According to the popularly cited enemy release hypothesis, exotic plants thrive in their introduced range because they are “released” from natural enemies (herbivores and pathogens) that they leave behind when they enter a new range. Enemy spillover is essentially the reverse of enemy release. With enemy spillover, exotic plants bring their natural enemies with them, and these enemies then “spill over” onto neighboring species. Because introduced enemies may impact new host species more than hosts that have evolved defense mechanisms, enemy spillover can reduce fitness of native plants more than that of the co-evolved introduced species.

Results/Conclusions

To test enemy release and enemy spillover, in 2006 we planted experimental communities containing differing mixtures of three perennial native and three perennial exotic grass species that co-occur naturally in the Willamette Valley of Oregon, USA. The native grasses are commonly used in restoration of seasonal wetland prairies in the region. The eight communities, each replicated five times, are as follows: three exotic species, three native species, three native species plus one exotic species, and three exotic species plus one native species. In 2007, we surveyed the seedlings for foliar damage by natural enemies. Two of the three native species showed more herbivore damage when exotic species were present in the community. Interestingly, one of the exotic species showed increased overall damage and herbivore damage when in a community with only the other two exotic species. Across all communities, exotic species had significantly more overall damage, herbivore damage, and pathogen damage than native species, in direct opposition to the prediction of the enemy release hypothesis. Our results are consistent with enemy spillover from exotic to native species.