How extreme drought events, introduced species, and disease interact to influence threatened amphibian populations

Over the past century, native amphibian species in California, USA, have declined in geographic range and population size, including precipitous declines of the California Red-legged Frog Rana draytonii Baird and Girard, 1852 and the California Tiger Salamander Ambystoma californiense Gray, 1853. Habitat loss, infectious disease (e.g., chytridiomycosis), and the introduction of invasive species are potential contributors to this decline, but it is unclear how these variables interact with extreme weather events, such as prolonged drought to affect amphibian population dynamics. We used a 7-y dataset (2013-2019) on amphibian breeding activity that encompassed a recent extreme drought event to assess 1) how breeding activities of these taxa differed during and after drought conditions and 2) the additive and interactive relationships of drought, chytrid fungal infection (Bd), and invasive species with the presence and abundance of larval amphibians. Based on 650 surveys of 110 lentic habitats in California, both amphibian species were highly sensitive to drought severity, such that the proportion of occupied sites during peak drought conditions decreased by 36% for R. draytonii and 87% for A. californiense relative to the initial year of the study. However, both taxa showed strong recovery, with increases in breeding occurrence as drought conditions eased in the following years (2016-2019). Invasive fishes and American Bullfrogs (Rana catesbeiana Shaw, 1802) also negatively predicted the occurrence and density of the 2 native taxa. Finally, we found that the relationship between Bd and R. draytonii larval density was influenced by drought conditions, suggesting that Bd's transmission rates or effects on host persistence are sensitive to climate. These findings illustrate the ability of native taxa to recover from recent drought conditions and reinforce the importance of controlling invasive species for improving breeding activities of threatened amphibians.