Efforts to protect native plant communities should consider the role of herbivores and pathogens (natural enemies), which may spread ahead of introduced plants and facilitate invasions. We examined natural enemy attack in a seasonal wetland prairie on three native (Agrostis exarata, Danthonia californica, and Deschampsia cespitosa) and three exotic (Anthoxanthum odoratum, Holcus lanatus, and Schedonorus arundinaceus) perennial grasses that commonly co-occur in the Willamette Valley in Oregon, USA. These species combined comprise 64% of total plant cover in the area where our field site is. The native species are commonly used in restoration. The exotic species are common throughout North America and potentially harbor enemies that could affect both crops and natural communities. We designed our study to test enemy spillover from exotic to native species. In 2006, we planted experimental communities containing mixtures of the native and exotic grass species. The eight communities were as follows: three exotic species, three native species, three native species plus one exotic species, and three exotic species plus one native species. Communities were replicated five times in a randomized block design, for 40 plots total. We surveyed for foliar damage by natural enemies in 2007, 2008, and 2009.
Results/Conclusions
In 2007, during the first growing season, two of the three native species showed more herbivore damage when exotic species were present in the community, indicating enemy spillover from the exotic species to the native species. In 2008, the trend of increased enemies on native species when in a community with exotic species was diminished. However, two of the three exotic species had significantly more natural enemies when in a community with only other exotic species than when with native species, and the third exotic species had a similar suggestive trend. This finding is consistent with dilution of enemies on exotic species by the native species. In 2009, there was no evidence of enemy spillover from the exotic to the native species, but dilution of enemies on the exotic species by the native species was stronger for all three exotic species. In the first two years, across all communities, exotic species had significantly more overall, herbivore, and pathogen damage than native species. This result is the opposite of what the enemy release hypothesis predicts. Our results are consistent with enemy spillover from exotic to native species, and dilution of enemies on the exotic species by the native species.