Diurnal versus nocturnal surveys for foothill yellow-legged frogs

Abstract

Foothill yellow-legged frogs (Rana boylii) are a declining species, and methodologies for assessing population occupancy have been associated with only diurnal work on the species. Diurnal visual encounter surveys are typically used to determine presence within suitable habitat; however, studies of ecologically similar species indicate a potential advantage of nocturnal surveys. At 5 sites in 4 counties in California, USA, we compared diurnal and nocturnal surveys (n = 53 paired surveys) for foothill yellow-legged frogs, conducted 2016-2023, to determine the value of each survey period. Generally, occupancy probabilities increased 1–22 times, detection probabilities 1–69 times, and estimated abundances 3–12 times during nocturnal versus diurnal surveys. Associated standard errors were also lower in nocturnal versus diurnal surveys. However, we noted 2 locations where diurnal surveys yielded higher detection probability or higher estimates densities compared to nocturnal surveys. We suggest that both diurnal and nocturnal surveys be required when assessing occupancy of foothill yellow-legged frogs.