacts of coral health and coral size on fish diversity must be examined over an entire reef. When the overall health of a reef is significantly compromised, reef fish diversity is negatively impacted as dead reefs lose both habitat structure and benthic communities (Alvarez-Filip et al 2009).

These conclusions have important implications for our understanding of coral reef ecology and conservation. Fish species did not prefer any particular level of live coral coverage class, indicating that health of individual colonies on a single reef may not impact the distribution of fish species as much as coral health of an entire reef. Therefore, reef preservation efforts aimed at promoting healthy and diverse fish communities should focus on preserving the presence of corals on a reef instead of individual colony health. ACKNOWLEDGEMENTS We would like to thank Celia Chen, Vivek Venkataraman, and Jessica Trout-Haney for their valuable feedback on our paper. We would also like to thank the damselfish for keeping us in line as we invaded their territory. AUTHOR CONTRIBUTIONS All authors contributed equally.

Figure 3. Reef fish functional feeding group was not associated with A. palmata percent live coral coverage. Herbivores and invertivores were most abundant in dead A. palmata and carnivores were never found in dead A. palmata.


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LITERATURE CITED Alverez-Filip, L., Dulvy, N.K, Gill, J.A., Cote,

I.M., Watkinson, A.R. 2009. Flattening of Caribbean coral reefs: region-wide declines in architectural complexity. Proceedings of the Royal Society 276: 3019-3025.

Bell, J.D., and Galzin, R. 1984. Influence of live coral cover on coral-reef fish communities. Marine Ecology Progress Series 15: 265-274.

Feary, D., Almany, G., McCormick, M., and Jones, G. 2007. Habitat choice, recruitment

and the response of coral reef fishes to coral degradation. Oecologia 153: 727-737.

Graham, N. A. J., et al. "Coral mortality versus structural collapse as drivers of corallivorous butterflyfish decline." Biodiversity and conservation 18.12 (2009): 3325-3336.

McCormick, M.I. 1995. Fish feeding on mobile

benthic invertebrates: influence onf spatial variability in habitat associations. Marine Biology. 121: 627-637.

Patterson, K. L., J. W. Porter, K. E. Ritchie, S. W. Polson, E. Mueller, E. C. Peters, D. L. Santavy, and G. W. Smiths. 2002. The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata. Proceedings of the National Academy of Sciences of the United States of America 99: 8725-8730.

Pratchett, Morgan S, A. S. Hoey, S. K. Wilson, V. Messmer, and N. A. Graham. 2011. Changes in biodiversity and functioning of reef fish assemblages following coral bleaching and coral loss. Diversity 3.3: 424-452.

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THE ROLE OF HETEROTYPIC SCHOOLING IN FORAGING BEHAVIOR OF PARROTFISH

EMILY R. GOODWIN AND AARON J. MONDSHINE


Abstract: Mixed-species aggregations offer organisms the benefits of anti-predatory defense while reducing competition between species through reduced niche overlap. Coral reef fish are often found foraging or resting in mixed-species aggregations, or heterotypic schools, near reef structures. We explored two aspects of heterotypic schools in reef fish: first, whether heterotypic school size varies with school behavior, and second, how participation in heterotypic schools affects foraging rate in parrotfish. We found that school behavior impacted school size but not species richness. Resting schools had significantly higher fish abundance than foraging schools. In addition, parrotfish had increased foraging rates within heterotypic aggregations than when foraging alone. However, for parrotfish foraging in aggregations, foraging rates decreased with increased group size. This suggests that moderately sized foraging groups are optimal for parrotfish. Heterotypic schooling, in contrast to homotypic schooling, allows parrotfish to benefit from increased safety from predators without sacrificing foraging rate due to competition. The tradeoff between protection from predation and feeding competition could play a role in the evolution of mixed-species associations. Key words: heterotypic schooling, mixed-species aggregations, parrotfish, schooling behavior INTRODUCTION In many organisms, the advantages of social groups include benefits related to reproduction, foraging, and predator avoidance. While many of these organisms are found in conspecific groups, mixed-species aggregations have also been observed in a variety of birds, primates, and fish (Morse 1977). These aggregations are also thought to confer advantages related to foraging or predator evasion, yet differ from single-species aggregations in that feeding competition is reduced in mixed-species aggregations due to reduced niche overlap between species (Ehrlich and Ehrlich 1973, Morse 1977, Lukoschek and McCormick 2000). Teleost fish schools in reef habitats represent a classic example of both single- and mixed-species aggregations (Pitcher and Parrish 1993). Fish occurring in homotypic, or single-species, schools may receive the anti-predatory benefits of increased group size. Heterotypic, or mixed-species, schools allow individual fish to decrease predation risk while minimizing costs of feeding competition through mechanisms such as niche partitioning. However, school behavior may also impact this trade-off between predator avoidance and competition. Coral reef fish often aggregate as heterotypic schools that rest or forage near underwater structures (Itzkowitz 1977, Ogden and Ehrlich 1977). Since there are no foraging benefits

for a resting aggregation, these groups are mainly driven by the need to maximize group size as an anti-predator defense. In contrast, foraging aggregations offer the benefits of group-driven anti-predator defense at the cost of increased feeding competition between species. This creates a situation in which resting schools may benefit from larger group size regardless of species, whereas foraging schools are driven to decrease competition through smaller groups with higher species diversity and decreased niche overlap.

This study assesses whether these patterns are occurring in heterotypic schools of Caribbean reef fish on Little Cayman Island. We hypothesize that resting schools will have higher overall fish abundance, whereas foraging schools will have smaller schools with increased species diversity. As a case study, we also examine how heterotypic schooling benefits parrotfish (Scaridae) on Little Cayman Island. Parrotfish are a common Caribbean reef fish that participates in mixed-species aggregations. Their feeding strategy requires rapid and noisy biting that can expose individuals to predation and incite aggression by notifying potential competitors of a food source (Ogden and Buckman 1973, Overholtzer and Motta 2000). Parrotfish can minimize these risks by schooling with conspecifics, but this increases feeding competition (Overholtzer and Motta 2000). Participation in heterotypic schools may


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therefore confer protection from predation while minimizing competition with other parrotfish. Therefore, we predict that parrotfish in heterotypic schools will have higher feeding rates than solitary parrotfish, since they can afford to invest less energy in predator detection and more energy foraging. METHODS Data Collection We observed 22 heterotypic aggregations and 53 parrotfish (32 in aggregations; 21 solitary) at Pirates Point, Cumber’s Cove, and Grape Tree Bay, Little Cayman, on 5-7 March 2014. We opportunistically observed aggregations at these sites and measured total fish abundance, species richness, species abundances, and school behavior. School behavior was determined over a 5-minute period and was characterized as either foraging or resting. Schooling behavior was found to be consistent over the entire 5-minute period. We also opportunistically measured foraging rates (bites in 1 minute intervals) of both solitary parrotfish and those in heterotypic schools. Statistical Analysis We used a principal components analysis (PCA) to compare the species composition of aggregations. We used a one-tailed t-test to test whether species richness and fish abundance differed by school behavior. We also used a t-test to compare the foraging rates of solitary parrotfish and those in heterotypic schools. For parrotfish in schools, we used a linear regression to test whether school size affected the foraging rates of individual parrotfish.

RESULTS We observed 20 different species of fish, seven of which were species of parrotfish. Haemulon carbonarium was the most abundant species, followed by Acanthurus coeruleus, Kyphosus sectatrix, and Haemulon flavolineatum (Fig. A). At least three parrotfish individuals were found in all of the schools. We did not observe any homotypic schools of parrotfish.

The PCA showed no clear trend with respect to composition of fish aggregations. Principal components 1 and 2 accounted for 16.1% and 15.6% of the total variance, respectively. The loading patterns of the eigenvectors did not appear to have any biological significance.

