Fungicide alters amphibian disease dynamics and community structure in an aquatic system

Background/Question/Methods

Chytridiomycosis, an emerging infectious amphibian disease caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), has played a major roll in amphibian declines. Studies have also shown that widespread pesticide use contributes to declines in amphibian health. These causes are not mutually exclusive; thus, studies must examine the effects of multiple concurrent perturbations on amphibian declines. We conducted a field study to determine the effects of the fungicide thiophanate-methyl (TM) and Bd on Southern Leopard frog (Lithobates sphenocephalus) tadpoles and aquatic ecosystems. We exposed tadpoles to Bd in the lab and then placed them in aquatic mesocosms, which were then exposed to TM (at three environmentally relevant concentrations) on a weekly basis. Aquatic community measures were recorded weekly. Frogs were collected upon metamorphosis and life history traits were measured. Based on previous studies, we predicted that Bd and TM would act in opposition to directly affect amphibian health, while TM would alter aquatic community composition, possibly negating the direct effects of either pressure on the amphibians.

Results/Conclusions

Counter to our predictions, Bd mitigated the effects of TM by “protecting” tadpoles from TM-induced mortality, and both Bd and TM altered aquatic community composition. However, we conclude that alterations to the measured aquatic community variables were not the cause of tadpole mortality, as has been recorded in previous studies. TM also interacted with Bd to affect the size of new metamorphs. Bd-unexposed tadpoles that were exposed to the low and high levels of TM metamorphosed larger than those exposed to the mid level of TM, results that were not observed in Bd-exposed individuals. Moreover, TM altered aquatic community composition, resulting in trophic cascades and dramatic aquatic community alterations. This research illustrates that interactions between multiple perturbations such as pesticides and disease impact amphibian health and aquatic community composition and further research is necessary to elucidate such effects.