Plant enemies (pathogens and herbivores) have been hypothesized to maintain plant species diversity by causing greater impacts on common species than rare species thus preventing competitive exclusion. While the role of plant enemies in producing distance-or density dependent mortality in seeds and seedlings is well-documented, the exclusive focus on these effects provides limited understanding of the effects of enemies on plant communities. Understanding the role of plant enemies in maintaining diversity requires a broader approach that considers how abundance of a plant species determines the composition of the community of enemies that attack it as well as relates the demographic effects of enemies on the growth of their host plant populations to the diversity of the entire plant community.

The accumulation of plant enemies is hypothesized to be a passive, sampling process which predicts more common plant species, i.e. larger in individual size and more wide-spread, are attacked by more enemy species than smaller-statured and more geographically-restricted species. I test this hypothesis in a comparative analysis of 490 plant species and find that, as predicted, the richness of fungal pathogens infecting a plant species increases with plant species' individual size and geographic distribution.

To test the hypothesis that plant enemies maintain plant diversity by limiting the growth of populations of competitively superior plant species through density-dependent disease or herbivory, I investigated the interactions between the host-specific fungal pathogen, Puccinia podophyllii and its host Mayapple (Podophyllum peltatum) as well as the effects of this interaction upon the species abundance and composition of the entire plant community in the forests of Washtenaw County, Michigan. I find that Mayapple is strong competitor reducing plant species seedling survival and the diversity of the plant community. Next, I find that the pathogen decreases host population growth by causing strong distance-dependent seedling survival and by reducing growth and reproductive rates. Furthermore, I show that rates of disease transmission and levels of disease severity increase with Mayapple density. The results strongly support the hypothesis that plant enemies maintain plant species diversity and provide the most complete demonstration, to date, of their diversity-enhancing role.