A stoichiometric exception to the competitive exclusion principle

The competitive exclusion principle (CEP) states that no equilibrium is possible if n species exploit fewer than n resources. This principle does not appear to hold in nature, where high biodiversity is commonly observed, even in seemingly homeogenous habitats. Although various mechanisms, such as spatial heterogeneity or chaotic fluctuations, have been proposed to explain this coexistence, none of them invalidates this principle. Here we evaluate whether principles of ecological stoichiometry can help in understanding the stable maintenance of biodiverse communities. Stoichiometric analysis recognizes that each organism is a mixture of multiple chemical elements (carbon, nitrogen, phosphorus) and that the relative relative proportions of such elements vary within and among species. We incorporate stoichiometric principles into a standard predator-prey model to analyze competition between two consumers on one autotrophic resource. The model tracks two essential elements, carbon and phosphorus, in each species. We show that a stable equilibrium is possible with two competitors on this single biotic resource. At this equilibrium both consumers are limited by the phosphorus content of the resource. This suggests that within a stoichiometric framework, the CEP may be violated because one biotic resource may support multiple competitors at a stable equilibrium. ∗Current address: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544-1033