Developmental changes in barrier web structure under different levels of predation risk inNephila clavipes (Araneae: Tetragnathidae)

Individuals of the orb-weaving spider Nephila clavipesbuild complex webs with a region used for prey capture, the orb, and tangle webs opposite either face, the barrier webs. Barrier webs have been hypothesized to serve a variety of functions, including predator defense, and the primary function of the barrier web should be reflected in the relative size of the barrier to the orb under varying conditions of foraging success and predation risk. To investigate the effects of predation pressure and foraging success on barrier web structure, I conducted a comparative study in three disjunct populations that differed in predation risk and foraging success. Although both the orb web and the barrier webs are silk, there was no indication of a foraging-defense trade-off. Barrier web structure did not change during seasonal shifts in orb web size related to changes in preycapture rate, and barrier web silk density and orb radius were positively correlated. The hypothesis that the construction of barrier webs is in part a response to predation pressure was supported. Barrier webs do deflect attacks by some predators, and barrier webs built by small spiders, suffering frequent predation attempts, had a higher silk density than barrier webs built by larger individuals. Additionally, barrier web complexity decreased at a later age in areas with higher predation risk.