The relationship between biodiversity and disease risk is complex and multi-faceted. Generally, it is predicted that communities high in biodiversity will experience lower disease risk compared to low-diversity communities. This relationship, referred to as the “dilution effect,” may be driven by changes in host density that occur as a consequence of altered biodiversity. In amphibians, the emerging infectious disease, chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), is responsible for many population declines worldwide. Despite the continued spread of Bd, its relationship with host diversity is unknown. This study experimentally manipulated host species richness (a taxonomic measure of biodiversity) and the density of a focal species, the western toad (Anaxyrus boreas), in the presence of Bd. Larvae of A. boreas were exposed to Bd at varying densities and combined with two other co-occurring amphibian host species (Pseudacris regilla and Rana cascadae).
Results/Conclusions
We quantified disease risk in five ways: infection prevalence (percent infected) for our focal species, infection prevalence for all species, infection severity (concentration of Bd infection) for our focal species, infection severity for all species, and total infection (the sum of Bd infection). We found a dilution effect where increased species richness decreased disease risk in 4 of the 5 measures. These trends were not driven by changes in host density. Total infection was the only measure of disease risk not predicted by species richness, and was best explained by the density of our focal species. These results indicate that global amphibian population declines and extinctions that reduce amphibian species richness may produce communities that are more at risk from Bd.