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 predictive model was constructed using information on species distribution collected by the Ministry of the Environment in 1978, 2003 and 2014, days of snow cover, forested and road areas, elevation, human population, and distance to the nearest occupied cell as covariates to calculate the probability of distribution change. Factors contributing to distribution expansion were elucidated and distribution expansion was predicted. Distance to the nearest occupied cell 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 negative effect and forest area and number of days of snow cover have positively affected. For wild boar, forest area, snow days and population had high importance. Predictions of future distribution showed that both species will be distributed nationwidely by 2050.