LB 39-328 - Positive effects of a predator on prey growth rate through reduction of prey foraging

Many prey species change their behavior in response to predation risk in a way that leads to a reduction in foraging rate. Although predator-induced foraging reduction can and has been shown to have clear and direct negative effects on prey growth rate, theory and a limited amount of empirical work predict that predators can also have net positive effects on prey growth rate when the combined reduction in foraging by the entire prey population leads to a large increase in resource availability. We found support for this counterintuitive prediction in the results of a manipulative field experiment in which risk of predation from sea stars (Pisaster ochraceus) caused a net increase in the growth rate of marine snails (Tegula funebralis). Although the snails’ increase in avoidance behavior and reduction in foraging rate in the presence of predators was likely to have negatively affected their growth, it was eclipsed by a positive effect on growth from a large increase in available resources. Further, this positive predator effect was dependent on prey size, predator abundance, and the temporal pattern of risk, highlighting the importance of ecological context in shaping the risk effects of predators. This study indicates that predator-induced reductions in prey foraging can have both negative or positive effects on prey growth, the sign and magnitude of which are dependent on relative predation risk, prey density, and resource dynamics.