Predation risk can drive trophic cascades and limit energy flow within food chains by causing changes in the behavior and physiology of prey. Such community- and ecosystem-level effects of predation risk should depend on how prey balance the costs and benefits of foraging. There is strong evidence that prey decision-making is state-dependent because antipredator behaviors such as increased refuge use and reduced feeding rates can increase the risk of starvation. The impact of predation risk on community structure and ecosystem function should therefore increase as prey are able to respond more strongly to risk. Using a rocky intertidal food web, we examined how starvation risk altered the responses of prey (snails, Nucella lapillus) to predation risk (from green crabs, Carcinus maenas) when feeding on two different basal resources: structurally simple but quickly consumed barnacles (Semibalanus balanoides) or structurally complex but slowly consumed mussels (Mytilus edulis).
Results/Conclusions
Increasing levels of predation risk caused reductions in the foraging effort, energy intake, and growth of intermediate consumer snails, and each of these effects was dampened by an increased risk of starvation. At low levels of predation risk, snails performed better—they grew more and had a higher return on investment (foraging effort)—when feeding on mussels than when feeding on barnacles, despite the shorter handling time of barnacles. However, the effects of resource type on net snail performance and returns on foraging investment diminished with increasing predation risk. Increasing predation risk also reduced trophic transfer efficiency within our experimental food chain. This loss of energy, or “trophic heat,” increased when snails were either feeding on barnacles or had high energy reserves. Taken together, these results indicate that the ability of prey to manage their risk of predation and risk of starvation can be shaped by the quality and quantity of available energy. Our findings suggest that the effects of predation risk will be most pronounced in systems with abundant, high quality resources and that risk-induced reductions in trophic transfer efficiency may help to explain variation in the lengths of natural food chains.