Risk‐Sensitive Foraging Strategies of Two Spider Populations

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Abstract
We found that foraging mode differed between habitats in the long—jawed orb—weaving spider Tetragnatha elongata. In a creek habitat, where relatively few prey (on average) were captured over extended intervals, spiders remained at the same foraging site for long periods. In a lake habitat, where average prey availability was relatively high, spiders changed web sites frequently. We first compared models differing in assumptions about a spider's ability to rank sites by their long—term average prey densities when prey availability varied through time at each site. When temporal variation in prey availability within foraging sites constrains a spider's ability to respond to spatial variation among sites, choice of foraging mode can be analyzed as a simple problem involving risk—sensitive decision making. The corresponding model associates the fixed—site strategy with risk—proneness, and associates the mobile strategy with risk—aversion. When average prey availability is less than the spider's requirement for successful reproduction, the fixed—site strategy induces the lower probability of a reproductive failure. When average prey availability exceeds the requirement, the mobile strategy induces the lower probability of a reproductive failure. In the laboratory, we estimated the prey biomass a female must consume in order to produce an egg sac. Given this estimate, a subsequent field experiment indicated that the risk—sensitive model adequately predicted observed variation in foraging behavior. The other two models we examined did not assume that a spider's response to spatial heterogeneity was inhibited by temporal variation; these models did not predict observed behavior.

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