Invasive plants have transformed ecological communities by causing large reductions in native plant abundances, effectively limiting the distribution of many native plants to small spatial refugia. It is unclear whether these refugia will allow native plant species to persist over the long-term, or conversely, if these transformations have created extinction debts. An extinction debt emerges in a metapopulation when colonization rates are not sufficient to compensate for local extirpations. Because of the long timelines involved with extinction processes and the difficulty of measuring colonization and local extirpation rates directly, empirical studies alone are unable to quantify extinction debts directly. In this study, we develop a theoretical framework that links local processes of competition, fecundity and dispersal to metapopulation processes of extirpation and colonization. We use this framework to predict the impacts of changes in abundances and spatial isolation of native metapopulations, and the relative likelihood of extinction debt emergence across species and landscape types. We then parameterize the model with results from experiments in a Californian serpentine plant community.
Results/Conclusions
Our study identifies two counter-intuitive principles. First, sink populations in matrix habitats can radically alter metapopulation persistence. In the California plant community, the reduction of sink populations in matrix habitat after invasion is estimated to have reduced colonization rates by up to ninety percent for some species. Second, the quality of the habitat lost to invasive species, relative to the quality of the habitat still occupied by native species post-invasion, is key to determining native species’ long-term persistence. For example, high quality habitat in the California ecosystem produces up to three times as much seed as low quality habitat, but this high quality habitat is the most heavily invaded. The resulting loss of seed production, coupled with more limited dispersal success per seed, has caused a large reduction in the colonization rates of native species in this landscape. Overall, we show that changes in plant metapopulation dynamics following invasion may induce extinction debts in a number of landscapes and species, and highlight this area of research as a priority for conservation ecology.
