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Plant-Insect Interactions in the Tropics
ZOL/ENT/PLB 485September 24, 2013
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Examples of Plant-Animal InteractionsPollination HerbivorySeed DispersalSeed PredationPathogensMicrobial Fungal Insect Mimicry
And on, and on…
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Types of Biotic Interactions
Mutualism
CommensalismPredation/Parasitism
Competition
Player 1 Player 2
Mutualism – both spp. benefit (but think of it as mutual exploitation)Commensalism – 1 spp. benefits, and other gets no benefit/harmPredation/Parasitism – 1 spp. benefits, and other is harmed/killedCompetition – both spp. (or individuals) negatively impact the other
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Types of Biotic Interactions
Player 1 Player 2
Mutualism
CommensalismPredation/Parasitism
Competition
+ +
Mutualism – both spp. benefit (but think of it as mutual exploitation)Commensalism – 1 spp. benefits, and other gets no benefit/harmPredation/Parasitism – 1 spp. benefits, and other is harmed/killedCompetition – both spp. (or individuals) negatively impact the other
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Types of Biotic Interactions
Mutualism
CommensalismPredation/Parasitism
Competition
+ +++
Player 1 Player 2
Mutualism – both spp. benefit (but think of it as mutual exploitation)Commensalism – 1 spp. benefits, and other gets no benefit/harmPredation/Parasitism – 1 spp. benefits, and other is harmed/killedCompetition – both spp. (or individuals) negatively impact the other
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Types of Biotic Interactions
Mutualism
CommensalismPredation/Parasitism
Competition
+ ++++
Player 1 Player 2
Mutualism – both spp. benefit (but think of it as mutual exploitation)Commensalism – 1 spp. benefits, and other gets no benefit/harmPredation/Parasitism – 1 spp. benefits, and other is harmed/killedCompetition – both spp. (or individuals) negatively impact the other
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Types of Biotic Interactions
Mutualism
CommensalismPredation/Parasitism
Competition
+ ++++
Player 1 Player 2
A B
Resource 1Re
sour
ce 2 B
A
Mutualism – both spp. benefit (but think of it as mutual exploitation)Commensalism – 1 spp. benefits, and other gets no benefit/harmPredation/Parasitism – 1 spp. benefits, and other is harmed/killedCompetition – both spp. (or individuals) negatively impact the other
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Why should we care?
Important in agriculture and maintaining biodiversity
Mechanisms of co-existence
Origins of diversity
They’re super cool!
Impo
rtan
t fo
r the
LD
G
“Only in the tropics…”
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Plant-Insect Interactions and Mechanisms of Co-existence
Species “niche”: the sum of all the environmental factors acting on an organism (Hutchinson 1944)
- An “n-dimensional hypervolume” (Hutchinson 1957)
- We can consider environmental axes that act as limiting factors as “niche axes”
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Plant-Insect Interactions and Mechanisms of Co-existence
http://proceedings.esri.com/library/userconf/proc99/proceed/papers/pap308/p30805.gif
Soil MoistureDry Wet
Sunl
ight
Soil Phosp
horous
High
Low
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Plant-Insect Interactions and Mechanisms of Co-existence
http://proceedings.esri.com/library/userconf/proc99/proceed/papers/pap308/p30805.gif
Herbivore PressureLow High
Sunl
ight
Soil Phosp
horous
High
Low
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Plant-Insect Interactions and Mechanisms of Co-existence
Biotic interactions can act as additional niche axes
Niche partitioning enables species co-existence among species
Figure 2 from Mayfield and Levine (2010) – Ecol Letters
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Plant-Insect Interactions and Mechanisms of Co-existence
Negative density dependence- Species population growth rates are limited by effects
associated with high density(frequency) of individuals
Mayfield and Levine (2010)
Competition/Crowding Predators & Pathogens
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Janzen-Connell Hypothesis: tree species richness is kept high due to the increased probability of mortality of seeds and seedlings growing nearer to their parent tree
Plant-Insect Interactions and Mechanisms of Co-existence
- Negative density dependence scenario
- Often, predators and pathogens are specialized
- Janzen 1970 and Connell 1971
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Janzen-Connell Hypothesis
Probability of Survival
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Janzen-Connell Hypothesis
Figure 1 from Janzen (1970) – AmNat(w/ my colorful adaptations!)
