COMPETITION THEORY AND THE STRUCTURE OF ECOLOGICAL COMMUNITIES

We develop a theory of competition based on two mechanisms that we call the cost of rarity (Mechanism R) and the cost of commonness (Mechanism C). These reduce the rate of population increase only at high densities (asexual organisms) or both at high and low densities (sexual species). The theory predicts that, in certain circumstances, the number of coexisting species in any assemblage will be finite and that these species will differ in their utilization of resources (and in associated morphological traits) more than expected by chance. Specifically, it predicts such nonrandom assortment in assemblages of three or more (i.e. multispecies) sexual species, especially in those communities where a few species are numerically dominant, but not in two-species associations and not in asexual forms. Unlike other theories, however, ours does not predict any specific value of morphological ratio or limiting similarity. We develop procedures to assess the degree of numerical dominance in an assemblage, and then test the predictions of the theory using data on morphological size ratios. The tests yield results that are consistent with the theory. Our analysis of multispecies assemblages Of granivorous rodents, bird-eating hawks, Anolis and sexual Cnemidophorus lizards, show that in these assemblages very small ratios are observed less often than expected by chance. Coexisting asexual Cnemidophorus lizards tend to be extremely similar in size. Nesting sparrow and flycatcher assemblages exhibit low numerical dominance which we predict will inhibit detection of regular assortment, and we find no regular pattern in these assemblages. Finally, we fail to detect regular assortment in most two-species associations as expected. We examine several alternative mechanisms that might account for morphological segregation among coexisting species but find little evidence that they have been important.