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

Can stone crabs provide biological control against southern oyster drills and increase eastern oyster survivorship?. F. Joel Fodrie, Matthew D. Kenworthy and Sean P. Powers Department of Marine Sciences, University of South Alabama & Dauphin Island Sea Lab ICSR 2008. Acknowledgements.

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

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  1. Can stone crabs provide biological control against southern oyster drills and increase eastern oyster survivorship? F. Joel Fodrie, Matthew D. Kenworthy and Sean P. Powers Department of Marine Sciences, University of South Alabama & Dauphin Island Sea Lab ICSR 2008

  2. Acknowledgements • 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.

  3. Drill predation: an obstacle to oyster restoration and population fitness 90% mortality reported (Butler 1985) • Adapted from Gregalis (2007)

  4. Relevant food-web interactions • Trophic cascade framework: crabs should “quite” drills and release oysters from top-down control

  5. Relevant food-web interactions • However crabs also consume oysters • What’s better for oysters: - drills foraging with impunity…or… - drills and omnivorous crabs foraging together? ?

  6. Testable questions • Are there numerical or behaviorally mediated interactions between drills and crabs that affect oyster mortality? [EXPS 1 & 3] • Are the effects of these multiple predators on oyster mortality risk enhancing, risk reducing, or independent? [EXP 2]

  7. Multiple Predator Effects • Consider two predators foraging alone: - Pred A eats a % - Pred B eats b % • Act independently when together: =a + b % • Generate enhanced risk when together: >a + b % • Generate reduced risk when together: <a + b % Sih et al. (1998) TREE

  8. Experiment 1: effect of crabs on drill mortality and behavior • Single factor: presence or absence of crab in high-density patch • Response variable: % of live drills in high-density patch High Density Low Density 0.5 m 2.5 m (height = 0.5 m)

  9. Expt 1 results: effect of crabs on drill mortality or behavior • Crabs influence drill habitat selection (p = 0.006) - crab absent: 80% of drills in the HD patch - crab present: <5% of drills in the HD patch • Implication: if drills will avoid a rich food supply to avoid crabs, then drills and crabs foraging together should result in reduced risk for oysters

  10. Experiment 2: oyster mortality in the presence of drills, crabs or both • 2 x 2 orthogonal design • Runs with loose or clumped oysters • Response variable: oyster mortality • MPEs revealed by significant interaction term drill crab

  11. Expt 2 results: oyster mortality in the presence of drills, crabs or both crab x drill interaction: p = 0.001 (loose); p = 0.05 (reef)

  12. Experiment 3: why facilitation, who benefits once an oyster shell is breeched? • 2 x 2 x 2 orthogonal design • Crabs and drills as before, plus oyster condition as additional factor: unbreeched and breeched • Response variable: oyster soft-body tissue consumed

  13. Expt 3 results: why facilitation? significant terms oyster condition: p = 0.002; drill x oyster condition: p = 0.04 Drills benefit from crabs handling oyster shells!

  14. Discussion points and concluding remarks • Field experiments demonstrated that drills and crabs together generated higher than expected oyster mortality based on each species foraging independently, even though crabs killed some drills • Crabs apparently facilitated drills by breeching oyster valves, thereby granting easy access for drills. Risk enhancement for a shared resource when its two consumers have their own predator-prey interactions is unusual • Applied to conservation biology, stone crabs are not an effective means of biological control of drills that will release oysters from top-down regulation

  15. Remaining questions • Extrapolating caging results to field patterns of drill density, crab density and oyster mortality… • How are drills managing their risk: can they gauge the danger/rewards related to crab proximity and oyster reef characteristics?

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