The effects of community structure on host-pathogen dynamics of Batrachochytrium dendrobatidis

Background/Question/Methods

The chytrid fungus, Batrachochytrium dendrobatidis (Bd), is a pathogen associated with numerous amphibian population declines and species extinctions worldwide. Flagellated aquatic zoospores of Bd infect keratinized tissues of amphibians, including the jaw sheaths and tooth rows of larval hosts. Infection is lethal to larvae of certain species, or can cause sublethal effects, including reduced foraging efficiency, reduced growth and development, and altered behavior. We showed in a previous study that the zooplankter Daphnia pulex consumes Bd zoospores, a finding which may have significance for Bd control. To more fully understand the implications of consumption of Bd zoospores by zooplankton within a community, we investigated interspecific interactions among amphibian larvae, zooplankton, and Bd in a 2x2x2 fully factorial experiment. We crossed the presence/absence of larval Rana cascadae with the presence/absence of zooplankton, and the presence/absence of Bd. The experiment lasted for 10 weeks and took place in 120L outdoor mesocosms, which simulated the natural aquatic environment. We measured growth, development, survival, and infection status of amphibians and took weekly measurements of zooplankton abundance, phytoplankton concentration, and periphyton biomass. 

Results/Conclusions

Our hypothesis that zooplankton would directly benefit larval amphibians through consumption of Bd zoospores was unsupported: qPCR analysis indicated that zooplankton presence did not influence Bd infection status or intensity of larval amphibians. Our hypothesis that effects of consumption of Bd zoospores by zooplankton would reverberate through the food web was partially supported. Competition between larval amphibians and zooplankton for phytoplankton resources reduced phytoplankton concentration, zooplankton abundance, and survival of amphibians. Interestingly, competitive effects were diminished in the presence of Bd, suggesting that zooplankton may have at least partially switched from phytoplankton to Bd zoospores, thus stimulating competitive release. However, competitive effects between zooplankton and larval amphibians overshadowed indirect positive benefits of zooplankton predation on Bd zoospores. This result represents a complex interaction between competition and predation.