The pesticide enigma: Varying effects of a fungicide and chytrid fungus on Southern Leopard frogs

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 laboratory and field studies to determine the interactive effects of the fungicide thiophanate-methyl (TM) and Bd in Southern Leopard frogs (Lithobates sphenocephalus). In the lab, we exposed tadpoles to water control, Bd, TM, or TM*Bd treatments. We measured life history traits as both larvae and adults. In the field, we replicated similar treatments in aquatic mesocosms while also implementing a high/low density treatment. Leaf litter, zooplankton, and snails were added to each tank to facilitate nutrient turnover. Tadpoles were added to the mesocosms after hatching and collected upon metamorphosis. In addition to metamorphic life history measures, we recorded final zooplankton abundance and diviersity as well as periphyton mass in each tank to test for the effects of TM on aquatic community parameters. We predicted that independent Bd or TM exposure would reduce life history traits while synergistically interacting to further reduce amphibian health.

Results/Conclusions

TM facilitated increases in tadpole mass, length, and shortened developmental times in the laboratory. Additionally, individuals exposed to both TM and Bd were heavier, larger, and developed faster compared to all other treatments. TM also cleared Bd in infected larvae. In the field, there was 100% anuran mortality in all TM+ tanks, independent of Bd exposure. Between control and Bd treatments, Bd+ individuals were lighter, shorter, and had longer developmental periods. We observed a significant Bd*density interaction on mass and SVL. At high densities, Bd+ individuals were lighter and shorter, but at low densities the opposite interactions were observed. There was a significant effect of TM on periphyton abundance with TM+ tanks having more periphyton than unexposed tanks. Zooplankton abundance and diversity was also affected by Bd, TM, and density. This disparity between lab and field results suggests that further research is necessary to delineate the direct and indirect effects of TM on anurans and overall aquatic community structure.