Can stone crabs provide Can stone crabs provide biological control against biological control against southern oyster drills and southern oyster drills and increase eastern oyster increase eastern oyster

survivorship?survivorship?

F. Joel Fodrie, Matthew D. Kenworthy and F. Joel Fodrie, Matthew D. Kenworthy and Sean P. PowersSean P. Powers

Department of Marine Sciences, University of Department of Marine Sciences, University of South Alabama & Dauphin Island Sea LabSouth Alabama & Dauphin Island Sea Lab

ICSR 2008ICSR 2008

AcknowledgementAcknowledgementss

                             

D Irwin, C Hightower, S Williams, E Miller, C Gustafson, J Herrmann, C Steeves, S Toro, M Johnson and D Booth

Auburn Shellfish Laboratory

National Marine Fisheries Service and National Science Foundation

K Heck, R Aronson, N Geraldi and 2 anonymous reviewers

Fodrie, FJ, MD Kenworthy and SP Powers (in press) Unintended facilitation between marine consumers generates enhanced mortality for their shared prey. Ecology.

Drill predation: an obstacle to Drill predation: an obstacle to oyster restoration and oyster restoration and

population fitnesspopulation fitness

Adapted from Adapted from Gregalis Gregalis (2007)(2007)

0

10

20

30

40

0 5 10 15 20

Drill Density (Drill m -2)

Oys

ter

Mor

talit

y (%

) Adults (> 3 cm)

Spat (< 3 cm)

90% mortality 90% mortality

reported (Butler reported (Butler 1985)1985)

Relevant food-web interactionsRelevant food-web interactions

Trophic cascade framework: Trophic cascade framework: crabs should “quite” drills and crabs should “quite” drills and release oysters from top-down release oysters from top-down controlcontrol

Relevant food-web interactionsRelevant food-web interactions

However crabs also However crabs also consume oystersconsume oysters

What’s better for oysters:What’s better for oysters:

- drills foraging with - drills foraging with impunity…or… impunity…or…

- drills and omnivorous - drills and omnivorous crabs foraging together?crabs foraging together?

?

Testable questions Testable questions

AAre there numerical or behaviorally mediated re there numerical or behaviorally mediated interactions between drills and crabs that interactions between drills and crabs that affect oyster mortality? [affect oyster mortality? [EXPS 1 & 3EXPS 1 & 3]]

Are the Are the effectseffects of these of these multiple predatorsmultiple predators on on oyster mortality risk enhancing, risk reducing, oyster mortality risk enhancing, risk reducing, or independent? [or independent? [EXP 2EXP 2]]

Multiple Predator Effects Multiple Predator Effects

Consider two predators foraging alone:Consider two predators foraging alone:

- Pred A eats - Pred A eats aa % - Pred B eats % - Pred B eats bb % %

Act Act independentlyindependently when together: when together: == a a + + b %b %

Generate Generate enhanced riskenhanced risk when together: when together: >> aa + + b %b %

Generate Generate reduced riskreduced risk when together: when together: << aa + + b %b %

Sih et al. (1998) TREESih et al. (1998) TREE

Experiment 1: effect of crabs on Experiment 1: effect of crabs on drill mortality and behaviordrill mortality and behavior

2.5 m (height = 0.5 m)

0.5 m

Single factor: presence or absence of crab in Single factor: presence or absence of crab in high-density patchhigh-density patch

Response variable: % of live drills in high-Response variable: % of live drills in high-density patchdensity patch

High Density Low Density

Expt 1 results: effect of crabs on Expt 1 results: effect of crabs on drill mortality or behaviordrill mortality or behavior

Crabs influence drill habitat selection (p = Crabs influence drill habitat selection (p = 0.006)0.006)

- crab absent: 80% of drills in the HD patch- crab absent: 80% of drills in the HD patch

- crab present: <5% of drills in the HD patch- crab present: <5% of drills in the HD patch

Implication: if drills will avoid a rich food supply Implication: if drills will avoid a rich food supply to avoid crabs, then drills and crabs foraging to avoid crabs, then drills and crabs foraging together should result in together should result in reduced riskreduced risk for for oystersoysters

Experiment 2: oyster mortality in Experiment 2: oyster mortality in the presence of drills, crabs or the presence of drills, crabs or

bothboth

2 x 2 orthogonal design2 x 2 orthogonal design Runs with loose or clumped Runs with loose or clumped

oystersoysters Response variable: oyster Response variable: oyster

mortalitymortality MPEs revealed by MPEs revealed by

significant interaction termsignificant interaction term

Present

Absent

Present

1,6 1,0

Absent

0,6 0,0

drill

cra

b

Expt 2 results: oyster mortality Expt 2 results: oyster mortality in the presence of drills, crabs or in the presence of drills, crabs or

bothboth

0

0.2

0.4

0.6

0.8

1

No Drill 6 Drill No Drill 6 Drill

Oys

ter

Mor

talit

y

Loose Oysters Oyster Reef

1 CrabNo Crab

Additve Model Prediction

Multiplicative Model Predictioncrab x drill crab x drill interaction:interaction:p = 0.001 (loose); p = 0.001 (loose); p = 0.05 (reef) p = 0.05 (reef)

Experiment 3: why facilitation, Experiment 3: why facilitation, who benefits once an oyster who benefits once an oyster

shell is breeched?shell is breeched?

2 x 2 x 2 orthogonal design Crabs and drills as before, Crabs and drills as before,

plus plus oyster conditionoyster condition as as additional factor: additional factor: unbreeched unbreeched and and breechedbreeched

Response variable: oyster Response variable: oyster soft-body tissue consumedsoft-body tissue consumed

Expt 3 results: why facilitation?Expt 3 results: why facilitation?

0

10

20

30

40

50

60

No Crab 1 Crab No Crab 1 Crab

Oys

ter

tissu

e

con

sum

ed

(g

)

No Drill

6 Drill

Damaged ShellIntact Shell

significant termssignificant termsoyster condition: p = oyster condition: p = 0.002; 0.002; drill x oyster condition: drill x oyster condition: p = 0.04 p = 0.04

Drills benefit from crabsDrills benefit from crabshandling oyster shells!handling oyster shells!

Discussion points and Discussion points and concluding remarksconcluding remarks

Field experiments demonstrated that drills and Field experiments demonstrated that drills and crabs together generated higher than expected crabs together generated higher than expected oyster mortality based on each species foraging oyster mortality based on each species foraging independently, even though crabs killed some independently, even though crabs killed some drillsdrills

Crabs apparently facilitated drills by breeching Crabs apparently facilitated drills by breeching oyster valves, thereby granting easy access for oyster valves, thereby granting easy access for drills. Risk enhancement for a shared resource drills. Risk enhancement for a shared resource when its two consumers have their own when its two consumers have their own predator-prey interactions is unusualpredator-prey interactions is unusual

Applied to conservation biology, stone crabs are Applied to conservation biology, stone crabs are not an effective means of biological control of not an effective means of biological control of drills that will release oysters from top-down drills that will release oysters from top-down regulationregulation

Remaining questionsRemaining questions Extrapolating caging results to field patterns of Extrapolating caging results to field patterns of

drill density, crab density and oyster drill density, crab density and oyster mortality…mortality…

How are drills managing their risk: can they How are drills managing their risk: can they gauge the danger/rewards related to crab gauge the danger/rewards related to crab proximity and oyster reef characteristics?proximity and oyster reef characteristics?