Seasonality, optimal foraging, and prey coexistence.

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