Early Spring and Early Vanishing Wetlands as Harbingers of the Future? The Climate Change Trap for Ephemeral Pond-Breeding Frogs

Abstract The decline of the northern red-legged frog, Rana aurora, in the Pacific Northwest has been attributed to invasive species, habitat loss, and climate change. Rana aurora lay eggs on emergent vegetation, and larval development occurs in shallow, often ephemeral wetlands. In 2015 and 2016 we investigated how habitat and water quality parameters influence breeding productivity and development of R. aurora. Early season temperatures in southwestern British Columbia during both years were warmer than in the previous two decades and egg-laying occurred much earlier than any previous records, allowing us to follow the development of R. aurora under unusual climatic conditions. We monitored 43 wetlands in the Little Campbell River Watershed and nearby areas in 2015 and selected six sites to monitor tadpole metamorphosis. Peak egg mass abundance was inversely proportional to concentrations of nitrates and orthophosphates, and colder water appeared to support greater egg mass abundance. Although warmer ambient temperatures facilitated earlier onset of R. aurora breeding, we observed some egg mortality due to subsequent frost. Sites with earlier peak egg mass abundance dates did not exhibit earlier peak metamorph dates, nor did warmer water appear to facilitate quicker larval development. In 2015, one wetland dried up before tadpoles completed metamorphosis. The frog's reproductive cycle may adapt to warmer temperatures to some degree, but erratic precipitation or frost events could reduce R. aurora survivorship beyond recovery. The disruption of larval development we observed under unusually warm spring conditions provides a useful vantage point for predicting future impacts of climate change.