Testing the resource-enemy release hypothesis: Pathogen release and nitrogen availability in plant invasion

High-resource availability, while clearly important to plant colonization, does not appear to explain the exceptional success of exotic plants. Here, we discuss the possibility that high resource availability may aid exotic plants more than natives, by increasing enemy release. Plant species adapted to high levels of available resources may be poorly defended, nutritious, and therefore susceptible to enemies. Consequently, these species may benefit most from a paucity of enemies in their new range. Examining this possibility for 267 European species that are naturalized in North America, we found that plant species from habitats with moderate to high N availability harbored the most pathogens in Europe, and lost the most pathogens upon moving to North America. These trends were observed despite the fact that high-N species lost a smaller proportion of their pathogens, and gained more pathogens in North America than did low-N species. High-N species were also most likely to be listed as noxious weeds in the United States. With respect to other resources, species from mesic habitats escaped the largest number of pathogens, but no clear trends were observed for species from different light environments. These correlations suggest that while native and exotic colonizing species share resource adaptations, these adaptations may provide additional benefits to exotic species. Consequently, factors influencing resource availability, such as disturbance, pollution, and restoration, may have particularly strong effects on exotic plants.