We found that fish abundance was significantly higher in resting aggregations (mean ± SE = 123.90 ± 21.05) than in foraging aggregations (mean ± SE = 50.40 ± 21.047, t1 = 2.47, P = 0.012; Fig. 1). However, there was no difference observed for species richness between resting (mean ± SE = 4.40 ± 0.60) and foraging aggregations (mean ± SE = 5.50 ± 0.60, t1 = 1.30, P = 0.11, Fig. 2).

Foraging rate was significantly higher for parrotfish in aggregations (mean ± SE = 20.69 ± 1.51 bites/min) than for solitary parrotfish (mean ± SE = 11.81 ± 1.87 bites/min, t1 = 3.69, P = 0.0003; Fig. 3). Parrotfish in mixed-species aggregations foraged less efficiently with increasing aggregation size (slope= -0.050 ± 0.022, P = 0.033, r2 = 0.14; Fig. 4).

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Figure 1. Fish abundance was significantly higher within resting aggregations than within foraging aggregations.

Figure 2. There was no significant difference in species richness between foraging and resting aggregations.

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DISCUSSION Heterotypic aggregations may provide increased protection from predators while minimizing the costs of conspecific competition. However, this tradeoff may be influenced by school behavior, species composition, and aggregation size. We found that resting schools had higher overall fish abundance than foraging schools (Fig. 1). This could be a result of (1) the anti-predatory benefits of resting in a large group, or (2) the habitat structure of a location being particularly suitable for a wide range of fish species. Because there is no feeding competition in resting schools, there is no tradeoff between protection from predation and feeding competition. We observed no difference in species richness between resting and foraging schools (Fig. 2), indicating that the pressures of conspecific competition are not necessarily driving greater species diversity in foraging schools compared to resting schools. Parrotfish exhibited behavior consistent with a tradeoff between predation risk and feeding competition. Foraging rate was significantly higher for parrotfish in aggregations as compared to those feeding alone (Fig. 3). This could be a result of either increased predator detection in a group setting, which allows individuals to devote more time to foraging, or group facilitation in locating patchily distributed resources. Increased predator detection is more likely because the macroalgae parrotfish foraged on was widespread. However, for parrotfish in aggregations, foraging

rate decreased as school size increased. This could be explained by increased feeding competition in larger groups. Therefore, parrotfish in moderately sized groups have the highest foraging rates. Since parrotfish in small aggregations have higher feeding rates than both parrotfish in large aggregations and solitary parrotfish, joining small aggregations appears to maximize foraging success. Our results suggest that the formation of heterotypic schools may be driven by both foraging benefits and predation risk. Parrotfish were never found in homotypic schools, but were found in all observed heterotypic schools. This suggests that parrotfish in homotypic schools may have an even smaller optimal group size than those in heterotypic schools due to increased feeding competition. Heterotypic schooling allows parrotfish to benefit from increased safety without sacrificing their foraging rate due to competition. This anti-predatory-competition tradeoff could play a role in the evolution of mixed-species associations. ACKNOWLEDGEMENTS We are indebted to C. Chen, V. Venkataraman, and J. Trout-Haney for their assistance at all stages of this study. We would also like to thank the FSP group for their valuable feedback. AUTHOR CONTRIBUTIONS Both authors contributed equally in the data collection and writing process.

LITERATURE CITED Ehrlich, P.R., and A.H. Ehrlich. 1973.

Coevolution: heterotypic schooling in Caribbean reef fishes. The American

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Figure 3. Foraging rate for parrotfish within an aggregation was significantly higher than for solitary parrotfish.

Figure 4. Foraging rate (bites/minute) decreased significantly with increased aggregation size.

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Naturalist 107: 158-160. Foster, S.A. 1985. Group foraging by a coral reef

fish: a mechanism for gaining access to defended resources. Animal Behavior 33: 782-792.

Itzkowitz, M.1977. Social dynamics of mixed- species groups of Jamaican reef fishes. Behavioral Ecology and Sociobiology. 2: 361-384.

Lukoschek, V., and M.I. McCormick. 2000. A review of multi-species foraging associations in fishes and their ecological significance. Proceedings of the International Coral Reef Symposium. Bali, Indonesia 23-27.

Morse, D.H. 1977. Feeding behavior and predator avoidance in heterospecific groups. BioScience 27: 332-339.

Ogden, J.C., and N.S. Buckman. 1973. Movements, foraging groups, and diurnal migrations of the striped parrotfish Scarus croicensis. Ecology 54: 589-596.

Ogden, J.C., and P.R. Ehlrich. 1977. The behavior of heterotypic resting schools of juvenile grunts (Pomadasyidae). Marine Biology 273-280.

Overholtzer, K.L., and P.J. Motta. 2000. Effects of mixed-species foraging groups on the feeding and aggression of juvenile parrotfishes. Environmental Biology of Fishes 58: 345-354.

Pitcher, T.J., and J.K. Parrish. 1993. Functions of shoaling behavior in teleosts. In: Pitcher T.J. (ed.), Behavior of Teleost Fishes. Chapman and Hall, London: 363-439.

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SALT POND MINNOWS ACCLIMATIZE TO SALINITY

REBECCA C. NOVELLO, AMELIA L. RITGER, AND ZACHARY T. WOOD

Faculty editor: Celia Chen Abstract: Acclimatization allows organisms to adjust to changes in their environment. Many fish can adjust to changes in components of water chemistry such as salinity. Ponds on Little Cayman Island are subject to large salinity ranges due to evaporation, seasonal rainfall, and marine water influxes. We examined how Gambusia spp. minnows are affected by variations in salinity levels and whether minnows have acclimatized to the salinity of their native ponds. We exposed fish from two ponds (high and low salinity) to a gradient of salinity treatments and calculated the 24-hour 50% lethal concentration (LC50). We performed a laboratory-based reciprocal transplant between fish of the two ponds and measured stress (via percent activity) to assess the effects of acclimatization to native-pond salinity upon salinity tolerance. Minnows from the high salinity pond had a higher upper salinity tolerance than those from the low salinity pond. Minnows were more stressed in water from the foreign pond than from their native pond. We conclude that minnows acclimatize to the salinity of their native ponds. Salinity levels higher or lower than that of native ponds results in impaired performance. Key words: acclimatization, Gambusia, LC50, minnows, salinity, tolerance INTRODUCTION Organisms that can physiologically adjust to changes in their environment by acclimatization can be more resilient to environmental change (Somero 2009). In aquatic ecosystems, organisms may acclimatize to changes in temperature, oxygen, pH, and salinity. Salinity, in particular, influences development, growth, and metabolism of aquatic organisms such as fish (Boeuf and Payan 2001). Fish use active transport, specialized chloride cells, and urination patterns to tolerate changes in salinity (Evans 2002, Uchida et al 2000, Loretz and Bern 1980). Fish can show varying levels of tolerance to changes in water chemistry; for example, estuary systems contain euryhaline killifish that are highly tolerant to changes in water salinity and therefore have a broader spatial range in the estuary than stenohaline species (Griffith 1974). Shallow inland ponds near seacoasts have large seasonal fluctuations in salinity due to evaporation, seasonal rainfall, and marine water influxes, and variation in basin morphometry may result in differences in salinity across ponds. Little Cayman Island contains many such ponds with differing salinities. During the dry season (January - April), local fishermen have reported salinities reaching 70 ppt (Tom Provost, personal communication). Despite variation in salinity among ponds, many contain populations of Gambusia spp. minnows. Therefore, these

minnows must have some mechanism for dealing with variation in salinity. We studied how fluctuations in native pond salinity affect salinity tolerance in Gambusia minnows. We tested whether minnows from a high salinity environment had higher upper salinity tolerances than minnows from a low salinity environment. If minnows wereacclimatized to the salinity of their environment, minnows from a high salinity environment would be more stressed than minnows from a low salinity environment when exposed to low salinities. Similarly, minnows from a low salinity environment would be more stressed than minnows from a high salinity environment when exposed to high salinities. METHODS We collected 150 minnows and 80 litres of water from two ponds of approximately one meter depth on Little Cayman Island on 6 March, 2014. Jackson's Pond, located on the north side of the island, had a surface area of about 110,000 m2 and a salinity of 35 ppt at the time of the study. Tarpon Lake, located on the south side of the island, had a surface area of about 190,000 m2 and a salinity of 52 ppt at the time of the study. Salinity tolerance We experimentally assessed the upper salinity tolerances of fish from Jackson's Pond and Tarpon


