Competition theory and practice are currently at loggerheads regarding the ecological implications of newly-introduced exotic species. If native and exotic species are all subject to the same biological fitness tradeoffs, competition models predict that a newly established exotic species should coexist with the established species with which it competes. However, a number of high-impact exotics make this interpretation difficult to accept. An open question is whether such insidious exotics are rare, or will become increasingly common as biogeographic exchanges are further increased by human activities. Here we propose a new variant on the 1969 Levins metapopulation model that addresses this issue. Specifically, we add a “priority effect” that decreases the ability of individuals to disperse into occupied patches regardless of competitive hierarchy. This two-way interaction has the long-term effect of increasing the prevalence of bad competitors (good dispersers) in the system, and can thus allow even highly competitive native species to be driven extinct by weedy exotic species.
Results/Conclusions
Our priority effect model reveals a simple mechanism through which frequent invasions by novel species from outside of the metapopulation can reduce the diversity of established communities. In simulated community assembly, regions that were invaded more frequently accumulated significantly fewer species (threefold on average) and were colonized by significantly worse competitors than communities that were allowed time to re-equilibrate between invasion events. This difference is driven by ephemeral communities of invading species which, though doomed to eventual deterministic extinction, nonetheless occupy space necessary for the survival of other species. In a process analogous to extinction debt, good competitors can thereby be driven extinct by highly abundant, but unstable, communities of weedy dispersers. These predictions accord well with field observations of grassland recovery following agricultural abandonment at Cedar Creek Ecosystem Science Reserve where the priority effect model significantly outperformed the classical Levins model in predicting both the order and relative abundance of species during transitory phases. These results therefore suggest that communities composed of species that have cohabited the same region for long time periods are indeed ecologically distinct from those assembled from recently invaded exotic species.