The role of community assembly on disease prevalence: Could there be an amphibian dilution effect?

Background/Question/Methods

The relationship between biodiversity and disease prevalence has recently been a growing topic among disease ecologists. The “dilution effect”, wherein disease risk in a community is lowered by higher species richness, remains debated in part due to the numerous species combinations and host susceptibilities that can change the outcome of an observed disease risk. The existence of a dilution effect is often assessed via lab experimentation, where host communities, disease load, and environmental factors are manipulated by the observer. Field studies that compare disease prevalence to biodiversity may remove the subjective bias that lab experiments are prone to. Here, we use field collection methods to determine if a dilution effect is at play in a network of amphibian metacommunities within a large watershed. We performed 87 pond sampling events of 13 possible community compositions to determine: a) how amphibian communities are most likely assembled as species richness increases, b) if observed disease risk is negatively correlated with species richness along the community assembly progression, and c) which amphibian species are the most likely disease carriers. We used quantitative polymerase chain reaction (qPCR) to determine disease prevalence and intensity of the widespread chytrid fungus Batrachochytrium dendrobatidis (Bd) among individuals within ponds.

Results/Conclusions

Pond disease prevalence was mostly influenced by species richness, life stage (larval, metamorph, or adult) and the presence of California red-legged frogs (Rana draytonii). Eight out of the 13 pond species combinations had Bd infections, and six of those were ponds with California red-legged frogs. Among the 13 combinations sampled, we found two very similar community assemblage progressions (starting with one species per pond and ending at four species per pond) that were most likely to occur. Both likely progressions had a negative correlation between species richness and disease prevalence, thus suggesting a dilution effect. When comparing all 13 possible species combinations together, we again found evidence of a dilution effect, with single-species ponds having the highest prevalence and two, three, and four-species ponds having incrementally lower prevalence as species richness increased. Bd was found to have infected individuals of all five species sampled, but the California red-legged frog had significantly higher Bd loads and prevalence than any other species. As the main carrier of Bd, red-legged frogs may be acting as a reservoir host within the watershed and responsible for maintaining pond infections between seasons. Nevertheless, increased biodiversity across the watershed appears to keep infection outbreaks to a minimum.