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Lake. At 16:30 on 6 March, we placed 10 minnows of similar size from each pond into one of 10 salinity treatments ranging from 52 ppt to 70 ppt at 2 ppt intervals. We altered salinity of treatments by adding NaCl to 3 L of water from the native pond and assessing salinity with a refractometer. We also placed 10 minnows from each pond into a control treatment of 3 L of water from the native pond without salt additions. We recorded minnow mortality at 40-minute intervals from 16:30 to 23:30 on 6 March and 04:30 to 16:30 on 7 March. For minnows from each pond, we fit a logistic model to proportional mortality (M) across salinities at 24 hrs.

(Equation 1)

This model has an upper asymptote at one and a lower asymptote at zero. As ß1 becomes more negative, the slope of the curve increases. The model is centered around point ß2, which is when M = 0.5 and is therefore the 50% lethal concentration (LC50) for salt. ß3 is the difference in LC50s between the high salinity (Tarpon) lake and low salinity (Jackson) pond. We compared LC50s between minnows from the two pond sources to determine whether upper salinity tolerances differed between ponds. We performed all analyses in R.

Reciprocal transplant We conducted a reciprocal transplant experiment to determine the acclimatization of minnows to the salinity of their native ponds. We placed twelve minnows from each pond into 6 L of water from their own pond (native) and twelve into 6 L of water from the other study pond (foreign). We gave the minnows two hours to acclimate to treatments. After the treatment acclimation period, we moved two minnows from haphazardly chosen treatments into 150 mL of the same treatment water every fifteen minutes and allowed them to acclimated for another fifteen minutes. We measured percent activity for the two minnows in those treatments. We defined percent activity as the percent of time spent swimming over the course of a minute and used it as a proxy for minnow stress, with more movement indicating higher stress. We compared percent activity across water and minnow sources using a two-way ANOVA. We performed this analysis in R. RESULTS Salinity tolerance Tarpon Lake minnows had a greater upper salinity tolerance (LC50) than Jackson's Pond minnows (Fig. 1). LC50 for Jackson's Pond minnows was 60.0 ppt, while that of Tarpon Lake minnows was 71.2 ppt (Table 1, P < 0.0001). 83% of all minnow deaths occurred in the first 4 hours of the study.

Figure 1. Minnows from a low salinity pond (Jackson’s Pond, 35 ppt) died at lower salinities than fish from a high salinity pond (Tarpon Lake, 52 ppt).

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Table 1. Regression coefficients for logistic model

Coefficient Est. ± Std. Err. P

β1 -0.44 ± 0.10 0.00038

β2 60.0 ± 0.6 < 0.0001

β3 11.2 ± 1.2 < 0.0001

Reciprocal Transplant Minnows were more active in water from the foreign pond than their native pond (Figure 2, Table 2, P = 0.009). Neither group of fish was more active, and neither water source caused greater fish activity (Table 2).

Table 2. ANOVA for fish activity on water and fish source

Effect df Mean Squares F P

Water 1 0.085 0.734 0.40

Fish 1 0.002 0.017 0.90 Water *

Fish 1 0.863 7.451 0.0091

Residuals 44 0.116

DISCUSSION We tested whether minnows acclimatize to different salinities by examining two types of responses: changes in upper lethal salinity levels and acclimatization to a particular salinity. Minnows in more saline lakes had higher LC50s, indicating that they have a higher upper salinity tolerance (Fig. 1). For both of our study ponds, minnows could tolerate salinities higher than those in their native ponds (Fig 2). This demonstrates that as salinity increases, minnows develop an increased salinity tolerance that buffers them against further rapid increases in salinity. However, as salinity increases, this buffer zone between experienced salinity and tolerable salinity decreases (i.e., from 25 ppt for Jackson’s Pond to 19 ppt for Tarpon Lake), suggesting that fish cannot increase their salinity tolerance ad infinitum. Fish were more active when placed in water of different salinity than that of their native pond (Fig. 2). Acclimatization to more saline water appears to cause fish to show poorer performance in low salinity water, and vice versa. High activity is metabolically costly. Therefore, fish that move will have decreased energy reserves, which could lead to decreased long-term fitness. Although minnows may not die within 24 hours at these lower or higher salinities, they may still exhibit stress responses. The ability to acclimatize to changes in salinity is important in light of anthopogenic effects. Lakes in cold temperate regions are becoming more saline due to road salt influxes (Novotny et al. 2008). Humans have altered the flow regimes in estuaries, changing where the boundaries between marine and freshwater occur (Colonnello and Medina 1998). Furthermore, wet and dry seasons are expected to become more pronounced (IPCC 2007), which will affect the degree to which salinity fluctuates seasonally in inland saltwater lakes and ponds. Therefore, selection for organisms that can acclimatize to changing salinity should become stronger in many different aquatic systems. Although many organisms can increase their salinity tolerance, our study suggests that this increase probably has limits. Furthermore, we have shown that the acclimatization of organisms to one salinity level can decrease functionality at other salinities, suggesting that acclimatization in areas of

Figure 2. Minnows were more active in water from the opposite pond than from their own pond.


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increasing salinity may limit the ability of organisms to leave these areas. ACKNOWLEDGEMENTS We would like to thank the Central Caribbean Marine Institute and their Little Cayman Research Centre for their hospitality and lab resources. We

would also like to thank Celia Chen, Jessica Trout-Haney and Vivek Venkataraman for driving us around Little Cayman and editing our manuscript. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Boeuf, G., and Payan, P. 2001. How should

salinity affect fish growth? Comparative Biochemistry and Physiology Part C: Toxicology and Pharmacology 130: 411-423.

Colonello, G. and Medina, E. 1998. Vegetation changes induced by dam construction in a tropical estuary: the case of the Manamo River, Orinoco Delta (Venezuela). Plant Ecology. 139.2: 145-154.

Griffith, R.W. 1974. Environment and salinity tolerance in the genus Fundulus. Copeia 1974: 319-331.

Evans, D.H. 2002. Cell signaling and ion transport across the fish gill epithelium. Journal of Experimental Zoology 293: 336-347.

International Panel on Climate Change. 2007. Climate Change 2007: Synthesis Report. Loretz, C.A., and Bern, H.A. 1980. Ion transport

by the urinary bladder of the gobiid teleost,

Gillichthys mirabilis. American Journal of Physiology- Regulatory, Integrative, and Comparative Physiology 239: 415-423.

Novotny, E.V., Murphy, D., and Stefan, H.G. 2008. Increase of urban lake salinity by road deicing salt. Science of the Total Environment 406: 131-144.

Somero, G.N. 2009. The physiology of climate change: how potentials for acclimatization and genetic adaptation will determine ‘winners’ and ‘losers.’ Journal of Experimental Biology 213: 912-920.

Uchida, K., Kaneko, T., Miyazaki, H. Hasegawa, S., and Hirano, T. 2000. Excellent salinity tolerance of Mozambique tilapia (Oreochromis mossambicus): elevated chloride cell activity in the branchial and opercular epithelia of the fish adapted to concentrated seawater. Zoological Science 17: 149-1600.

