Uncovering how top-down factors regulate plant diversity is a theoretical and empirical challenge for ecology. In particular, the impact of pathogens on plant diversity is largely unknown. Unseen pathogens that attack seeds or below-ground tissue may influence population dynamics without producing obvious above-ground effects. Shared pathogens can regulate species interactions when they differentially harm the more abundant species in a community, thereby stabilizing coexistence. I explored how shared generalist pathogens influence diversity in a storage effect model, where competing annual plant species coexist by partitioning the environment in time. Specifically, I investigated the effects of pathogen attack on two stages in the plants’ life cycle: the dormant seed bank and the germinating seeds.  

Results/Conclusions

I found that attack on dormant seeds undermines coexistence by reducing a population’s capacity to buffer against unfavorable years. Two species with only a small difference in fecundity coexisted stably in a variable environment via the storage effect. Adding pathogen attack on dormant seeds undermined this storage effect, causing the species with the lower fecundity to have lower growth rates when rare, and increasing it chances of extinction. Pathogen attack also decreased the population density of the remaining species. By contrast, attack on germinating seeds had no effect on coexistence in the model. While the germinant pathogen depressed both populations, attack rate had no relationship with growth rate when rare. This latter result is unexpected, given that attack depends on the density of germinants, and is therefore analogous to competition, a key ingredient of the storage effect. My results suggest that understanding when in the life cycle pathogens attack seeds is key to predicting their effects on coexistence.