How do marine snails (Nucella lapillus) respond to the frequency of high predation risk and how should we expect them respond?

Background/Question/Methods

Prey are usually exposed to temporal variation in predation risk as they and predators move around in their environment. How they respond to this variation in risk affects the relative strength of consumptive and non-consumptive effects of predators on their communities. When prey have access to quick and reliable information about the current level of predation risk in their vicinity it is typically advantageous for them to vary their behavior in response to changes in predation risk. Lima and Bednekoff (1999) produced the Risk Allocation hypothesis that predicted how prey behavior should vary as the frequency of relatively low and high risk periods varies. The original risk allocation model assumed that the prey fitness function is a step function with prey only achieving positive fitness if they achieve a threshold size, but fitness does increase beyond the threshold size. Two of the predictions from the model are that as the frequency of high risk periods increases that 1) prey foraging effort during low and high risk periods will increase, but 2) average effort averaged across all time periods will be unaffected.

Results/Conclusions

We first present data from an experiment where marine snails (Nucella lapillus) were exposed to varying cues of predation risk from green crabs (Carcinus maenas). We observed that when periods of higher risk were more common that the amount of resources consumed and the growth of the snails were reduced, unlike the model prediction. We present an individual-based model to explore the generality of the risk hypothesis predictions. In particular we present how prey responses to changes in the frequency of high risk periods depend on 1) the reliability of information prey have about changes in predation risk and 2) the shape of the prey fitness function. We present that the predictions of the risk allocation hypothesis are sensitive to assumptions about the shape of the prey fitness and the reliability of information available to prey about predation risk. Opposite to the previous predictions, when prey fitness increases linearly with their achieved size and they have imperfect information about predation risk as the frequency of high risk periods increases 1) prey foraging efforts during low and high risk periods decrease, and 2) average effort averaged across all time periods also decreases.