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ENERGY ALLOCATION VARIES THROUGHOUT GROWTH AND BETWEEN SEXES OF PTEROIS VOLITANS

MAYA H. DEGROOTE AND ALEXA E. KRUPP

Faculty Editor: Celia Chen

Abstract: The rapid invasion of lionfish (Pterois volitans) in the Caribbean poses significant threats to fish communities through competition with native piscivores. Understanding the life history strategies of lionfish, which include rapid growth rate and high fecundity, may help explain the mechanisms underlying their competitive advantage. In Little Cayman, we examined how spine, fin and body length changed with body weight and how investments in fat and gonad mass differed between mature males and females. As lionfish weight increased, their relative spine, fin, and body length decreased. Both female and male lionfish invested more in relative gonad weight as their body weight increased, but only males exhibited increasing fat content with body size. These findings suggest that as lionfish grow, their reproductive capability continues to increase and larger males with more energy reserves may have a greater ability to withstand stressors. As increasingly larger lionfish are found throughout their non-native ranges, higher reproductive and survival capabilities in larger adults may contribute to a higher rate of lionfish proliferation. Key words: invasive, lionfish, Pterois volitans, resource allocation INTRODUCTION Lionfish (Pterois volitans), native to the Indo-Pacific were introduced to southern Florida in the early 1990s and have spread prolifically throughout the Caribbean (Albin & Hixon 2008). As top predators, lionfish can adversely affect reef ecosystems through competition with native piscivores and consumption of a variety of native reef fishes, crabs and shrimps (Hare & Whitfield 2003). The competitive advantage of this species may be attributed to their rapid growth rate, high fecundity, and venomous spines (Morris et al. 2011). All organisms face trade-offs in how resources are allocated toward reproduction, defense, and growth. The principle of allocation (Cody 1966) suggests that organisms evolve to allocate resources in ways that maximize their fitness. Juvenile and adult fish allocate resources differently based on the predatory threats and reproductive demands associated with their life stages. Juvenile fish prioritize rapid growth in part to escape predation, whereas adults invest more heavily in traits relating to reproductive success. Juvenile lionfish may invest more in morphological features such as long venomous spines and protruding fan-like pectoral fins that increase their apparent size and likely reduce predation. Female and male lionfish may also have different strategies of resource allocation due to

differing reproductive energy requirements. Mature female lionfish in the Caribbean can reproduce multiple times per month (Morris et al 2011); this short, frequent reproductive cycle is energy intensive, and may explain why females have less body fat in relation to total body length than males (Costello et al. 2012).

We studied how lionfish invest in defensive characteristics, fat storage, and reproduction throughout ontogeny and between sexes. Since lionfish exhibit indeterminate growth, allocation to morphological features may vary with size, particularly before and after sexual maturity. We tested the hypothesis that smaller and more vulnerable juvenile lionfish invest more heavily in morphological features related to defense relative to larger lionfish. Alternatively, these morphological traits such as spine length and pectoral fin length grow isometrically with body size. Due to the frequency in which females reproduce we also predicted that females allocate more resources to reproduction than males and will thus have lower relative body fat and larger gonad size. METHODS We measured twenty-seven juvenile lionfish collected in February-March 2014 from Mary’s Bay, Pirate’s Point Dock, and South Hole Sound in Little Cayman. We also dissected ten lionfish


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culled from Joy’s Bay on March 5, 2014. We weighed all lionfish and measured total body length, pectoral fin length, and both dorsal and pectoral spine length. Total length was measured as the distance from the mouth to the tip of the caudal fin. Pectoral fin length was measured from the base of the fin to its longest point. The longest dorsal spine and the first pectoral fin were measured using SPI 2000 calipers.

In mature fish, we determined sex and then removed and weighed gonads. We also determined body fat mass by removing and weighing all fat in the body cavity. All weights were made using an AdventurerTM OHAUS. We also used data collected in Little Cayman in March 2012 by Costello et al. (2012) in our analysis. Statistical analysis All morphological measurements and the total body weight of lionfish were log transformed. We used regressions to examine the relationship between morphological traits and body weight. If the length of a feature plotted against weight in log-log space exhibited a slope >1 (positive allometry), this indicates that the feature increases disproportionately relative to body size, whereas a slope < 1 (negative allometry) indicates that the feature decreased disproportionately relative to body size. If the length of a feature was proportional to body weight, the slope = 1 (isometric relationship). To examine resource allocation throughout ontogeny, we performed a regression analysis of body length, pectoral fin length, pectoral spine length, and dorsal spine length, with respect to body weight. To test for differences between sexes in allocation toward body fat and reproduction, we regressed fat and gonad mass with weight in males and females. We performed an analysis of

covariance (ANCOVA) of gonad mass and fat mass in males and females. All analyses were performed using JMP® 10 statistical software (SAS Institute, Cary, NC). RESULTS The body weight of lionfish ranged from 0.6 g to 720.1 g and body length ranged from 57 mm to 381 mm. Lionfish total body length scaled in a negatively allometric fashion with weight, indicating the rate of body length increase was lower than that of weight gain (slope = 0.28 ± 0.02, P < 0.0001, r2 = 0.99; Fig. 1). Likewise, length of the pectoral fin, dorsal spine and pectoral spine scaled negatively with lionfish weight (slope = 0.20 ± 0.10, P < 0.0001, r2 = 0.54; slope = 0.031 ± 0.07, P < 0.0001, r2 = 0.94; slope = 0.24 ± 0.06, P < 0.0001, r2 = 0.94 respectively; Fig 2).

Male and female lionfish differed in their investment in fat storage (ANCOVA; F1,53 = 3.87, P = 0.05). Heavier females had lower relative body fat, while heavier males had higher relative body fat (females: slope = 0.77 ± 0.44, P = 0.07, r2 = 0.16; males: slope = 1.45 ± 0.26, P < 0.0001, r2 = 0.79; Fig. 3). Gonad mass increased with body mass in a positive allometric fashion in both sexes (female: slope = 2.58 ± 0.32, P < 0.0001, r2 = 0.82; male: slope = 1.77 ± 0.17, P < 0.0001, r2 = 0.89; Fig. 4), with females exhibiting a greater rate of increase than males (F1,53 = 9.85, P = 0.003). DISCUSSION Understanding how lionfish invest energy into growth, reproduction, and defense may be useful in assessing which factors contribute to their competitive advantage over native fish species.

Figure 1. Lionfish length had a negative allometric (slope = 0.28 ± 0.02) relationship with weight.

Figure 2. Pectoral fin, dorsal spine, and pectoral spine lengths all had negative allometric relationships with lionfish weight (slope = 0.20 ± 0.10; slope = 0.031 ± 0.07; slope = 0.24 ± 0.06, respectively.

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We found that lionfish allocate resources differently depending on their body size, ontogenetic stage, and sex. Heavier lionfish appear to invest relatively less energy into body length and defensive traits than juveniles (Fig. 2). This suggests that rapid growth and investment in predator defense are more important elements to juvenile survival. Larger lionfish have likely reached a size threshold at which predation risk is low. In both male and female lionfish, gonad weight and fat storage increase with body mass (Figs. 3 & 4). However, the allometric scaling of fat storage and gonad weight with body mass differed between males and females. As males get heavier, they exhibit a disproportionate increase in fat storage (Fig. 3), whereas females invest more in gonad weight than males at the expense of fat. The high level of investment in reproduction in female lionfish has major implications for the reproductive success of this invasive species in the Caribbean and central Atlantic. Male and female lionfish each allocate resources in a way that confers a competitive advantage against native fish. Both lionfish exhibit high fecundity and

invest energy disproportionately to reproduction (Heino & Kaitala 2001). In addition to investment in greater reproductive capability, males invest in enhanced fat storage, buffering them against periods of low food availability and other environmental stressors (Fishelson 1997). These life history strategies have the potential to increase lionfish reproductive success and proliferation, indicating the management and eradication efforts should target larger lionfish since they have a greater reproductive capacity. ACKNOWLEDGEMENTS We would like to thank Katie Lohr and Caitlin Kuempel for outstanding lionfish catching abilities and unwavering enthusiasm. We would like to thank Robin Costello, Nina Frankel, and Madeline Gamble for their project in 2012 and helping make our project possible. Also, thank you Celia Chen who sat with us looking at graphs for hours, and Vivek Venkataraman and Jessica Trout-Haney who helped us attempt to count gonads. AUTHOR CONTRIBUTION All authors contributed equally. Adam Schneider identified all fish by species.

