Food Habits and Body Condition of the Sika Deer Population in the Tanzawa Mountains, Central Japan

We reviewed studies on population density, food habits, and body condition of the sika deer population in the Tanzawa Mountains, central Japan, to understand the backgrounds of sustaining the high-density deer population in this heavily browsed habitat. Since the culling program under the Kanagawa Prefecture sika deer management plan commenced in 2003, the estimated total number of sika deer within the entire area of the Tanzawa Mountains has shown a decreasing trend. However, in some areas where the deer density estimates locally exceeded 20 individuals/km2 in 2015, the understory vegetation was still deteriorating. An analysis of the diet composition of sika deer using rumen content samples collected during 2015–2016 revealed that the sika deer population largely depended on canopy subsidies such as fallen leaves in all seasons. These food items are known to be stably abundant even under intensive browsing pressure by sika deer. Moreover, the crude protein content values of the rumen content samples satisfied its minimum requirement estimates for maintenance even in the winter. Along with the progress of the culling program, winter body mass and hindfoot length of fawns in the Tanzawa Mountains increased. In the winter of 2014–2015, these body condition indicators reached a similar level to another Japanese sika deer population that lived in habitats relatively less deteriorated than the Tanzawa Mountains. We concluded that canopy subsidies were likely a contributing factor in sustaining the high-density sika deer population in a heavily browsed habitat.