Positive frequency dependence undermines the success of restoration using historical disturbance regimes.

Abstract

Anthropogenic alterations of historical disturbance regimes (e.g. suppressing floods and wildfires) is a primary mechanism by which exotic species can come to dominate native communities. Unfortunately, reinstating historical disturbance regimes to restore native communities has achieved mixed success. The presence of positive frequency dependence (PFD) is commonly invoked to explain why exotic plant invasions are so difficult to eradicate. However, models examining PFD have not considered the effect of reintroducing disturbances. Using a spatially explicit individual-based model, we consider how magnitude and direction of frequency dependence of native and exotic species affects the success of reintroducing disturbances that favour fitness of natives over exotics. Our model illustrates why restoration is difficult; there is a narrow range of parameters that allows for native species to eliminate or coexist with exotics once they have established. Dominance by exotic invaders occurs with moderate initial frequencies of exotic individuals, aggregation of these individuals, or an exotic propagule production advantage. Reintroducing disturbances allows native dominance only when PFD of the exotic is weaker than that of the native species, disturbance intervals are short, and/or exotics are not initially frequent. Our framework provides guidelines for conditions in which the reintroduction of disturbances will effectively restore invaded habitats.