Competition and Coexistence: The Effects of Resource Transport and Supply Rates

Abstract

Classical resource competition theory can be generalized to apply to a variety of specific resource types and specific supply media (e.g., soil, water, or air). We develop a general model that relaxes the assumptions that (1) resources and organisms are sufficiently mixed that all organisms experience the same resource concentration and (2) the organisms themselves regulate the resource concentration of their shared environment. These assumptions are shown to apply to a limited subset of conditions in which the resource input rate is low and the resource transport rate in the environment is high. Under such conditions, the coexistence criteria of our general model converge with those of classical resource competition models. Such conditions may be met in some aquatic environments, but under other conditions, in which resource transport rates may be low or input fluxes high, the general model makes predictions that differ radically from those of the classical models. Specifically, our model predicts that, instead of a 1:1 ratio between limiting resources and locally coexisting species, a large number of species can coexist on a single limiting resource under steady-state conditions. Shifts from limitation by one type of resource to limitation by another type can dramatically alter the nature and intensity of competitive interactions. This phenomenon is proposed as the explanation for the ubiquitous unimodel curve of autotroph diversity along productivity gradients.