Diurnal Versus Nocturnal Surveys for California Red-Legged Frogs

Abstract

The United States Fish and Wildlife Service (USFWS) federally listed the California red-legged frog (Rana draytonii; Shaffer et al. 2004) as a threatened species in June 1996 (USFWS 1996). The listing justification stated that a variety of human activities, including urban encroachment, construction of reservoirs and water diversions, introduction of exotic predators and competitors, livestock grazing, and habitat fragmentation, had extirpated redlegged frogs from 70% of their former range and continue to be a threat (see also Fellers 2005). Because the Endangered Species Act of 1973 now protects this frog, biologists must conduct surveys prior to any federally permitted habitat modifications or any activity on federal lands that has the potential to affect this species. In February 1997, USFWS provided guidelines for conducting red-legged frog surveys, including general guidelines for conducting both diurnal and nocturnal surveys (USFWS 1997). The USFWS updated these guidelines in 2005 (USFWS 2005) and gave detailed requirements for both the qualifications of biologists conducting surveys and the techniques for carrying out field surveys. The goal of our study was to compare diurnal and nocturnal surveys for California red-legged frogs to determine whether there was a difference in detections. We conducted paired diurnal and nocturnal surveys for adult and subadult California redlegged frogs at sites in the California Coast Range and Sierra Nevada foothills.