Density estimation for an island population of raccoon dogs in Japan using fecal DNA

Estimation of the population size is essential for understanding population dynamics. Estimating animal density using multiple methods and/or multiple attempts is required for accurate estimations. Raccoon dog Nyctereutes procyonoides is native to East Asia, including Japan, and has become an invasive species in Europe. Information on raccoon dog density in their native range is important to understand their invasion; however, relatively few studies have been conducted on raccoon dog density in their native range. In this study, we extracted DNA from fecal samples of raccoon dogs inhabiting a small island in Japan and conducted density estimation over two periods using DNA capture‐recapture methods: CAPWIRE and SECR. We also investigated sex ratio using genetic sex identification. Density estimates using SECR were approximately threefold different between the two study periods: 17.2 individuals per km2 in 2018 and 49.0 individuals per km2 in 2020. In contrast, estimates using CAPWIRE were relatively stable: 21.7 individuals per km2 in 2018 and 24.3 individuals per km2 in 2020. A drastic increase or decrease is not expected during the study period, and thus, density estimates using CAPWIRE are more reasonable than those using SECR. The small number of samples per individual might result in low accuracy of density estimates by SECR. The density estimated by CAPWIRE was similar to that in the main island in Japan and higher than that in Europe. Feeding competition with other omnivorous carnivores and/or predation risk by wolves might maintain the low density in Europe. The sex ratio of raccoon dogs was 1:1, which was similar to the values in invasive raccoon dogs and other canids. Further genetic census, including sex identification in various landscapes in their native and invasive range, will enable us to understand not only the ecology of raccoon dogs but also their adaptations to their invading areas.