Lots of seed/seedling mortality
Less seed/seedling mortality
Probability of seed dispersal decreases with increasing distance from parent
Seedling Sweet Spot
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Plant-Insect Interactions and Origins of DiversitySelective pressures that are the result of biotic interactions drive evolution, and ultimately speciation
BPopulation
APopulation
ASpecies
ASpecies Species
B
(Selective Agent)
(Selective Target)
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Plant-Insect Interactions and Origins of DiversityWe can use a phylogenetic approach to view past evolutionary events
Circle flower shape
Ancestral state = Square flower shape
A
A
B
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Plant-Insect Interactions and Co-evolutionIf there are reciprocal selective pressures exerted by both interactors in the relationship, you can get co-evolution
Selective Target
Selective Agent
Selective Agent
Selective Target
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Plant-Insect Interactions and Co-evolutionAgain, let’s take a look at this past evolution using a phylogenetic approach
Ancestral state
Ancestral state
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Plant-Insect Interactions and Co-evolution
We can see how co-evolution can drive species diversification (ie: lineage splitting), but note that it can also drive continued evolution within a lineage without leaving many descendants
- Note, these two scenarios are really not mechanistically different, but we may observe different patterns of species diversity today
“Evolutionary Arms Race”
Red Queen Hypothesis]
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“The tubes of the corollas of the common red and incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty glance appear to differ in length; yet the hive-bee can easily suck the nectar out of the incarnate clover, but not out of the common red clover, which is visited by humble-bees alone” (Darwin, On The Origin of Species).
Just so you know…Darwin has almost always said it first…
Top: https://news.brown.edu/files/article_images/Darwin1.jpgBottom: https://upload.wikimedia.org/wikipedia/commons/4/41/Humle.jpg
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CAUTION! When is it co-evolution?Janzen, Daniel H. 1980. When is it coevolution? Evolution 34: 611-612.
1. Just because a pair of species have traits that are mutualistically congruent, doesn’t mean they have co-evolved
2. Parasites/predators could have evolved along with the plant they parasitize, or elsewhere, and then dispersed to their new host plant that is not “evolutionary informed” of this newly arrived predator’s tactics
3. “…it is likely that many defense traits of plants were produced through co-evolution with animals no longer present…” (Janzen 1980)
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Just a few (very few) examples…
1. Inga diversification in response to herbivores2. Bursera3. Complex relationships of figs and their fig wasps4. Ant-Acacia relationships: The Ant Defenders!!!5. Lepidoptera evolution
With these examples, keep in mind:
a) How did these interactions arise?b) What do these interactions mean with regard to
species diversity and co-existence?c) Is there enough evidence to support conclusions?
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Plant – Herbivore Interactions
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Plant DefensesPhysical Defenses• Thorns/prickles• Trichomes• Toothed leaves• Tough leaves• Exudate/latex
Compositional Defenses• Chemistry
o Alkaloids, tannins, phenolics, cyanogenic glycosides, etc…
• Fiber content/nutritional contentBehavioral Defenses
• Ant defense• Timing of
leafing/masting
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Inga (Fabaceae)(ie: the “pea family”)
Over 300 species
Neotropical in range
Recent and rapid diversification (Richardson et al. 2001)
- Lineage only 10 million years old
- Many species arising only 2 mya
Variety of herbivore defense strategies
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Inga EvolutionRichardson et al. 2001. Rapid diversification of a species-rich genus of Neotropical rain forest trees. Science 293: 2242-2245.
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Inga – A pairwise study in defense strategiesColey et al. 2005. Divergent defensive strategies of young leaves in two species of Inga. Ecology 86: 2633 – 2643.
Question: Is there a difference in defense strategies between two closely related species of Inga?
Data Collected:
Herbivore-host associationsAnts at EFNsLeaf size and growth rateLeaf secondary metabolites
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Inga – A pairwise study in defense strategiesMain Results:
• The two species compared had similar levels of herbivory
• There was a difference in defense strategy: Escape vs. Defense
Escape (I. umbellifera)
• Lower levels of defense compounds• Lower investment in recruitment of ants• Synchronous leafing• Faster leaf expansion• Lower chlorophyll content
Defense (I. goldmanii)
• Opposite patterns of I. umbellifera
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Inga – Genus wide chemical defenses
Kursar et al. 2009. The evolution of antiherbivore defenses and their contribution to species coexistence in the tropical tree genus Inga. PNAS 106: 18073 – 18078.
Study Objectives: evaluate the evolution of antiherbivore defenses and their possible contribution to Inga coexistence
Approach: • 37 spp. in Panama & Peru • Characterized defense
mechanisms• Evaluated evolution of these
mechanisms in a phylo context
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Figure 2
Variation in antiherbivore defense
• In all, 13 distinct “chemotypes”
• Variation in leaf expansion and chlorophyll content of new leaves (Fig 2)
• Much variation in ant abundance and EFN visitation (20-fold difference!)
