Assembly Rules for Communities of Nutrient-Limited Plants and Specialist Herbivores

When local communities are assembled from a regional species pool, assembly rules state which of the species from this pool can coexist. I consider how herbivory and resource competition determine these assembly rules, assuming that all plants are limited by a single resource and that all herbivores are specialists. Assembly rules for these communities, which consist of parallel food chains, are derived from invasibility conditions. In particular, a rule that generalizes Tilman's R* is found. Equilibrium nutrient concentrations of the subcommunities in the assembly sequence for any persistent community follow an ordering relation, expressing a rule that a new plant invades if its competitive ability is neither too large nor too small. If opportunities for colonization are unlimited, species richness can be as high as the regional species pool allows. Otherwise, most communities will be species poor and dominated by highly competitive plants and their herbivores. For any community, herbivores that eat highly competitive plants are "keystone" species, in that their removal leads to a cascade of further extinctions.