Plant pathogens are extremely common in natural plant communities, and can affect plant population dynamics by reducing growth, fecundity, or survival. Pathogens that differentially harm plant competing species may impact the outcome of competition, and therefore plant diversity within host communities. However, the range of potential pathogen impacts on plant communities remains poorly understood. In this work, I examine how pathogens can affect the diversity of plant species by (i) reviewing the literature, (ii) surveying fungal seed mortality in the field, (iii) analyzing a two-host-pathogen model, and (iv) parameterizing the model in the field. The review yielded a range of mechanisms by which pathogens affect plant communities, but no study actually demonstrated pathogen impacts on key criteria for coexistence.

One mechanism, pathogen spillover, in which a reservoir host species builds up high densities of a pathogen and drives infection in a second host species, was particularly poorly understood both theoretically and empirically. Intuitively, one might expect that spillover would benefit the reservoir host to the detriment of the non-reservoir host. To test this hypothesis, I examined the impacts of spillover in cheatgrass-invaded grasslands in the western U.S., using a theoretical model that I then parameterized with empirical data. Cheatgrass reaches extremely high densities and promotes a fungal seed pathogen that also infects the seeds of native grass species. The model revealed, counter-intuitively, that the pathogen can favor the reservoir host (cheatgrass), the non-reservoir host (native grass), both species, or neither species, and that species-specific transmission and tolerance are the key traits determining the outcome.

The empirically-fitted model demonstrates that the pathogen benefits the native grass and harms cheatgrass, despite pathogen spillover. More broadly, this suggests that pathogen spillover does not necessarily imply a net benefit the reservoir host. Because the field survey of seed mortality showed an important role of soil fungal pathogens---many of which may be generalists---pathogen spillover may be an important and overlooked driver of the outcome of competition in many plant communities. Understanding the pathogen impacts on plant communities is an important goal for conservation and for mitigating the impacts of invasive species.