THE RESOURCE-BASED ORGANIZATION OF COMMUNITIES

The argument is developed that the resource base, on which food webs are built, controls many aspects of trophic structuring. With emphasis on food webs based on plants, important effects are represented: the plant's influence on three-trophic-level interactions; the cascading effects of plant quality on herbivore attack and performance; the resource-based influence on the probability of competition in communities; how within-species variation in resources selects for high specificity in resource utilization by foraging females and the consequences for intraspecific competition at low herbivore densities; and, how minor differences between plant species cause differences in herbivore behavior and consequently the number of parasitoids in the food web. Phylogenetic constraints on herbivores are superimposed on plant variation. Insect species in which females actively forage for rare high-quality resources are constrained to be latent species, while those in which larval feeding sites are reached more passively tend to generalize on plant quality and can become epidemic periodically-eruptive species. Thus the resource base and the responses of organisms to this base have major organizing influences on populations and communities.