LITERATURE CITED Albins, M.A. and Mark A Hixon. 2013. Worst

case scenario: potential long-term effects of invasive predatory lionfish (Pterois volitans) on Atlantic and Caribbean coral-reef communities. Environmental Biology of Fishes 96: 115-1157.

Brönmark, C. and J.G. Miner. 1992. Predator- induced phenotypic change in body

morphology in crucian carp. Science 258: 1348-1350.

Cody, M.L. 1966. A general theory of clutch size. Evolution 20: 174-184.

Costello, R.A., N.B. Frankel, and M.M. Gamble. 2012. Allometric scaling of morphological feeding adaptations and extreme sexual dimorphism in energy allocation in invasive lionfish (Pterois volitans).

Figure 4. Gonad mass scaled in a positively allometric fashion with weight in both females (slope = 2.58 ± 0.32) and males (slope = 1.77 ± 0.17).

Figure 3. Fat mass had a negative allometric (slope = 0.77 ± 0.44) relationship with weight in females and a positive allometric (slope = 1.45 ± 0.26) relationship in males.


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Darling, E.S., S.J Green, J.K. O’Leary, and I.M Cote. 2011. Indo-Pacific lionfish are larger and more abundant on invaded reefs: a comparison of Kenyan and Bahamian lionfish populations. Biological Invasions 13: 2045-2051.

Fishelson, L. 1997. Experiment and observations on food consumption, growth, and starvation in Dendrochirus brachypterus and Pterois volitans (Pteroinae, Scorpaenidae).

Environmental Biology of Fishes 50: 391-403. Heino, M. and V. Kaitala. 1999. Evolution of resource allocation between growth and reproduction in animals with indeterminate growth. Journal of Evolutionary Biology 12: 423–429. Morris, J.A, Jr., C.V. Sullivan, and J.J. Govoni.

2011. Oogenesis and spawn formation in the invasive lionfish. Scientia Marina 75: 147-154.

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IMPACT OF PROTECTED AREAS ON SPINY LOBSTER (PANULIRUS ARGUS) DISTRIBUTION AND POPULATION STRUCTURE

ALLEGRA M. CONDIOTTE, NATHANAEL J. FRIDAY, AND CLAIRE B. PENDERGRAST


Abstract: Overfishing of economically important tropical reef species has led to widespread declines in fishery yield and reef ecosystem stability. Populations of Panulirus argus, a highly valued but frequently overexploited top predator in reef ecosystems, have experienced declines throughout the Caribbean. In response to declining P. argus abundance in the Cayman Islands, protective regulations such as seasonal closure of fisheries and the creation of marine protected areas have been implemented. These regulations aim to maintain stable populations of P. argus inside protected areas and enhance fishery yield by replenishing populations through recruitment from protected to fished areas. We examined the effect of protection status on P. argus population density, size, and habitat preference on Little Cayman Island. We found a greater abundance of P. argus in protected versus unprotected areas, but observed no relationship between body size and protection status. We observed no preference for grass, sand, or patch reef substrate in lobsters in protected areas, while lobsters in unprotected areas were exclusively located on patch reefs. Finally, we used our observed population densities in protected and unprotected areas to estimate the carrying capacity, current population size, and annual harvest of P. argus on Little Cayman Island. Our results suggest that marine park protection is an effective and essential conservation tactic for maintaining healthy P. argus populations, but that current populations are unlikely to support increased harvest. Continued management and enforcement of protective regulations are essential for the health and sustainable harvest of spiny lobster on Little Cayman Island. Keywords: marine protected area, fishery management, Panulirus argus, population structure INTRODUCTION Management and protection of marine environments are vital to the stability of ecological and human systems in coastal areas. Tropical reef environments are renowned for their biological diversity and complexity, but they also provide essential resources and ecosystem services for coastal communities. As coastal populations have grown rapidly in recent decades, pressure on tropical reefs from pollution, development, and overfishing has increased dramatically. Efforts to preserve the ecological health and stability of fishery resources through management of reef environments have had varied success (Roberts and Polunin 1993). Economically important species in tropical reef fisheries are often large roaming herbivores and carnivores that have a strong top-down influence on the trophic structure and community composition of reef ecosystems (Agardi 2000). Unfortunately, overexploitation and population decline of highly valued species occurs in fisheries worldwide, often resulting in severe losses in both ecological resilience and economic value of fisheries for coastal communities (Bellwood et al. 2004).

Declining yields from tropical reef fisheries has resulted in a variety of mitigation efforts, ranging from total prohibition of fishing activity, species-specific catch limits and size regulations, to seasonal closure (Lundquist and Granek 2005). Marine Protection Areas are of particular importance in areas where coastal marine tourism is a central economic driver and where coastal communities depend on local fisheries for food and income (Brown et al. 2001). Protected areas provide a refuge that allows species to rebound from overexploitation. When they are in close proximity to unprotected areas, they may also assist in replenishing unprotected populations by facilitating movement between these zones (Gell and Roberts 2003).

Residents of the Cayman Islands have historically relied on marine systems for food and income; up until the late 1900s, shipbuilding, fishing and turtling were primary sources of income on the island. In recent years a concerted effort has been underway to promote tourism development in the Cayman Islands, primarily through the expansion of ecotourism ventures such as scuba, snorkel, and fishing activities on the island’s relatively pristine coral reef ecosystems


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(World Travel Guide, 2014). Spiny lobster is a popular dish in the Caymans, particularly for tourists seeking “authentic” Caribbean cuisine (Marc Pothier, pers. comm). Thus it is likely that as tourism on the island increases, so will the demand for this highly valued dish, incentivizing an increase in lobster harvest beyond current levels (Conservation). In addition to its economic value, P. argus is a prominent predator species in reef ecosystems and plays an important role in the top-down control of the reef community (Shears and Babcock 2002). However, the spiny lobster’s slow growth rate (P. argus estimated age at maturity is two years and life span is 12 years) and ease of harvest has resulted in the overexploitation of populations across the Caribbean (Chavez 2001, Acosta and Robertson 2003).

Reports of overexploitation and reduced abundance on Little Cayman Island led to a 1978 Marine Conservation Law imposing P. argus catch limits, minimum size regulations, technology restrictions, and closed seasons for lobster harvest (Cayman Department of Environment, 2011). In 1985, two replenishment zones and two marine parks were created, prohibiting lobster harvest throughout the year in these areas (Brunt and Davies). As of 2014, 50.7% of Little Cayman shoreline (19.8 km) is protected as either Marine Parks, where all marine species are protected, or as replenishment zones, where only lobster harvest is prohibited (Bothwell pers. comm). In the remaining 49.3% of shoreline (19.3

km), lobsters must exceed 6 inches in tail length, and harvest may not exceed three lobsters per person or six per boat between 1 December and 28 February (Little Cayman History).

