Consequences of pathogen spillover for plant species diversity

Background/Question/Methods

Pathogen spillover is increasingly recognized as an important driver of disease spread through host communities. Disease-tolerant reservoir hosts can enhance transmission to less tolerant host species. As a result, increasing host diversity increases disease prevalence through spillover to susceptible hosts. Less apparent is how pathogen spillover feeds back to influence host diversity. Because tolerant hosts promote pathogen attack that differentially harms less tolerant species, one might expect pathogen spillover to generate positive feedbacks between host species abundance and population growth rates. These feedbacks would ultimately decrease diversity by increasing the dominance of already abundant species. To test these predictions, I have constructed a model of two competing plant species that share a pathogen. In the model, the plant species can differ in their ability to survive while infected and in their susceptibility to infection.

Results/Conclusions

The model reveals that pathogen spillover can stabilize or destabilize host communities, depending on the differential ability for species to transmit the pathogen to each other. Surprisingly, most cases of asymmetric pathogen transmission between species generate stabilizing negative feedbacks between per capita growth rates and relative abundance, while at the same time imposing a fitness cost to the more susceptible species. Such negative feedbacks can stabilize coexistence given a tradeoff between fecundity and pathogen susceptibility. Pathogen spillover only imposes destabilizing positive feedbacks when a species promotes pathogen attack that is more harmful to individuals of another species than to individuals of the same species. This model highlights the need to carefully study within- and between-species transmission in order to understand the effect of pathogen attack on host community dynamics.