Exotic plant invasions have many possible drivers. Yet most invasion hypotheses offer single-factor explanations, including release from natural enemies, increased resource availability, and low community diversity. These factors are often studied alone, but likely interact to influence exotic success. In particular, species-rich communities may be less regulated by enemies, limiting the realized advantage to exotics of enemy release, and reducing exotic success. Likewise, species-rich communities may draw down resource availability, potentially reducing exotic success in these communities following resource addition. Moreover, exotic species in high resource environments may realize more benefits from enemy release than exotics in low resource environments. To test for interactive effects of these three drivers, we planted communities with six native herbaceous species at two diversity levels – monocultures of all six focal species and five-species polycultures each lacking one focal species – totaling 12 planted communities. We then manipulated soil nutrients (ambient, + NPK) and aboveground enemies (ambient, exclusion via biocide spraying) and allowed these communities to be naturally invaded for four years. To quantify exotic success, we measured the cover of all species within each plot. This is the first study to examine the interactive and independent influences of resources, enemies, and diversity in invasions.
Results/Conclusions
Overall, we found no evidence that resources, enemies, and diversity interacted to influence exotic success. Rather, resource addition and enemy exclusion each independently influenced exotic success, overwhelming any effects of diversity in explaining exotic success. Specifically, exotic dominance (exotic / total cover) did not differ with diversity (P = 0.43). Despite this, native colonizer (natives not planted in a plot) abundance was considerably higher in monocultures than polycultures (P < 0.001), suggesting that niche overlap between native residents (natives planted in a plot) and exotics was low. Furthermore, fertilization increased exotic dominance (P < 0.001) and reduced native resident and native colonizer absolute abundance in years three and four. This indicates that exotics were strong competitors in fertilized plots. Moreover, spraying reduced exotic dominance significantly (P < 0.001), primarily because spraying slowed the decline in native resident abundance over time (P < 0.01). In fact, spraying only reduced exotic absolute abundance in 2014, suggesting that except for 2014, enemies influenced exotic success minimally. Rather, the large benefit of spraying to native residents was primarily responsible for reducing exotic dominance. Together, these results suggest that the effects of resources and enemies operate additively and not interactively within our study system.