Effects of sika deer (Cervus nippon yakushimae) population growth on saplings in an evergreen broad-leaved forest

An increase in size of sika deer populations has caused severe damage to forest vegetation in many regions of Japan. In a Natural World Heritage area on the western part of Yakushima Island, existing high densities of deer are thought to be increasing, but the impact of deer on forest vegetation has not been estimated. Roadside censuses of deer between 1988 and 2006 showed that encounter rates are continuing to increase. Long-term monitoring of individual tree saplings in a 0.13 ha area of the forest between 1992 and 2005 showed that many saplings still remain despite increases in sika deer density. However, differences in growth and mortality rates of preferred, neutral (neither preferred nor avoided) and avoided sika deer food species varied over time. As a group, preferred species continuously had a low growth rate, while their mortality rate increased through time. The growth rates of the neutral and avoided groups decreased especially in the second half of our study period at the same time that their mortality rates increased. These data suggest that in spite of food species preference, direct feeding pressures as well as indirect physical effects by increasing sika deer density are affecting growth and mortality of tree saplings. Clearly, it is difficult to predict how plant communities will respond to possible continued sika deer population growth, and it is therefore important to continue monitoring these forest communities in the interest of preserving the unique ecosystems on Yakushima Island.