ONTOGENIC SHIFTS WITHIN THE SELFISH HERD - PREDATION RISK AND FORAGING TRADE-OFFS CHANGE WITH AGE IN COLONIAL WEB-BUILDING SPIDERS

We examine costs and benefits associated with spatial position relativ e to spider age (size) in colonial web-building spiders. Predator atta ck and capture rates vary with position in the colony, and suggest tha t risk is higher for the smallest and the largest spiders on the perip hery, and lower in the central core of the colony. Foraging success is greater on the periphery for small and medium spiders but does not di ffer significantly with position for larger spiders. Decreased predati on risk may be the reason why larger spiders aggressively seek and def end positions in the colony core, demonstrating a ''selfish herd effec t'' (Rayor and Uetz, 1990). Smaller (immature) spiders, unable to comp ete for protected web positions in the core, must trade-off potentiall y higher risk of predation to take advantage of higher prey availabili ty on the periphery. Increased foraging success on the periphery may a llow juvenile spiders to achieve the larger size necessary to compete successfully for protected core positions as adults. Spatial variation in size-related fitness trade-offs between predation risk and foragin g success may explain why colonies are dynamic entities - with individ ual spiders exhibiting ontogenetic shifts in web location as they grow larger and mature-accounting for the characteristic age (size) struct ure of Metepeira incrassata colonies.