Wildlife mortality caused by vehicle collisions ("roadkill") is recognized as a major anthropogenic impact and a significant challenge in wildlife conservation. In the northern part of Okinawajima Island, located in the central Ryukyu Archipelago of Japan, the roadkill threatens the populations of endemic species such as the Okinawa rail (Hypotaenidia okinawae, Rallidae) and the Ryukyu long-furred rat (Diplothrix legata, Muridae). However, the impact and patterns of roadkill on other terrestrial vertebrates, such as amphibians and reptiles, remain unclear.

To address the impact of the roadkill on terrestrial vertebrates, we conducted a nighttime survey (for ca. 3 hours starting 2 hours after sunset) for 95 days from February 2024 to January 2025 in Yambaru National Park in Kunigami Village, Okinawa Prefecture. The study encompassed a total survey distance of 6,042 km along a 31.8 km road segment spanning Okinawa Prefectural Routes 2 and 70, which traverse the core zone. We recorded the frequency of roadkill, and then analyzed species composition, the detection rate of roadkill per surveyed km, and the spatiotemporal patterns of roadkill. In addition to the above analyses, we examined the relationship between the roadkill frequency and environmental variables (mean temperature and mean humidity).

During the survey, we recorded 908 carcasses by roadkill, the majority of which were amphibians (756 individuals, 83.3%). Moreover, we found 0.150 roadkill per surveyed km, which is considered a high impact compared to previous research in other countries. The results suggest that roadkill may exert some ecological pressure on amphibians in our research area. Moreover, we found clear hotspots in the spatial distribution of roadkill, and that the roadkill frequency changed with the seasons (lower in summer and higher in winter). GLM detected a statistically significant negative effect of temperature on roadkill.

In our survey, the two most frequently road-killed species were the Ryukyu tip-nosed frog (Odorrana narina, Ranidae) (253 individuals, 27.9%) and the Okinawa green tree frog (Zhangixalus viridis, Rhacophoridae) (223 individuals, 24.6%). While both showed similar spatial distributions, the former occurred throughout the year except winter, whereas the latter was limited to fall and winter. Temperature and humidity positively affected the former, while temperature negatively affected the latter.

Our results represent the first comprehensive assessment of terrestrial vertebrate roadkill in the northern part of Okinawajima Island. To develop conservation strategies to mitigate roadkill impact, future investigations should focus on factors explaining the local hotspots of roadkill．