Effects of host diversity and resource availability on foliar parasite diversity

Background/Question/Methods

Host diversity and resource supply to hosts play important roles in determining parasite prevalence in natural systems, yet their importance in shaping the diversity of parasite communities remains poorly understood, in part due to the metacommunity structure of parasite assemblages. Within host communities, increased host diversity can increase parasite species richness (PSR) by increasing the potential parasite species pool. Within host individuals, processes that increase parasite prevalence can also increase the likelihood of co-infection. Thus increased resource supply to hosts may increase PSR, and higher host diversity may decrease PSR via decreased prevalence of specialist parasites. Alternatively, increased resource availability to pathogens within hosts may lead to a decrease in PSR by shifting competitive outcomes between parasites. In an old field (Duke Forest, Durham, NC) dominated by herbaceous plants, we manipulated host diversity (1, 5 species) and soil resources (+NPK, control) across a pool of 6 native, perennial host species. We visually estimated percent damage to leaf tissue and classified each parasite by host symptoms. We expected host community-level PSR to be lowest in monocultures and highest in 5-species mixtures. Within host individuals, we expected PSR to be lowest in unfertilized polycultures and highest in fertilized monocultures.

Results/Conclusions

Neither host diversity nor soil resource availability significantly predicted total parasite prevalence. However, within plots, host species exhibited significant differences in parasite prevalence (F_5,116=7.7815,  p<0.001). At the host community level, PSR was significantly higher in 5-species mixtures than in monocultures (F_1,116=123.94, p<0.001). This was not surprising, since 20 of 27 parasites observed only infected one host species. However, among parasites infecting multiple host species, there was still a positive relationship between PSR and host diversity (F_1,116=47.466, p<0.001). Host species also differed significantly in PSR, (F_5,116=59.30, p<0.001), and within each host species, PSR did not respond to host diversity treatment. This supports the hypothesis that higher PSR in 5-species mixtures is the result of sampling from a larger parasite species pool.  As predicted, PSR within individual hosts was positively correlated to total parasite prevalence (F_118,1244=3.48, P<0.001). PSR within individuals did not respond to host diversity treatment (F_1,116=1.53, p>0.3), and was significantly lower in fertilized plots than in unfertilized plots (F_1,116=23.09, p<0.001). Since neither host diversity nor fertilization influenced parasite prevalence, this decrease indicates that increased resource availability within hosts may shift competitive outcomes between parasites, decreasing PSR within hosts.