Identifying decline mechanisms is often the first step in developing effective conservation strategies for threatened species. To assess two potential amphibian decline mechanisms, we studied a species of conservation concern in the northern U.S. (Blanchard’s cricket frog, Acris crepitans blanchardi). The climatic anomaly hypothesis suggests that unusually cold and dry winters have caused widespread mortality and localized extinction in this species because of its marginal over-wintering ability. The epidemic disease hypothesis suggests that the recent appearance of a novel pathogen (Batrachochytrium dendrobatidis) has been responsible for the decline of this once common species. To assess the climatic anomaly hypothesis for Blanchard’s cricket frog declines, we analyzed long-term data (1948 – 2003) on nine climatic variables from ten monitoring stations in central Ohio (where cricket frogs were historically abundant but are now largely absent) and ten monitoring stations in western Ohio (where cricket frogs remain relatively abundant). To test the epidemic disease hypothesis, we used the polymerase chain reaction (PCR) to amplify B. dendrobatidis DNA from cricket frog skins swabs. Skin swabs were taken from 21 sites both in areas where declines have occurred, as well as from areas where they have not.
Results/Conclusions
Historical information puts the beginning of the decline in Ohio in the 1960’s or 1970’s and this corresponds to the occurrence several unusually cold and dry winters. In addition, our results detected significant differences between the western and central portions of the cricket frog’s range in the mean number of days less than 0°C (p = 0.003) and the mean number of days less than -17.8°C (p = 0.012). Significant differences in climatic variability were also detected between regions. However, in all cases, these analyses indicated that climates in western Ohio were significantly colder and more variable than those in central Ohio. Our PCR assay detected the presence of B. dendrobatidis in Blanchard’s cricket frogs from areas where declines have occurred, but also from areas where declines have not been reported. In fact, all sites where ten or more individuals were sampled were infected with B. dendrobatidis. Thus, the spatial patterns of climatic anomalies and disease prevalence do not coincide with the spatial patterns of decline. While synergistic interactions between these variables cannot be ruled out, there is presently little evidence to implicate climate anomalies or disease epidemics as drivers of the decline of Blanchard’s cricket frog.
