The Individual and Combined Effects of Warming and Atrazine on <i>Lithobates pipiens</i> Phenotypes: Implications for Frog Declines

Amphibians are the most threatened vertebrate class globally. Climate change, agrochemicals, and/or pathogens and parasites are implicated in contributing to amphibian declines, either singly or in combination. We investigated individual and combined effects of elevated temperatures and atrazine (2.0 μg/L) on Lithobates [formerly Rana] pipiens phenotypes, in a mesocosm experiment. We sampled tadpoles after 2 weeks, and other individuals at the completion of metamorphosis for endpoints relative to development, locomotor performance, immunity, and stress. Temperatures ranged from 7.18°C to 31.27°C over the experimental period, with a significant ~2°C difference between temperature treatments: warming and ambient. Whereas we found no effect of atrazine alone, we found strong effects of temperature, and some evidence of an interaction between atrazine and temperature on various phenotypic attributes. In tadpoles, elevated temperatures were associated with increased growth, accelerated development, and may have reduced stress, but decreased locomotor performance. Elevated temperatures also interacted with atrazine, offsetting an atrazine-mediated delay in tadpole development. In metamorphs, elevated temperatures accelerated development at the cost of reduced size, but did not influence locomotor performance. However, warming was associated with lowered immunity, reflecting a trade-off between growth and immune function. Elevated temperatures and atrazine also combined to affect metamorph neutrophil to lymphocyte ratios, reducing immunocompetency. Our results highlight the importance of incorporating multiple environmentally relevant stressors, thought important to amphibian declines in ecotoxicological studies, and of assessing multiple developmental stages.