Risk-Sensitivity: Foraging Mode in an Ambush Predator

We present a stochastic model for foraging mode, where the mode is idealized as a "sit—and—wait" strategy or as a strategy of active movement between foraging sites. We direct the model's assumptions toward the behavior of certain (female) orb—web spiders, where both temporal and spatial stochasticity govern the number of prey taken by a predator. At a given web site, prey are captured randomly through time according to a Poisson process. Spatial heterogeneity is incorporated by allowing the probabilistic rate of prey capture to vary randomly among potential web sites. We assume that an efficient predator should minimize the probability of capturing fewer prey than required to survive and reproduce during a single season. The model shows that in the same environment the two strategies yield the same expected number of captured prey, but the variance in the number of prey captured is greater for the sit—and—wait predator. An approximation to the model's solution indicates that a predator should be risk—averse and employ the mobile strategy in an environment where the expected number of prey captures exceeds the requirement. But in an environment where the requirement exceeds the expected number of prey captures, a predator should be risk—prone and employ the sit—and—wait strategy.