Anthropogenic effects have propelled us into the sixth mass extinction and amphibians are among the groups most affected. The causes of global amphibian population declines are varied, complex, and context-dependent. Two stressors of particular importance, agricultural contaminants and diseases, may interact to contribute to amphibian population declines and extinctions. For example, sublethal levels of contaminants could negatively affect the immune system of amphibians, making them more susceptible to infectious diseases. We examined the combined effects of a pesticide (carbaryl), a fungal pathogen (Batrachochytrium dendrobatidis [Bd]), and interspecific competition between larvae of the pacific treefrog (Pseudacris regilla) and the Cascades frog (Rana cascadae) through a 2x2x3 factorial experiment conducted in replicated outdoor mesocosms. Carbaryl was applied every week at a concentration of 10ppb and Bd was applied every two weeks at a concentration of 2.08x10⁴ zoospores/L for 7 weeks. We assessed responses of early and late stage tadpoles separately by sampling animals near the midpoint and at the end of the experiment. Response variables included mass, snount-vent length (SVL), time to metamorphosis, developmental stage, and infection level, which was assessed via qPCR. We also documented the response of the pond community to experimental treatments by sampling zooplankton, phytoplankton, and periphyton.

Results/Conclusions

We found main effects of each of the three treatments on larval growth and development of both species. Carbaryl increased time to metamorphosis, mass, and SVL of both species early and late in the larval period. The abundance of herbivorous cladocerans was also significantly reduced in pools contaminated with carbaryl, which in turn had positive effects on phytoplankton. Exposure to Bd extended the larval period and reduced mass and SVL of R. cascadae early in the larval period. Preliminary results of qPCR analysis indicate that exposed R. cascadae exhibited higher levels of Bd infection than exposed P. regilla. The presence of interspecific competitors decreased time to metamorphosis and reduced mass and SVL of P. regilla both early and late in the larval period. The magnitude of the carbaryl effect was greater than the magnitude of the interspecific competition effect for P. regilla and similar in magnitude to the Bd effect for R. cascadae. No interactive effects of the treatments were observed, perhaps because carbaryl had effects opposite to those of Bd and interspecific competition.