We examined the impact of harvesting activity on spiny lobster population density, body size, and habitat preference by surveying P. argus in protected and unprotected areas on Little Cayman Island. We hypothesized that P. argus would be more abundant in protected areas than in unprotected areas as a result of differences in fishing pressure. We also predicted a larger mean body size in the protected area, as larger lobsters are more susceptible to capture, have greater economic value, and exhibit increased foraging activity in exposed environments. Finally, because they are under greater fishing pressure, we expected lobsters in unprotected areas to occur more frequently in the reef where shelter is more abundant.

METHODS Data Collection We surveyed P. argus populations between 2030 and 2330 on 4-6 March 2014 in the Marine Park unprotected zone of Grape Tree Bay on the north coast of Little Cayman Island. Sites in the park and in the unprotected zone were several thousand meters apart. We surveyed in the marine park on 4 and 5 March, and in the open zone on 5 and 6 March, immediately following the March 1 termination of lobster season.

Figure 1: Representative sampling block, divided into near shore, mid-distance, and back-reef.

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We set up twelve sampling blocks: six continuous blocks in the park and six continuous blocks in the open zone. Each block extended 50 meters along the coast and 80 meters to the edge of the back reef (Figure 1). The sampling blocks crossed three types of substrate: turtle grass, sandy stretches, and reef. Each sampling block contained roughly equivalent amounts of turtle grass, sand, and patch reef substrate. We exhaustively scanned each block recording abundance of spiny lobsters, estimated carapace length (Figure 2), and type of substrate on which the lobster was located. We searched the perimeter and top of patch reefs but were unable to exhaustively survey interior crevices.

Cayman Islands DoE regulations prohibit the removal of P. argus with tail lengths of less than

six inches (152.4 mm) (Cayman Islands Department of Environment). We used Equation 1 to calculate the minimum carapace length for harvest (Matthews et al. 2003), roughly corresponding to the body size at which lobsters reach maturity (Smith and Herrkind 1992). Population size, carrying capacity, and annual harvest estimations We estimated the overall abundance of P. argus in the island’s near-shore environments from the densities observed in our study sites. Protected areas comprise 19.8 km of the island’s shoreline and extend roughly 80 meters from shore, resulting in a protected area of roughly 158.7 hectares. Similarly, 19.3 km of unprotected shoreline results in an unprotected area of 154.5 hectares. We multiplied each area by the density of lobsters observed in areas of equivalent protection status in Grape Tree Bay to provide rough estimates of population size in Little Cayman’s protected and unprotected areas (Equations 2, 3). Adding these two population estimates provides the total population of P. argus under current marine protection conditions (Equation 4). By multiplying the entire island’s circumference by 80 meters and by the protected area density, we calculated Little Cayman’s P. argus carrying capacity (Equation 5). Subtracting current total population from this carrying capacity provides an estimate of how many P. argus were harvested during this past lobster season on Little Cayman (Equation 6).

Figure 2: Carapace length on P. argus. Tail begins at the rear end of the carapace.

(Equation 1)

) (Equation 2)

(Equation 3)

(Equation 4)

(Equation 5)

(Equation 6)


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Statistical Analyses We performed an ANOVA comparing density between sampling blocks within an area to eliminate the possibility of a block effect in our results. To examine the difference in population density between protected and unprotected zones,

we performed a T-test. We performed an ANOVA and used Tukey’s HSD to compare mean body size by substrate. Finally, we performed an ANOVA to compare lobster abundance by substrate in the protected area. RESULTS Lobster density was 3.4 times greater in protected areas than unprotected areas (T1,30= -2.959, P= 0.006; Figure 4). Lobster size ranged from 4 to 24 cm (carapace length) and did not differ significantly between protected and unprotected areas (T1,11= -1.028, P= 0.33). All but five individuals exceeded the minimum capture size limit (81.3 mm carapace), indicating that our

Figure 3. Protection status of Little Cayman Island coastal areas. 50.7% of Little Cayman’s 39 km coastline is comprised of Marine Park or Replenishment Zone, in which P. argus harvest is prohibited. Protected survey site was located in the Grape Tree Bay Marine Park, and unprotected site was roughly 1 km east.

Figure 4: Density of lobsters per hectare in the protected area (mean ± SE = 11.25 ± 2.2) and density in the unprotected area (mean ± SE = 3.33 ± 1.5).

Table 1: The mean carapace lengths and calculated tail lengths for lobsters in the protected area, in the unprotected area, and combined.

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observations reflected predominantly mature lobsters (Table 1).

Within the protected area, lobster density did not differ significantly by substrate (F2,17 = 2.36, P = 0.13; Figure 5). However, in the unprotected area, every P. argus we observed was on patch reef substrate. We found a significant effect of substrate on carapace length, with smaller lobsters more frequently on the reef (F2,34= 10.57, P= 0.0003; Figure 6).

We observed a density of 11.25 lobsters per hectare in the protected area. If Grape Tree Bay is representative of other protected areas on Little Cayman, then we estimate that roughly 1750 lobsters are present in protected areas across the island (Equation 2). The density of lobsters in unprotected areas (3.33 lobsters per hectare) leads to an estimated 500 lobsters located in unprotected areas (Equation 3). The current spiny lobster population is estimated to be 2300 (Equation 4), while the carrying capacity for this population is 3524 lobsters (Equation 5). By these estimates, 1223 lobsters, or 35% of carrying capacity, are removed from unprotected areas during lobster season each year (Equation 6). DISCUSSION

P. argus is a valuable fishery species in the Caribbean, where it serves as a major source of income (Gongora 2010). P. argus were more abundant in the marine park than in the unprotected zone (Fig. 4), indicating that the marine park effectively protects lobster populations from harvest. P. argus are known to move up to 200 meters in a night, but rarely leave their home reefs (Bertelsen and Hornbeck 2009); considering the distance between the study block within the park and the unprotected area (approximately one kilometer), it is unlikely that migration occurred between the park and the unprotected zone in the four days between the end of lobster season and the beginning of our study. There was no difference in average P. argus size between the marine park and the unprotected zone (Table 1). The similarity in size of P. argus at the two sites suggests that adult lobsters have not been entirely removed from the unprotected zone. All but five of the lobsters measured in this study were above the minimum catch size for lobster season, with a carapace longer than 813 mm. Spiny lobsters reach maturity between a carapace length of 76 and 82 mm (Goni et al. 2003), suggesting that the majority of lobsters we measured were adults. Juvenile P. argus spend their early lives hiding in macroalgae before

Figure 5: Density of lobsters per hectare by substrate in the turtle grass (mean ± SE = 13.75 ± 4.1), density in the sand (mean ± SE = 5.0 ± 2.5), and density in the reef (mean ± SE = 15.0 ± 3.9).

Figure 6: Carapace length of lobsters by substrate in the turtle grass (mean ± SE = 166.36 ± 10.8 mm), in the sand (mean ± SE = 182.0 ± 12.4 mm), and in the reef (mean ± SE = 109.47 ± 10.2 mm).


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moving to deep reef dens, making them difficult to locate. Therefore juveniles may not have been present at the sites we surveyed, where there was little macroalgae. Additionally, previous studies have found no difference in the density of juvenile spiny lobsters in protected and unprotected areas (Shears and Babcock 2002). As protective cavities in patch reefs appeared roughly equivalent in both sites and minimum size regulations prohibit the harvest of juveniles, we expect juvenile P. argus populations to vary little between the two sites. However, further investigation of distribution and recruitment of juveniles across protective areas is essential to determining the stability of Little Cayman’s P. argus populations.

All of the P. argus in the unprotected area were found on reef substrate, while lobsters in the protected area were equally distributed across grass, sand, and reef substrate (Fig. 5). Lobsters on the reef are less visible than those in the open, and may be sheltered from human harvesting during lobster season. It is possible that the clustering of unprotected P. argus on the reef is due to pressure from fishing, as those lobsters who spend more time hiding in the reef are less likely to be caught and removed (Wynne and Cote 2007).

