Coexistence destabilizing mechanisms from nonlinear predation in a variable environment

Background/Question/Methods

Fundamental to community ecology is the question of how similar species can coexist. Recently it has been shown that generalist predators may often undermine coexistence by weakening the mechanisms that promote coexistence, such as resource partitioning. Positive effects of generalist predators have been found in the presence of complex predator behavior, but these studies have not considered the common nonlinearities that arise from a Type II functional response, or the complex behavior predicted by optimal foraging theory. We modeled seed predators with a Type II functional response consuming seeds of annual plants. The plants compete for a single limiting resource, and coexist by responding differently to environmental factors that fluctuate over time, which generates a storage effect. We considered predators with fixed seed preferences, and predators that change preferences with seed availabilities to maximize their calorie intake per unit time (“optimal foragers”). We studied these models both analytically, and with simulation. 

Results/Conclusions

Predators with a Type II functional response cause less of a reduction in the storage effect because they have reduced hunting efficiency. However, predator satiation from the Type II functional response results in species at high density suffering less per capita predation than species at low density, generating a negative storage effect from predation and weakening coexistence. Moreover, this negative storage effect can cause a priority effect where a weak species can exclude a stronger species by establishing first. Optimally foraging predators are capable of creating a new stabilizing mechanism if they preferentially attack competitively dominant species. However, this stabilizing effect may not always compensate for the weakening of the storage effect due to competition and the presence of a negative storage effect due to predation.