A graphical-mechanistic approach to spatial resource competition.

Ecological communities are structured by processes operating at multiple spatial scales, which results in an often daunting complexity. Here we present a simple graphical theory to study the interaction of two fundamental community processes: resource competition at the local scale and dispersal at the regional scale. We consider a metacommunity model with two habitat patches in which consumer species compete for a spatially distributed resource. We introduce a graphical construction of the equilibrium metacommunity composition, analogous to traditional competition theory for two resources. As in the nonspatial case, the zero net growth isoclines (ZNGIs) play a central role in the analysis. We show that a consumer species' ZNGI depends on its dispersal characteristics, and this dependence leads to a unification of various dispersal-based coexistence mechanisms. We illustrate this unification using four specific mechanisms: species-specific dispersal rates, spatially asymmetric dispersal, resource-dependent dispersal, and competition between habitat specialists and generalists.