The number of Japanese deer has been increasing for the following reasons: Warm winter climates in recent years have decreased the number of deaths of young or old deer that are vulnerable to severe winter climates, there exists no animal to prey on them in their natural environment, and hunting is prohibited in the research forest.
また、ニホンジカが、近年の暖冬傾向で冬場に抵抗力のない子供や老年の個体が死亡する数が減っていることや天敵がいない自然環境に加え、研究林が禁猟区域であることから個体数が増加傾向にある。