Pathogens promote plant diversity through a compensatory response

Numerous models, observations, and experiments suggest that pathogens can promote diversity in plant communities by preventing competitive exclusion. Previous studies have focused primarily on single-plant single-pathogen interactions, yet the interactions between multiple pathogens and multiple hosts may have non-additive impacts on plant community composition. Here we report that both a bacterial and a fungal pathogen maintained the diversity of a four-species plant community across generations; however, significant interactions between the pathogens resulted in less diversity in the two-pathogen community than in the fungal pathogen community. Contrary to assumptions in standard models, diversity was not maintained because infection caused a disproportionate fitness cost to the dominant plant species. Instead, pathogens maintained plant diversity because the rare species produced more seeds through a compensatory response to pathogen infection. Finally, we found that the influence of pathogens on plant diversity in our experiment was 5.5 times greater than the influence of nutrient resource heterogeneity, suggesting that pathogens may be a major factor in maintaining plant diversity in natural communities.