Plant pathogens reduce the performance of their hosts and therefore may contribute to ecological mechanisms that either increase or decrease the potential for coexistence. In Chesson’s framework, pathogens will contribute to stabilizing mechanisms when they intensify negative intraspecific interactions, and they will contribute to equalizing mechanisms when they reduce differences in performance among species. In a coastal grassland in California, we studied the impact of leaf diseases on seventeen coexisting species in the genera Trifolium and Medicago (collectively referred to as “clovers”), for which we also collected data on plant growth rate and relative abundance over 5 years of surveys.
We planted randomized arrays of the 17 species into the natural grassland community in 3 years of large-scale common garden experiments. We measured disease prevalence, and on a subset of plants, disease severity. We also used a fungicide experiment to measure the fitness effects of reducing foliar fungal attack. We assessed the potential role of foliar disease in negative density-dependence by testing whether disease on outplants was positively related to abundance in the community. We looked for life history tradeoffs by testing whether there was greater susceptibility to pathogens in the most rapidly growing species.
Results/Conclusions
We found modest evidence that disease was related to background levels of local abundance, but only in one of three years. The effect of fungal infection as measured by response to fungicide was not higher in more abundant species. Contrary to expectation, we found that disease was not higher in the larger, faster-growing species. However, those faster-growing species showed a significantly lower effect of fungal infection on growth, as measured by the fungicide experiment. This result may signal greater tolerance to infection and is consistent with predictions about plant growth strategies. Our results suggest that the combination of density dependent processes and life-history tradeoffs together may shape the impacts of disease that can facilitate species coexistence.