Testing differences in physiological tolerance between invasive bullfrog tadpoles (Rana catesbeianus) and non-invasive green frogs (Rana clamitans)

Physiological traits may play a key role in the establishment of non-native species in a new habitat. Still, there are few empirical studies that link physiological traits with invasion success. The American bullfrog, Rana catesbeianus, and the green frog, R. clamitans, are functionally and morphologically similar congeners that largely occupy the same native range in the eastern United States, but have drastically different invasion histories. Bullfrogs have established in much of the western United States, which is generally much hotter and drier than their native range. We hypothesized that intrinsic differences in heat and desiccation tolerance in the tadpole stage may explain the contrasting invasion histories of the two species. We tested this hypothesis by measuring critical thermal maximum (CTMAX), thermal acclimation capacity, and hydric stress tolerance in bullfrog and green frog tadpoles occurring in their native habitat. We found that species do not have intrinsic differences in thermal tolerance, but bullfrog tadpoles have slightly greater thermal acclimation capacity than green frogs. Green frogs are more sensitive to warmer temperatures but can withstand greater hydric stress. In addition to being stronger competitors, we surmise that other factors, such as rapid evolution of thermal tolerance or combinations of responses to novel abiotic and biotic conditions may ultimately predispose bullfrog tadpoles to successfully persist in hotter and drier habitats in the western U.S.