Effects of life stages on the occurrence of two frog species on paved roads in northeastern Japan

Amphibian roadkill mitigation has primarily targeted adult individuals; however, it is important to also focus on juveniles to restore populations and to understand the actual effects of roadkill on other species traits, such as demographic or metapopulation dynamics. This study aimed to determine the effects of different life stages on the occurrence of two frog species with different ecological characteristics on paved roads. We conducted transect surveys in the Shonai region of northeastern Japan to search for adult and juvenile Pelophylax nigromaculatus (Hallowell, 1861) and Rana tagoi tagoi Okada, 1928 on paved roads. We analyzed the effects of the surrounding landscape within various radius buffers and weather conditions on the number of adult and juvenile individuals. The juveniles of both species were dependent on neighboring paddy fields or forest areas, whereas the effects of forest edges differ between life stages, and juveniles were less likely to occur on roads with complex landscapes. Additionally, the optimal spatial scale for juveniles was narrower than that for adults in both species. Our results suggest that ignoring life stages may lead to incorrect estimates when assessing roadkill risk for frogs and that future roadkill mitigation measures should consider juveniles as well as adults.