This study will evaluate the role of specialist and generalist pathogens in interactions between native and introduced grass species. The enemy release hypothesis argues that the demographic success of introduced plant populations relative to natives results from greater suppression of the natives by natural enemies. Current data suggest that introduced plants are more completely released from specialist than generalist enemies. Thus, introduced populations may achieve greater demographic success than natives by indirectly suppressing them through pathogen-mediated apparent competition, the enemy of my enemy hypothesis. The proposed work will use an experimentally tractable system of native and introduced old-field grasses to test these hypotheses. Large-scale surveys will be used to quantify variation in ambient pathogen species richness and infection rates on native and introduced grasses in the North Carolina Piedmont. Experimental monocultures of each species will be established to quantify pathogen species richness and infection rates in a common garden. Soil fertility, infection by fungal pathogens, and infection by viral pathogens will be manipulated in a factorial design. Together, the data from the survey and experiment will be the first two steps in one of the most comprehensive tests to date of the role of pathogens in biological invasions. Biological invasions are estimated to cost the US economy billions of dollars per year and be a leading threat to native biodiversity. This work may suggest mechanisms for the control or mitigation of harmful invasive species. Additionally, several of the pathogens to be studied in these native and introduced populations are also agricultural pests. This project may suggest novel approaches to the ecological control of their outbreaks in crop systems. Finally, this research seeks to shed light on fundamental principles of disease ecology that may be applied to animal and human populations.
