Ontogenetic 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 relative to spider age (size) in colonial web-building spiders. Predator attack and capture rates vary with position in the colony, and suggest that risk is higher for the smallest and the largest spiders on the periphery, and lower in the central core of the colony. Foraging success is greater on the periphery for small and medium spiders but does not differ significantly with position for larger spiders. Decreased predation risk may be the reason why larger spiders aggressively seek and defend positions in the colony core, demonstrating a "selfish herd effect" (Rayor and Uetz, 1990). Smaller (immature) spiders, unable to compete for protected web positions in the core, must trade-off potentially higher risk of predation to take advantage of higher prey availability on the periphery. Increased foraging success on the periphery may allow 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 foraging success may explain why colonies are dynamic entities -- with individual spiders exhibiting ontogenetic shifts in web location as they grow larger and mature-accounting for the characteristic age (size) structure of Metepeira incrassata colonies.