Effects of native diversity, soil nutrients, and natural enemies on exotic invasion in experimental plant communities

Many factors can promote exotic plant success. Three of these factors-greater pressure from natural enemies on natives, increased soil nutrient supply, and low native species richness-may interact during invasions. To test for independent and interactive effects of these drivers, we planted herbaceous perennial communities at two levels of native richness (monocultures and five-species polycultures). We then factorially manipulated soil nutrient supply and access to these communities by aboveground foliar enemies (fungal pathogens and insect herbivores), and allowed natural colonization to proceed for four years. We predicted that nutrient addition would increase exotic success, while enemy exclusion and increasing native richness would reduce exotic success. Additionally, we expected that enemy exclusion would reduce the benefits of nutrient addition to exotic species most in species-poor communities, and that this effect would be weaker in species-rich communities. In total, we found no evidence that nutrient supply, enemy access, and native richness interacted to influence exotic success. Furthermore, native richness had no effect on exotic success. Instead, nutrient addition increased, and enemy exclusion decreased, exotic success independently. As predicted, enemy exclusion reduced exotic success, primarily by slowing the decline in abundance of planted native species. Together, these results demonstrate that multiple drivers of exotic success can act independently within a single system.