Biological invasions are causing worldwide ecosystem degradation and economic loss, with such effects predicted to increase. To mitigate and control biological invasions, it is critical to understand whether exotic species and native species respond similarly to disturbances. If this is the case, then most of our knowledge of ecology and evolution of native species can be used. Species-area relationships, one of the most common ecological patterns, has been used as a proxy of species' overall responses to environmental conditions. However, the phylogentic diversity-area relationships have received much less attention even though they may provide additional information about species' behaviors. To test whether exotic and native species have similar overall responses to environmental gradients, we compared and contrasted the species-area and phylogenetic diversity-area relationships between native and exotic plants and birds across the National Parks of the United States. We then used multiple regressions with climate, visitations, population, and other variables as predictors to explain patterns of species richness and phylogenetic diversity of plants and birds.
Results/Conclusions
Native birds, native plants, and exotic birds (but not exotic plants) all have significantly positive SARs. For both birds and plants, SARs of native species had higher slopes then those of non-natives, suggesting that they respond differently to environmental conditions. However, they have similar PDARs between native and non-native species, suggesting that native and non-native species respond similarity to processes that influence phylogenetic diversity. The predictors we used explained a decent proportion of variations in species richness (19% - 52%) and phylogenetic diversity (26% - 42%, but only 3.5% for exotic birds). Area was the most important variable for native plants richness, which explained ~30% of variations in richness. On the contrary, the number of visitors was the most important variable for non-native plant richness, which explained ~13% of variations. For native bird species richness, longitude and area are the most important variables while the percentage of natural area was the most important predictor for non-native bird species richness. Climatic variables are the most important variables, explaining ~15% of the variation of phylogenetic diversity. For native birds, area still was the most important predictor of phylogenetic diversity. All variables together only explained 3.5% of variation in exotic bird phylogenetic diversity. Our results thus highlight the importance to study biological invasions at multiple levels. To do this, SARs and PDARs can be used as quick proxies.