Diversity and abundance of foliar parasites in an experimental plant metacommunity
Changes in local host diversity and resource supply to hosts may influence subsequent changes in the number and characteristics of other host species colonizing from the broader metacommunity, which in turn may also influence the abundance and diversity of parasites. To test this hypothesis, we planted 1m2 plots denuded of vegetation with seedlings of six native herbaceous species at two diversity levels – monocultures of six focal species and five-species polycultures lacking one focal species – totaling 12 planted community types. Planted richness and density were maintained through July 2012, then we manipulated resource supply to hosts (ambient, 10 g m-2 yr-1NPK). The plots were naturally colonized. In 2014, we visually quantified the cover of all plant hosts within each plot and the relative abundance of all parasite morphospecies infecting leaves. We predicted that initial host diversity and resource supply would indirectly affect parasite species richness and abundance through their effects on subsequent host community structure. We tested our predictions using a structural equation model (SEM), and here report the strongest path effects from the SEM.
The experimental manipulations of host diversity and resource supply to hosts had no direct effects on parasite species richness or parasite abundance. Instead, treatment effects on the parasite community were mediated by their influences on host community structure. The effect of resource supply on parasite abundance was mediated by colonization by exotic species. Specifically, fertilization increased the relative cover of exotic species in the host community (p<0.001, R2 = 0.27), which in turn increased parasite abundance (p<0.001, R2=0.13). The host diversity treatment had no indirect effect on parasite abundance, but indirectly influenced parasite species richness through its effect on plant species richness. Specifically, plant species richness was higher in plots that were originally planted as polycultures than plots that were planted as monocultures (p<0.001), and parasite species richness increased with plant species richness (p<0.001). Finally, an indirect effect of resource supply on parasite species richness was mediated by a negative effect on plant species richness (p<0.001) and its positive effect on parasite abundance (parasite species richness combined R2=0.32). Together, these results show that increasing resource supply to hosts can have strong effects on host community structure, which lead to predictable changes in parasite richness and abundance.