Leaf quality, predators, and stochastic processes in the assembly of a diverse herbivore community

Ecological communities are structured by both deterministic, niche-based processes and stochastic processes such as dispersal. A pressing issue in ecology is to determine when and for which organisms each of these types of processes is important in community assembly. The roles of deterministic and stochastic processes have been studied for a variety of communities, but very few researchers have addressed their contribution to insect herbivore community structure. Insect herbivore niches are often described as largely shaped by the antagonistic pressures of predation and host plant defenses. However host plants are frequently discrete patches of habitat, and their spatial arrangement can affect herbivore dispersal patterns. We studied the roles of predation, host plant quality, and host spatial proximity for the assembly of a diverse insect herbivore community on Quercus alba (white oak) across two growing seasons. We examined abundances of feeding guilds to determine if ecologically similar species responded similarly to variation in niches. Most guilds responded similarly to leaf quality, preferring high-nitrogen, low-tannin host plants, particularly late in the growing season, while bird predation had little impact on herbivore abundance. The communities on the high-quality plants tended to be larger and, in some cases, have greater species richness. We analyzed community composition by correlating indices of community similarity with predator presence, leaf quality similarity, and host plant proximity. Birds did not affect community composition. Community similarity was significantly associated with distance between host plants and uncorrelated with leaf quality similarity. Thus although leaf quality significantly affected the total abundance of herbivores on a host plant, in some cases leading to increased species richness, dispersal limitation may weaken this relationship. The species composition of these communities may be driven by stochastic processes rather than variation in host plant characteristics or differential predation by insectivorous birds.