Effects of chronic browsing on life-history traits of an irruptive large herbivore population

Abstract

Highly abundant ungulate populations are widely observed in heavily browsed habitats; however, few studies have investigated the effects of chronic browsing on the life-history traits of large herbivores. We assessed the nutritional quality (crude protein [CP] contents) of staple foods, reproductive success, fawn recruitment, winter body mass, and body size in an introduced population of sika deer (Cervus nippon Temminck, 1938) on Nakanoshima Island, Lake Toya, Japan, which underwent repeated irruptions with shifting alternative foods and maintained high densities (>20 deer/km2). Our evaluation covered each irruptive dynamical phase, including initial irruption (Phase 1), second irruption (Phase 2), and subsequent phase (Phase 3). CP contents of staple foods (plum yew and litterfall in Phases 2 and 3, respectively) were at the same level as that of dwarf bamboo in Phase 1 during summer but were lower in autumn and winter, suggesting a decline in diet quality after initial irruption. The deer herd exhibited density-dependent changes in life-history traits, including delayed sexual maturity, lower fawn recruitment, and delayed body growth. The mean adult female (≥3 years old) body mass in Phases 2–3 was marginally above the 50% pregnancy rate threshold. The deer herd tolerated substantial declines in food quality, which involved trade-offs between survival, growth, and reproduction as adaptations to life-history traits. This phenotypic plasticity is the main factor in maintaining high deer densities under severe food resource limitations. This suggests that the sika deer population will not reach equilibrium with their food resources without hunting or predators.