Group size and predation risk in colonial web-building spiders: analysis of attack abatement mechanisms

Higher rates of encounter with wasp predators are a consequence of group living for Metepeira incrassala (Araneae: Araneidae), a colonial orb-weaving spider from tropical Mexico. Field observation of wasp attacks on these stationary prey groups, which vary widely in size, allows separation of attacks at the colony and individual level and provides evidence of a complex attack-abatement effect. No predator attacks were observed for solitaries and small groups of two to eight spiders. In groups of 10 spiders or more, predator encounter rate increases with group size, although at a decreasing rate. This nonlinear relationship suggests an encounter avoidance effect that may be due in part to a visual apparency effect, wherein the target area presented by these three-dimensional colonies does not increase proportionately with increasing group size. Despite increased encounter rates in larger colonies, individual risk decreases with colony size, but not entirely similar to the manner predicted by a numerical dilution effect. Dilution of attack risk per individual may be offset by the foraging behavior of wasp predators, as they concentrate their foraging and sequentially attack more spiders in larger groups. Even so, wasp capture efficiency decreases withcolony size, as spiders become aware of attacks on others, suggesting an early warning effect from web vibrations. As a result of these combined effects, in colonies of 10 of more spiders, overall predation risk from wasps decreases with increasing group size.