While residents of Little Cayman Island no longer depend on harvest from local waters for their livelihoods, spiny lobster is a popular dish among locals and visitors. Sale of P. argus to the Hungry Iguana Restaurant serves roughly 300 spiny lobster meals annually, generating over $9,000 CI for local restaurants and over $3,000 CI for local fishermen (Marc Pothier, pers. comm). However, should Little Cayman’s population and tourism industry continue to grow, demand for

spiny lobster may increase, incentivizing increases in annual harvest that current populations are likely unable to tolerate.

As a predator species, P. argus is also fundamental to the shallow reef communities in which it lives, contributing to top-down regulation by controlling herbivore populations and influencing the trophic structure of the reef system (Shears and Babcock 2002). Overfishing of lobsters has led to sharp declines in coastal populations, resulting in the disruption of trophic cascades and community structure (Acosta and Robertson 2003). Our results indicate that the protected status of P. argus in marine parks is an effective and vital conservation tactic for maintaining viable lobster populations in the Caribbean. However, P. argus populations may not be able to endure increases in harvest beyond current levels. Continued monitoring and enforcement of protective regulations is essential to the ongoing stability of both lobster populations and their environments on Little Cayman Island. ACKNOWLEDGMENTS We thank Celia Y. Chen, Jessica Trout-Haney, and Vivek Venkataraman for their guidance and assistance in the writing process. We also thank Amelia Ritger and Aaron Mondshine for their assistance in data collection. AUTHOR CONTRIBUTIONS All authors contributed equally in the data collection and writing process. Claire B. Pendergrast developed and designed the project, and performed all economic modeling.

LITERATURE CITED Acosta, C. and Robertson, D. 2003. Comparative

spatial ecology of fished spiny lobsters Panulirus argus and an unfished congener P. guttatus in an isolated marine reserve at Glover’s Reef atoll, Belize. Coral Reefs 22:1–9.

Agardi, T. 2000. Effects of fisheries on marine ecosystems: a conservationist’s perspective. ICES Journal of Marine Science 57: 761-765.

Bellwood, D., T. Hughes, C. Folke, and M. Nyström. 2004. Confronting the coral reef crisis. Nature. 429: 827-833.

Bertelsen, R.D. and Hornbeck, J. 2009. Using

acoustic tagging to determine adult spiny lobster (Panulirus argus) movement patterns in the Western Sambo Ecological Reserve (Florida, United States). New Zealand Journal of Marine and Freshwater Research 43: 35-46.

Brown, K. 2001. Trade-off analysis for marine protected area management. Ecological Economics 37.3: 417-434.

Brunt, M. and J. Davies, ed. (1994). The Cayman Islands: Natural History and Biogeography, Volume 71. Norwell MA: Kluwer Academic Publishers.

Cayman Islands Department of Environment.

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2000. Investigations into the recruitment, abundance, and distribution of the Caribbean spiny lobster, Panulirus argus, into the North Sound, Grand Cayman, BWI.

Cayman Island Department of the Environment. 2011. Rules for Cayman Islands Marine Parks. Published online at http://www.doe.ky/wp-content/uploads/2011/07/Marine-Parks-July-2011.pdf, accessed 05 March 2012.

"Conservation." Cayman Islands Legislation. Web. 08 Mar. 2014. <http://reefresearch.org/ conservation/cayman-islands-conversation-legislation/>.

Gell, F. R., and C. M. Roberts. "Benefits beyond boundaries: the fishery effects of marine reserves." Trends in Ecology & Evolution 18.9 (2003): 448-455.

Gongora, M. 2010. Assessment of the spiny lobster (Panulirus argus) of Belize based on fishery-dependent data. Marine Fisheries Institute 4: 5-38.

Lundquist, C. and Granek, E. 2005. Strategies for Successful Marine Conservation: Integrating Socioeconomic, Political, and Scientific Factors. Conservation Biology 19:1771–1778.

"Little Cayman History." LittleCayman.com. Little Cayman Resort. 8 Mar. 2014. <http://www. littlecayman.com/our-island/island-history/>.

Marx, J.M. and Herrnkind, W.F. 1985. Macroalgae (Rhodophyta: Laurencia spp.) as habitat for young juvenile spiny lobsters, Panulirus argus. Bulletin of Marine Science 36: 423-431.

Matthews, T.R., Hunt, J.H., and Heatwole, D.W. 2003. Morphometrics and management of the Caribbean spiny lobster, Panulirus argus. Proceedings of the Gulf and Caribbean Fisheries Institute 54:156-174.

Shears, N.T. and Babcock, R.C. 2002. Marine reserves demonstrate top-down control of community structure on temperate reefs. Oecologia 132:131-142.

Roberts, C. M., and N. V. C. Polunin. 1993. Marine reserves: Simple solutions to managing complex fisheries? Ambio 22: 363-368.

Smith, K.N., and Herrkind, W.F. 1992. Predation on early juvenile spiny lobsters Panulirus argus (Latreille): influence of size and shelter. Journal of Experimental Marine Biology and Ecology 157: 3-18.

Wynne, S. and Cote, L. 2007. Effects of habitat quality and fishing on Caribbean spotted spiny lobster populations. Journal of Applied Ecology. 44: 488-494.


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MY ANEMONE, MY RULES: NICHE PARTITIONING IN ANEMONE COMMUNITIES

LARS-OLAF HÖGER, ELLEN R. PLANE, AND ADAM N. SCHNEIDER


Abstract: Coral reefs are known for their immense species diversity. However, many coral reef species are functionally redundant, suggesting that high diversity is maintained through mechanisms that reduce competition. We studied functionally similar shrimp and crabs inhabiting Bartholomea annulata and Stichodactyla helianthus anemones at Little Cayman Island to determine how these crustaceans are able to coexist. We found that anemone size does not have an effect on the abundance of inhabitants, suggesting that space is not a limiting factor for anemone inhabitants. We also found that species in the same functional feeding group never inhabit the same anemone, indicating that competition between inhabitants of the same functional feeding group limits the distributions of species. We conclude that spatial niche partitioning plays a key role in maintaining the diversity of coral reefs. Key words: anemone, functional feeding groups, niche partitioning, shrimp INTRODUCTION Coral reefs are widely recognized for their immense species diversity. Within a reef ecosystem, many animal species fill the same or similar functional niches (Bellwood et al. 2003), suggesting that mechanisms are operating to reduce competition. Different species of anemone shrimp, which inhabit a variety of anemone species and live amongst the tentacles for protection, often coexist and appear to fill similar feeding and protective niches (Smith 1977, Wirtz 1997). Anemones host a number of cleaner shrimp species, as well as scavengers and predators. For example, both Periclimenes yucatanicus and Ancylomenes pedersoni are cleaner shrimp that commonly inhabit Bartholomea annulata anemones (Silbiger and Childress 2008). Competition between inhabitant species for protective space, food scraps, or cleaner clients may limit the number of inhabitant species that can successfully reside on one anemone.

We conducted an observational study of inhabitant species of the anemones Bartholomea annulata (corkscrew anemone) and Stichodactyla helianthus (sun anemone) to examine the factors affecting species distribution and coexistence within a reef ecosystem. We expected that larger anemones would host more shrimp because they have more usable surface area and thus a higher carrying capacity. These anemones should host more cleaner shrimp in particular, as larger anemones are more visible signals to fish that require cleaning services (Chadwick and Huebner,

2012). We also hypothesized that anemone inhabitants that fill the same functional feeding niche would exhibit spatial niche partitioning to avoid direct competition. We predicted that shrimp of different functional feeding groups (i.e. scavenger and cleaner) would be found coexisting on the same anemone, but multiple shrimp species of the same functional group (i.e. cleaner and cleaner) would not. It is also possible that inhabitant species may specialize to specific species of anemone, which would fulfill the same niche partitioning function on a species-wide rather than individual anemone level. Both of these methods of niche partitioning should reduce interspecific competition for shelter and food. METHODS We haphazardly sampled 15 corkscrew anemones and 28 sun anemones in Grape Tree Bay and South Hole Sound, Little Cayman, on 5 and 6 March 2014. We counted anemones that were in the process of splitting, or had joined discs, as a single anemone. We measured the length across the longest axis for each anemone disc and noted the substrate that it was growing on (rock, sand, seagrass, coral, or conch). We then counted the number of shrimp of each species on the anemone, and recorded the number of crabs (unidentified species) present. Since many anemones were surrounded by hermit crabs, we also recorded whether or not hermit crabs were present.