Inga – Genus wide chemical defensesMain Results
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IngaInga – Genus wide chemical defensesMain Results
Figure 3
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Inga – Genus wide chemical defensesMain Results
Evaluation of Coexistence:
• NOTE: Negative values mean members in the community are similar, positive values mean they are dissimilar
• At both sites, the species were more different in defensive traits than expected by chance
Figure 4
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Inga – Genus wide chemical defensesMain ConclusionsInga species display much variation in all three “trait syndromes” (ie: developmental, chemical, and ant defense strategies)
There is evidence of much trait convergence for chemical and ant defenses, but not for developmental defenses
All three defenses are orthogonal, meaning they potentially represent 3 independent niche axes important for evolution
Species co-occurring at a site are more dissimilar in defense traits than expected, suggesting niche partitioning
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Plant – Pollinator Interactions
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Figs and Fig Wasps (and their “friends”…)
Photo by Diana Durance
Figs (Ficus – Moraceae) and their fig wasps are global in distribution
There are over 750 species worldwide!
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http://www.youtube.com/watch?v=JfkiYfrStrU
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“…were a human to inhabit such a place it would be an utterly dark and crowded room filled with jostling people, some of whom would be homicidal maniacs wielding sharp knives” (Kricher, paraphrasing Hamilton, 1979)
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Figs and non-pollinating wasps
Study Objectives: To evaluate the role that Idarnes, a non-pollinating fig wasp, has on the overall fitness of its host figs.
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Figs and non-pollinating wasps
Main Conclusions: Fig fitness (as measured by fruit crop production) was much lower for figs with Idarnes
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Plant – Ant Defense Interactions
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Ant-Acacia Interactions
http://www.youtube.com/watch?v=Xm2qdxVVRm4
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Ant-Acacia Interactions
Palmer et al. (2008) - Science
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Ant-Acacia InteractionsStudy Objectives: To evaluate how the removal of large herbivores in an African savanna impacted the dynamics of an ant-Acacia mutualism
Crematogaster mimosae : very aggressive; needs domatia
C. sjostedti: less aggressive; does not use domatia, but plant stems for housing
Under natural conditions, C. mimosae is the most abundant ant symbiont, occupying ~52% of all trees at our sites, whereas C. sjostedti occupies ~16% of host plants. C. nigriceps occupies ~15% and T. penzigi occupies ~17%.
Crematogaster nigriceps: a defender; prunes axillary buds and kills apical meristems, which reduces likelihood of contact with treesoccupied by hostile colonies
Tetraponera penzigi, an intermediate protector; destroys itshost-plants’ nectaries: a “scorched-earth” strategy to reduce competition
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Ant-Acacia Interactions
Figure 1
Grey bars represent presence of herbivores, white represent absence
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Figure 2 Figure 3
Figure 4
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Ant-Acacia Interactions
Main Conclusions:
Removal of large herbivores in this community can greatly affect the mutualism between ants and their plants, and results in decreased fitness of the Acacia trees.
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Plant – Insect Interactions (herbivory, pollination,
ant defense, oh my!)
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Lepidopterans – Heliconius & Passiflora“Lepidopterans are (to plant species) evolutionary examples of Dr. Jekyll and Mr. Hyde” (Kricher, pg. 308)
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Heliconius & PassifloraA Suite of Biological Interactions:
Heliconia butterflies pollinate PassifloraHeliconia caterpillars are Passiflora herbivores, and can greatly reduce fitness due to folivary
Passiflora has many defenses to reduce impact of herbivory by Heliconia• Chemical compounds in leaves• Production of extrafloral nectaries• Egg mimics on leaves
But…not only are the caterpillars undeterred by the chemical compounds, it is thought that these compounds are sequestered and used as a defense in adult butterflies
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Plant – Insect Interactions on a Global Scale
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Swallowtail Biodiversity
Study Objectives: Use a phylogenetic approach to investigate the evolutionary process responsible for the LDG in swallowtail butterflies (Papilionidae)
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Distributions across the globe
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Correlated Evolution
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Why should we care?
Important in agriculture and maintaining biodiversity
Mechanisms of co-existence
Origins of diversity
They’re super cool!
Impo
rtan
t fo
r the
LD
G
“Only in the tropics…”
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Biotic Interactions and the LDG
Mittelbach et al. 2007. Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography. Ecol Letters 10: 315-331.
Tropics have more “niche space” to occupy than do the temperate zones
Tropics have higher diversification rates
There has been a longer time for diversification to occur
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Biotic Interactions and the LDGStudy Objective: Review the literature and determine if studies showed importance of interactions (a) greater at lower lats, (b) greater at higher lats, (c) no evidence of a difference
Main Results: From 39 studies, found only one instance where the biotic interaction was deemed “more important” in temperate regions
But, obviously this is a limited dataset, and only a review of the literature. Much more work needs to be done!