Japan's sika deer population has roughly doubled over the last 20 years, creating a surge in landslides caused by over-grazing [1].
This ecological shift threatens human safety and infrastructure. As deer strip away ground-cover plants, the soil loses the root systems necessary to remain stable during heavy rains, transforming forests into hazardous zones.
Researchers, including Hayato Abe of Kyushu University, found that deer are consuming vegetation across entire mountains [1]. Abe said the animals eat the leaves growing from the ground, which causes soil to erode and roots to become exposed. This process weakens trees, making them susceptible to final collapse when typhoons or lightning strike [1].
In Nara Park, the impact of population growth is evident. The deer count there was approximately 1,200 head 10 years ago but has risen to over 1,600 head [1]. The resulting lack of vegetation leaves mountains "naked," stripped of the greenery that prevents erosion [1].
The consequences have already manifested in deadly terrain. On one surveyed mountain, three landslides were reported in July 2026 [2]. In Yonehara City, located in Shiga Prefecture, a deer-related landslide prompted an emergency safety order for 313 people [3].
Field investigations this week in Miyazaki Prefecture and other regions confirm that this is a nationwide issue. The loss of biodiversity, and soil stability, indicates that the deer population has exceeded the land's carrying capacity [1, 2].
“Deer numbers have roughly doubled in the past 20 years.”
The crisis highlights a failure in wildlife management and a dangerous feedback loop between ecological imbalance and natural disasters. By removing the primary biological defense against erosion—root-stabilized soil—the deer population is effectively priming the Japanese landscape for more frequent and severe landslides as climate-driven typhoons intensify.
