Dynamics of regional coexistence for more or less equal competitors

Abstract

Summary Competition between two species in a metapopulation involves each inhibiting the other's ability firstly to colonize an already occupied area and then to persist in it. Models for regional competition of this kind have 3‐D dynamics, from which it has proved difficult to extract useful predictions except for special conditions. We introduce a 2‐D general model for species that are equally vigorous at inhibiting the ability of others to remain in an occupied patch as to arrive there. The model covers the full spectrum of competitive interactions, from weak to strong and symmetrical to asymmetrical. Its Lotka–Volterra dynamics extend the general theory of competitive coexistence by generating clear predictions for community structure, amenable to cross‐system comparisons and experimental manipulations. Previous 2‐D models of interactions between dominant and fugitive species are special cases of the 2‐D general model. Moderately asymmetrical competition has two outcomes distinctly different from dominant–fugitive interactions, at both the scale of metapopulation and population: (i) slow growing and weak competitors coexist with faster growing superior competitors, albeit at reduced densities; and (b) habitat removal always yields relative gains in abundance for species with higher growth capacity, but the gains are absolute only for species subjected to competitive impacts that exceed within‐species impacts. Extinctions of slow growing and weak competitors provide the most sensitive indication of habitat degradation, and their losses also have the least effect on community structure. The 2‐D general model further predicts that highly productive communities will tolerate differences between species in their capacity for population growth, whereas less productive communities will tolerate stronger competitive interactions between species. This prediction applies equally to a population of resource consumers as to a metapopulation of colonists. The model explicitly links local and regional population dynamics to r–K selection in community structure by predicting a prevalence of growth‐orientated species in resource‐poor habitats and competition‐orientated species in resource‐rich habitats.