Competition between native ungulates: negative effects of sika deer (<i>Cervus nippon</i>) on foraging behavior, physiological stress, and population characteristics of Japanese serow (<i>Capricornis crispus</i>)

Abstract In coevolved native ungulate assemblages, the mechanisms underlying competition remain unclear because, to date, little direct evidence of competition exists. In high‐elevation areas in Japan, the number of native Japanese serows ( Capricornis crispus ) has been decreasing as the number of native sika deer ( Cervus nippon ) increases. Here, we assessed the effects of deer on vegetation, vigilance and foraging behavior, physiological stress, and relative abundance and ratio of young individuals of serows throughout the warm season across two study sites with different deer densities and similar landscapes in high‐elevation areas of Mount Asama, Central Japan. The abundance and diversity of palatable forage for serows (i.e., herbaceous forbs) were significantly lower in areas with abundant deer, suggesting that deer abundance reduced the vegetation quality for serows. Serow vigilance rate was significantly greater in areas with abundant deer, suggesting that the increased probability of encountering deer or of intraspecific aggression increased vigilance. Serow bite rate was significantly lower and their step rate was greater in areas with abundant deer, suggesting that lower availability of herbaceous forbs and higher vigilance reduced serow foraging efficiency. Levels of serow fecal cortisol metabolites were significantly greater in areas with abundant deer, suggesting that deer abundance and decreased foraging efficiency increased physiological stress of serows. The relative abundance and ratio of young individuals of serow populations were significantly lower in areas with abundant deer, suggesting that deer abundance negatively affected the serow population through both exploitative and interference competition. This is a rare case to show direct evidence and mechanisms of interspecific competition in native ungulate assemblages. Human alteration of ecosystem processes, such as the elimination of top predators and decreased human hunting pressure on deer, may determine changes in ecological relationships between serow and deer.