SEASONALITY, OPTIMAL FORAGING, AND PREY COEXISTENCE

Several empirical studies suggest that herbivores may promote coexiste nce between plants by relaxing the strength of resource competition. I n contrast, recent mathematical models predict that food-limited herbi vory instead cause exclusion through apparent competition, regardless of whether herbivore selectivity is constant or density dependent. Thi s study extends existing theory to consider a strongly seasonal system . Herbivores with fixed diet preferences have the same effect regardle ss of seasonality, but there is a marked difference when the diet sele ctivity of herbivores conforms to a simple optimal-foraging model. An optimally foraging herbivore in a seasonal environment is able to prom ote plant coexistence among many species. The mechanism involves diet switching, occurring over narrow density intervals. For this to have a n effect in a nonseasonal model, equilibrium resource densities must b e in this interval, which requires close parameter fitting. In seasona l environments, resource densities change through the year and may fre quently move across narrow regions in which diet changes occur. The po tential of gray-sided voles to promote coexistence between two arctic dwarf shrubs is evaluated in terms of the model. For this system, it i s shown that vole herbivory has the potential to reverse competitive d ominance.