Despite spillover, a shared pathogen promotes native plant persistence in a cheatgrass-invaded grassland

Background/Question/Methods

How pathogen spillover influences host community diversity and composition is poorly understood.  Spillover occurs when transmission from a reservoir host species drives infection in another host species.  In cheatgrass-invaded grasslands in the western U.S., the fungal seed pathogen black fingers of death (Pyrenophora semeniperda) spills over from exotic cheatgrass (Bromus tectorum) to native perennial bunchgrasses such as squirreltail (Elymus elymoides). Previous theoretical work based on this system predicted that pathogens that spill over could favor either host coexistence, the exclusion of either host species, or priority effects, depending on species-specific transmission rates and pathogen tolerance. Here, I test these model predictions by parameterizing a population growth model with field data from Skull Valley, Utah.

Results/Conclusions

Across the observed range of demographic variation, the pathogen most often provides a net benefit to squirreltail and a net cost to cheatgrass, though both effects are relatively weak. Although cheatgrass is more tolerant, squirreltail is far less susceptible to infection, and its long-lived adult stage buffers population growth against seed losses to the pathogen. This work shows that despite pathogen spillover, the shared pathogen promotes native grass persistence by reducing exotic grass competition. Counter-intuitively, the reservoir host does not necessarily benefit from the presence of the pathogen, and may even suffer greater costs than the non-reservoir host. Understanding the consequences of shared pathogens for host communities requires weighing species differences in susceptibility, transmission, and tolerance using quantitative models.