Density-dependent disease, life history tradeoffs, and the effect of leaf pathogens on a suite of co-occurring close relatives

1. Plant pathogens reduce the performance of their hosts and therefore may contribute to ecological mechanisms of coexistence. In Chesson’s framework, pathogens contribute to stabilizing mechanisms when they intensify negative intraspecific interactions, such as density-dependent disease. Additionally, pathogens contribute to equalizing mechanisms when they reduce differences in performance among species. Life history tradeoffs predict higher susceptibility to pathogens in rapidly growing species, which could equalize performance among fast- and slow-growing species in the presence of pathogens.
2. In a coastal prairie in California, we studied the impact of leaf diseases on the performance of seventeen coexisting species of Trifolium and Medicago (“clovers”). We transplanted clovers in randomized arrays into the natural prairie community in three years of common garden experiments. We quantified infection rates by isolating fungi from leaves, and we measured disease severity as perce...