Temperature Variability and Salt Pollution Interact to Alter Subsequent Multi‐Parasite Susceptibility in Larval Amphibians

ABSTRACT Wild populations face unprecedented pressure from an assortment of anthropogenic environmental changes and parasites. We sought to understand how host–parasite interactions are affected by the interactive effects of multiple environmental stressors and subsequent parasite infection. We focused on American bullfrog tadpoles ( Lithobates catesbeianus ) that can become infected by several parasite species (e.g., trematodes in the family Echinostomatidae genus Echinstoma, and Batrachochytrium dendrobatidis ; “ Bd” ) and are affected by abiotic stressors including road salt and variable temperatures attributed to climate change. In a multi‐phase laboratory experiment, we exposed tadpoles to one of two salt treatments (0 and 1.5 g L −1 added NaCl sublethal salt) and one of two temperature treatments (constant 23°C and fluctuating between 20°C and 25°C). Next, we exposed these tadpoles to one of four parasite treatments (parasite absent, Bd only, trematodes only, and Bd & trematodes together) at a constant temperature (23°C) without added salt. We then recorded morphological measurements (length, mass, developmental stage) and quantified infection intensity. Tadpoles exposed to Bd were larger in mass than the other parasite treatments, and the effects of trematode exposure on length varied by temperature. We did not detect any effect of our treatments on developmental stage. We found an interaction between the salinity and temperature treatments where tadpoles exposed to Bd only or to fluctuating temperatures and Bd and trematodes together had higher Bd infection in the no salt added treatment compared to the sublethal salt treatment. Tadpoles in stable temperatures and no salt treatments had lower subsequent Bd infection loads than those in stable temperatures and elevated salt. With trematode infection, we found that when no salt was added, tadpoles exposed to trematodes only had higher parasite abundance compared to the other treatments (i.e., a salinity by parasite interaction). Our results suggest that the interaction of temperatures and salt pollution in freshwater can increase subsequent Bd infection and that the parasites ( Bd & echinostome trematodes) can interact with each other, altering infection levels.