Allee Effects and Coexistence

Allee effects are common to diverse taxa, but their consequences for coexistence are rarely considered by ecologists. Recent research has suggested that Allee effects are incompatible with modern coexistence theory or that their impacts on coexistence are no different from other sources of positive density dependence that generate alternate stable states through priority effects. We use a graphical approach that builds on mathematically robust theory to develop simple conditions for coexistence and alternate stable states when an Allee effect is present. We show that weak Allee effects (those that do not depress population growth rates below zero in the absence of competition) can be integrated with modern coexistence theory but often produce outcomes distinct from other priority effects. This integration allows us to determine how Allee effects alter stabilizing and fitness differences. Importantly, we characterize a high-density criterion for a third alternate stable state that indicates species coexistence even when mutual invasibility is not met. Strong Allee effects (those that preclude invasibility even in the absence of competitors) permit coexistence only when the high-density criterion is satisfied. Our model offers an intuitive extension of modern coexistence theory that accounts for more than two alternative stable states and provides a guide for empirical research on how Allee effects structure ecological diversity.