Linking manipulative experiments to field data to test the dilution effect

Background/Question/Methods

The dilution effect, the hypothesis that biodiversity reduces disease risk, has received support in some systems. However, few dilution effect studies have linked mechanistic experiments to field patterns to establish both causality and ecological relevance. We first conducted a series of laboratory experiments and tested the dilution effect hypothesis in an amphibian-Batrachochytrium dendrobatidis (Bd) system. We then tested for consistency between our laboratory experiments and field patterns of amphibian species richness, host identity, and Bd prevalence across the U.S., a scale to which the dilution effect has never been tested. 

Results/Conclusions

We show that tadpoles can filter feed Bd zoospores and that the degree of filter feeding was positively associated with their dilution potential. The obligate suspension feeder, Gastrophryne carolinensis, generally diluted the risk of chytridiomycosis for tadpoles of Bufo terrestris and Hyla cinerea, whereas tadpoles of B. terrestris (an obligate benthos feeder) generally amplified infections for the other species. In addition, G. carolinensis reduced Bd abundance on H. cinerea more so in the presence than absence of B. terrestris and B. terrestris amplified Bd abundance on H. cinerea more so in the absence than presence of G. carolinensis. When ignoring species identity, species richness was a significant negative predictor of Bd abundance. Amphibian species richness was a significant negative predictor of Bd prevalence at the scale of the entire U.S. Consistent with our laboratory findings, the presence of Bufo spp. was a significant positive predictor of Bd prevalence whereas the presence of Hyla spp. and Gastrophryne spp. were negative predictors of Bd prevalence. The results from our laboratory and field data support the dilution effect hypothesis and also suggest that dilution and amplification are predictable based on host traits.