The effects of variable predation risk on foraging and growth: less risk is not necessarily better

There is strong evidence that the way prey respond to predation risk can be fundamentally important to the structuring and functioning of natural ecosystems. The majority of work on such nonconsumptive predator effects (NCEs) has examined prey responses under constant risk or constant safety. Hence, the importance of temporal variation in predation risk, which is ubiquitous in natural systems, has received limited empirical attention. In addition, tests of theory (e.g., the risk allocation hypothesis) on how prey allocate risk have relied almost exclusively on the behavioral responses of prey to variation in risk. In this study, we examined how temporal variation in predation risk affected NCEs on prey foraging and growth. We found that high risk, when predictable, was just as energetically favorable to prey as safe environments that are occasionally pulsed by risk. This pattern emerged because even episodic pulses of risk in otherwise safe environments led to strong NCEs on both foraging and growth. However, NCEs more strongly affected growth than foraging, and we suggest that such effects on growth are most important to how prey ultimately allocate risk. Hence, exclusive focus on behavioral responses to risk will likely provide an incomplete understanding of how NCEs shape individual fitness and the dynamics of ecological communities.