Stoichiometrically explicit competition between grazers: species replacement, coexistence, and priority effects along resource supply gradients.

Assuming key trade-offs among interactors, several models (resource ratio, keystone predation, intraguild predation) predict changes in species composition over resource supply gradients. Ecological stoichiometry could also predict compositional shifts of grazers over gradients of nutrient and light supply through a mechanism involving (mis)matches between elemental body composition of grazers and plants. This hypothesis is explored here using a suite of two-grazer, one-plant models that incorporate three key components: plant production depends on light and nutrients, nutrient content of plants can vary, and homeostatic grazers can be carbon or nutrient limited. The results from this suite closely resemble the classical resource ratio model describing plant competition for two resources. Here, the models predict shifts of grazer composition along resource supply gradients if species trade off competitive abilities for plant carbon and nutrients. Given this trade-off, superior nutrient competitors should dominate low nutrient environments, and superior carbon competitors should dominate high nutrient environments. At intermediate nutrient supply, species can coexist at a stable equilibrium, or alternative stable states emerge, depending on how grazers impact their resources. These results depend on food web architecture, however. For instance, predators can alter or reduce possibilities for stoichiometry-mediated coexistence of grazers.