Large ungulates will be present in most of Japan by 2050 owing to natural expansion and human population shrinkage

The aims of this study were to elucidate factors contributing to the expansion of the distributions of sika deer and wild boar in Japan and to predict the expansion of their distributions by 2025, 2050, and 2100. A site occupancy model was constructed using information on species distribution collected by the Ministry of the Environment in 1978, 2003, 2014, and 2018, days of snow cover, forested and road areas, elevation, human population, and distance from occupied grid cells as covariates to calculate the probability of distribution change. Factors contributing to distribution expansion were elucidated and distribution expansion was predicted. Distance from occupied grid cells had the strongest influence on distribution expansion, followed by the inherent ability of each species to expand its distribution. For sika deer, human population had a strong negative effect and elevation and number of days of snow cover were important. For wild boar, forest area and elevation had high importance. Predictions of future distribution showed that both species will be distributed over 90% of Japan by 2050 and over 100% by 2100.