Department of Ecology and Evolutionary Biology, Princeton University, United States
Community ecology typically assumes that competitive exclusion and species coexistence are unaffected by evolution on the time scale of ecological dynamics. However, recent studies suggest that rapid evolution may facilitate species coexistence, setting up the need for general theory that incorporates this process in parallel with purely ecological mechanisms when predicting competitive outcomes. In modern coexistence theory, niche and competitive ability differences are two forces driving competitive outcomes, but the use of these metrics is currently restricted to situations in which species' traits and vital rates are fixed. Whether these metrics even apply in cases where rapid evolution enables eco-evolutionary cycles of coexistence could be questioned.
Here, we argue that coexistence theory can be "rescued" in eco-evolutionary contexts by considering invasion growth rates with adaptive evolution. We define eco-evolutionary niche and competitive ability differences by considering how invading and resident species adapt to conspecific and heterospecific competitors. We show that our theory of eco-evolutionary niche and competitive ability differences accurately evaluates the potential for stable coexistence in previous theoretical studies, where eco-evolutionary processes drive population cycles. Finally, we show how this theory better frames recent empirical assessments of rapid evolution effects on species coexistence, and how empirical work on this problem can progress.