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Data analyses We performed linear regressions between anemone disc length and number of anemone inhabitants and number of cleaner shrimp for each species of anemone. We also conducted a Principal Components Analysis (PCA) on the abundances of each species to obtain a measure of community composition. Finally, we conducted analysis of variance (ANOVA) to examine the effect of substrate on the abundance of inhabitants. RESULTS We observed four shrimp species (Alpheus sp., Periclimenes rathbunae, Thor amboinensis, and Palaemonetes sp.), in the two species of anemone we studied, as well as several unidentified crab species. Alpheus sp. is a predatory species, P. rathbunae a cleaner species, and T. amboinensis and Palaemonetes sp. are scavenger species (Humann and DeLoach 1992). Anemone size did not have an effect on number of inhabitants (Fig. 1), or number of P. rathbunae, the only cleaner shrimp species we observed. We tested for a size effect only in sun anemones because corkscrew anemones hosted a maximum of two inhabitants. Substrate type did not have an effect on the abundance of any inhabitant species.

We observed that 73% of corkscrew anemones and 96% of sun anemones had at least one shrimp or crab inhabitant. Alpheus sp. was only present in corkscrew anemones, and P. rathbunae and T. amboinensis were only present in sun anemones. Palaemonetes sp. and crabs were most frequently in sun anemones, but also occasionally in corkscrew anemones. We found each species alone on at least one anemone. Alpheus sp. was the sole inhabitant of an anemone more frequently than any other species (Fig. 2). Crabs and P. rathbunae coexisted on the same anemone more frequently than any other two species (Fig. 2) and we never saw T. amboinensis and Palaemonetes sp. on the same anemone. The presence of hermit crabs did not have an effect on abundance of any other inhabitant species. The first two principal components (PC1 and PC2) accounted for 36% and 22 % of the

variance, respectively. According to the first component of a PCA on community composition, Alpheus sp. was unlikely to be associated with any other inhabitant species. The second component showed that P. rathbunae, Palaemonetes sp., and crabs were likely to be associated with one another, but not with T. amboinensis (Table 1, Fig. 3). Table 1. Loading matrix for first two principal components of community structure.

Figure 1. Anemone length did not have an effect on the number of inhabitants. DISCUSSION Our results indicate that spatial niche partitioning between functionally similar species is playing a role in maintaining diversity on coral reefs. The two species of scavenger shrimp were never found on the same individual anemone, suggesting that they may be actively separating based on competition within their functional feeding group. This example of spatial niche partitioning may help to explain how direct competition can be avoided within the reef by choosing different residence locations to avoid overlap. The frequent

PC1 PC2P. rathbunae 0.75 -0.07Alpheus sp. -0.73 -0.21T. amboinensis 0.16 0.89Palaemonetes sp. 0.33 -0.47Crab 0.77 -0.12

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association of crabs with cleaner shrimp supports this theory as well; many crabs are scavengers and opportunistic hunters (Johnston and Freeman 2005), so their coexistence with non-scavenging shrimp suggests further niche partitioning. Spatial niche partitioning may also occur within the anemone as all crabs were found sheltering

underneath the disc rather than in the tentacles with the shrimp. Although niche partitioning may be occurring in cleaner and predatory functional groups, we were unable to observe it because our study only included one species in each of these groups.

Figure 2. Alpheus sp. was frequently found alone, while other species were often found coexisting with other species on the same anemone. Black bars are single-species assemblages, white bars two-species assemblages, and hatched bars three-species assemblages. Only anemones with inhabitants were included. Our results demonstrate that species in the same functional feeding group are generally found on separate individual anemones, but not on different species of anemones. The two scavenger species both showed a preference for sun anemones, athough one Palaemonetes sp. shrimp was found on a corkscrew anemone, indicating that it is also a viable habitat for these shrimp. Alpheus sp. showed a strong preference for

corkscrew anemones and no Alpheus sp. shrimp were found on sun anemones. Corkscrew anemones have longer tentacles (Humann and DeLoach 1992), which may be more adequate for sheltering the large Alpheus sp. than the smaller sun anemone tentacles. We never saw Alpheus sp. coexisting with other shrimp species. This is most likely due to their predatory nature; Alpheus sp. hunt smaller shrimp and crabs and would not be

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expected to share anemone disc space with potential prey.

Since almost all of the anemones sampled contained at least one shrimp or crab, it is likely that these anemones are a limiting resource. However, space within the anemone is not limited; in sun anemones, inhabitant abundance was not affected by anemone size, which suggests that competition for anemone protection among inhabitants is not density-dependent. Anemone size also had no effect on cleaner shrimp abundance, indicating that larger anemones are not a more desirable resource for cleaner shrimp. It is possible that signaling client fish from a distance is irrelevant to the shrimp on these particular anemones because they were found at shallow depths, or that the shrimp are relying on other features of the anemone, such as color or shape, to signal client fish. Substrate did not have an effect on anemone community composition, suggesting that the anemone itself, rather than the environment it lives in, has the greatest effect on the species that inhabit it. Niche partitioning in the scavenger functional group could be an indication of what is occurring in the rest of the reef ecosystem, as other animals may also be separating spatially based on feeding strategy. This partitioning does not seem to be affected by anemone size or abundance of anemone inhabitants, but rather by an avoidance of overlap between identical functional groups. This mechanism of species coexistence, employed as a strategy to avoid competition, could help explain

how such a high level of diversity is maintained within coral reefs.

Figure 3. The first principal component of a PCA showed that Alpheus sp. was unlikely to cohabitate with any other anemone inhabitant species, and the second principal component showed that T. amboinensis was unlikely to cohabitate with P. rathbunae, crabs, or Palaemonetes sp. ACKNOWLEDGEMENTS We would like to thank Celia Chen, Jessica Trout-Haney, and Vivek Ventakaraman for their guidance in assembling this paper. AUTHOR CONTRIBUTIONS All authors contributed equally.

LITERATURE CITED Huebner, L.K., and N.E. Chadwick. 2012. Reef

fishes use sea anemones as visual cues for cleaning interactions with shrimp. Journal Experimental Marine Biology and Ecology 416−417: 237−242.

Humann, P., and N. DeLoach. 1992. Reef Creature Identification: Florida, Caribbean, Bahamas. New World Publications, Inc., Jacksonville, Florida. Page 93.

Johnson, D., Freeman, J. 2005. Dietary preference and digestive enzyme activities as indicators of trophic resource utilization by six species of crab. The Biological Bulletin. 208:36-46

Silbiger, N.J., and M.J. Childress. 2008. Interspecific Variation in Anemone Shrimp Distribution and Host Selection in the Florida Keys (USA): Implications for Marine Conservation. Bulletin of Marine Science 83: 329-345. Rican landscapes.

Smith, W. L. 1977. Beneficial behavior of a symbiotic shrimp to its host anemone. Bulletin of Marine Science 27: 343–346.

Dartmouth Studies in Tropical Ecology 2012, in press.

Wirtz, P. 1997. Crustacean symbionts of the sea anemone Telmatactis cricoides at Madeira and the Canary Islands. Journal of Zoology 242: 799–811.

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