Changes in the population dynamics of Japanese serow and sika deer as a result of competitive interactions in the Ashio Mountains, central Japan

Abstract

Changes in the population dynamics of the Japanese serow Capricornis crispus and the sika deer Cervus nippon were studied in the Ashio Mountains of Nikko National Park, Tochigi Prefecture, Japan, from March 1982 to December 1995. Although the geographical distributions of the two species overlap in many areas of Japan, wherever the snow is not deep, Japanese serow and sika deer are usually found living allopatrically. In Ashio, however, they occurred sympatrically, but their populations have changed drastically and they have segregated during the 1980s and 1990s. I propose two factors that may contribute to their shift from sympatry towards an allopatric existence. The primary contributing factor may be the overlap in the food habits of the two species. Japanese serow were found to feed on the leaves of deciduous trees and herbaceous shrubs during the summer, while in winter they feed mainly on the needles of coniferous trees. In contrast, sika deer were found to feed mainly on graminoid species year round. Only in severe winters did the deer extend their diet to include the needles of coniferous trees, and the twigs and bark of deciduous trees. During such winters their diet then overlaps that of the serow, and this overlap may result in a rapid depletion of limited food supplies. The deer, with their more flexible dietary habits may be better able to tolerate such poor foraging conditions, whereas the serow may suffer from an unbalanced diet and face a deficiency of food thus forcing them to move away from areas where deer are present. A secondary cause may be the increased interactions between the species. Serow are territorial and essentially solitary, and so are presumed to be sensitive to both intra-and inter-specific crowding, whereas sika deer are not territorial, occur in herds and are usually very tolerant of other animals. In order to avoid encounters with sika deer, the sensitive serow may emigrate from areas with high deer population densities. This competitive/avoidance relationship between the Japanese serow and sika deer may help to explain why the two species generally occur allopatrically in Japan.