Occurrence of California Red-Legged (Rana draytonii) and Northern Red-Legged (Rana aurora) Frogs in Timberlands of Mendocino County, California, Examined with Environmental DNA

Abstract

Effective species management requires knowledge of species distributions, but surveys for cryptic species near the boundaries of their geographical ranges can be difficult. We used environmental DNA (eDNA) and occupancy modeling to examine the distribution of Northern Red-legged Frogs (Rana aurora) and federally threatened California Red-legged Frogs (Rana draytonii) in a sample of 60 forested stream sites near where their ranges meet in southern Mendocino County, California, USA. For both species, the probability of occurrence (ψ) in forest streams in our study area was very low: California Red-legged Frog ψ was <0.01 (95% credible interval = <0.01–0.05), and Northern Red-legged Frog ψ was 0.07 (0.02–0.15). DNA from both species was found at 1 pond site, suggesting either co-occurrence or introgression. Our results suggest that abundance, stream use, or both are very low for red-legged frogs in forested streams in southern Mendocino County.