Occupancy of breeding sites by wood frogs determined by winter conditions

Background/Question/Methods

Climate change portends changes in the timing, frequency, duration, or intensity of weather events.  When assessing impacts, vulnerabilities, and adaptation to climate change, it is important to discern which impacts produce vulnerabilities, and when or in which life stages vulnerabilities occur.  Our overall goal is to identify how weather conditions in key periods of amphibian life-cycles (breeding, post-breeding, and over-wintering) affect occupancy of breeding sites and population persistence at regional scales.  Here, we determine whether winter weather conditions influence occupancy at breeding sites.  We tested several hypothesized mechanisms for how winter conditions may reduce survival of adult or juvenile wood frogs (Lithobates sylvatica).  We predicted that increased frequency of winter thaws, shorter duration of snow cover, or freeze events in spring would reduce occurrence at breeding sites.  Furthermore, if winter conditions affected adult survival then reduced breeding would be detected that spring.  If winter conditions affected juvenile survival, then we expected a lag effect with reduced occurrence in the second spring.  We used daily weather data from Climate Data Online to model frozen ground conditions throughout Wisconsin and wood frog occurrence information from a 25-yr, statewide call survey of >100 routes in Wisconsin.  We paired frog occurrence information on each route with our estimates of historic daily frozen ground conditions in an occupancy model framework. 

Results/Conclusions

The energy reserve depletion hypothesis was the most supported hypothesis.  We found that duration of winter thaw events is the best predictor of occupancy at spring breeding sites the following spring.  While much research on amphibian response to climate change is focused on shifts to breeding phenology, we demonstrate that occupancy of breeding sites can be influenced by winter conditions during the non-breeding season.  Thus, knowledge of climate change vulnerabilities during all life stages and all seasons within an annual cycle are important for developing conservation actions.