Pathogen impacts on plant communities: unifying theory, concepts, and empirical work

Pathogens, like other consumers, mediate the outcome of competitive interactions between their host species. Ongoing efforts to integrate pathogens into plant community ecology could be accelerated by greater conceptual unification. Research on plant pathogens has mainly focused on a variety of disparate mechanisms-the Janzen-Connell hypothesis, plant-soil feedbacks, competition-defense trade-offs, escape of invasive plants from their enemies, and epidemic-driven community shifts-with limited recognition of how these mechanisms fit into the broader context of plant coexistence. Here, I extend an emerging theoretical framework for understanding species coexistence to include various pathogen impacts on plant communities. Pathogens can promote coexistence by regulating relative abundance or by reducing the disparities between species in fitness that make coexistence more difficult. Conversely, pathogens may undermine coexistence by creating positive feedbacks or by increasing between-species fitness differences. I review the evidence for these pathogen-mediated mechanisms, and I reframe the major hypotheses in a community ecology context in order to understand how the mechanisms are related. This approach generates predictions about how various modes of pathogen attack affect plant coexistence, even when direct impacts on host relative abundance are difficult to measure. Surprisingly, no study gives direct empirical evidence for pathogen effects on mutual invasibility, a key criterion for coexistence. Future studies should investigate the relationship between pathogen attack and host relative abundance, in order to distinguish between mechanisms.