title: Alien vs. predator: effects of a native predator on two invasive oyster drills and oysters in Washington State
Creators: name: Grason, Emily W.
Publisher: name: Western Washington University
Year: 2010

description: <p>I investigated the consumptive (CEs) and non-consumptive effects (NCEs) of a native crab predator (the red rock crab, Cancer productus) on a system of two invasive oyster drills (the Atlantic drill, Urosalpinx cinerea, and the Japanese drill, Ocinebrina inornata), a native oyster (the Olympia oyster, Ostrea lurida), and an introduced but commercially valuable oyster (the Pacific oyster, Crassostrea gigas). In the presence of chemical effluent from crabs eating conspecifics, drills increased hiding behavior by 2 to 6 times, and reduced the number of oysters consumed by 50 to 67%. This is consistent with an adaptive behavioral response and has the potential to transmit large positive indirect NCEs of the crab to oysters. Neither species of drill uses a density-dependent risk assessment, as the effect of predation effluent was similar at both high and low densities of conspecific drills. The response specificity of the Atlantic drill was further investigated to determine which cue sources (i.e., predator or prey) were eliciting these defenses. Drills responded equivalently to the consumptive predation cue (predators consuming injured conspecific prey) and injured conspecific cue alone, suggesting injured conspecifics alone are sufficient to elicit the greatest magnitude of defense. A smaller defense was elicited by cue from an unfed crab, indicating that drills have the ability to recognize a relatively novel predator. Cues of injured conspecifics and unfed predators do not have an additive effect on drill behavior. Finally, digestive byproducts, both general chemicals produced by the crab and altered cues from conspecifics, did not increase the magnitude of drills' defenses. I explored the consumptive effects of crabs on drills and oysters by estimating crab feeding rates on and preferences among all prey types. Crabs consumed both species of oyster at similar rates and did not express a preference for either oyster. However, while crabs can consume both drills and Pacific oysters at similar rates, they expressed a strong preference for Pacific oysters over either species of drill. As a result, it is likely that crabs can have strong negative consumptive effects on oysters, and that drills may be released from crab predation in oyster beds. I have identified several mechanisms which might have facilitated invasion by drills: (1) inducible defenses against a novel predator, (2) general cue recognition strategy, and (3) simultaneous introduction of prey preferred by native predators. All three mechanisms weaken the efficacy of biotic resistance. Therefore, without greater human intervention, drills will continue to pose a problem for oyster culture and restoration efforts in Washington State. Behavioral defenses and crab preferences offer drills a refuge from predation that will enable them to persist in a stable trophic chain with red rock crabs and oysters. However, if the alarm cues that trigger defenses in Atlantic drills can be identified and isolated, application to oyster beds could reduce drill predation on oysters. Taken together, these findings suggest that, in general, inducible defenses might facilitate invasion by defended prey. Further, the strong response by Atlantic drills to cues of injured conspecifics suggests a mechanism whereby these invasives, and potentially others, can recognize and respond appropriately to novel predators. Continued exploration of this system can offer opportunities to test hypotheses about the evolution of inducible defenses.</p>