Is group living an antipredator defense in a facultatively communal webspinner?

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I tested whether predator attack rate is a function of expanse of silk for colonies ofAntipaluria urichi, a facultatively communal webspinner. The avoidance effect hypothesis predicts that the probability of a predator detecting a group does not increase proportionately with an increase in expanse of silk, and therefore, larger groups are relatively less likely to be attacked. I counted the number of holes inflicted on silk over 3 weeks, an indirect measure of predation, for 47 colonies. Supporting the avoidance effect, holes per perimeter of silk accumulated at a rate lower than would be predicted by size alone. Further analysis using predation risk, computed as holes per individual per silk perimeter, revealed that risk was extremely variable for colonies with little silk (<200 cm) but, on average, did not vary as silk expanse increased. Overall, predators of webspinner colonies appear to be influenced by the avoidance effect, but whether the occupants of the silk are safer is unresolved.