Irruptive Dynamics of Sika Deer: Search for the Mechanism

To understand the process and mechanism of irruptive behavior, we reviewed previous studies on this behavior in two contrasting sika deer populations: an introduced population on Nakanoshima Island (NKI herd) and a naturally colonizing population on Cape Shiretoko (CS herd) and the consequences on the habitat over 30 years. Both populations had achieved peak abundance, followed by a crash (i.e., mass mortalityMass mortality) with the elimination of “surplus” foragesSurplus” forages, resulting in significant effects on the vegetation. However, there were marked differences in post-crash behavior between the two populations. Following the initial population reduction, the NKI herd recovered with a slower growth rate reaching a higher peak abundance than the first irruption, while the CS herd showed repeated irruptions and crashes with no decline in peak abundance. The NKI herd shifted their winter staple food from dwarf bamboo to woody vegetation and litterfallLitterfall after that time. The nutrition of these alternative resources decreased in winter; however, they maintained a high nutrition level in summer as buffering resources, restricting starvation or malnutrition, and maintaining high reproductivity and survival of adult female deer. In contrast to the NKI herd, the CS herd had high-quality foods available throughout the year after the initial population crash, particularly a large amount of Gramineae herbs in summer and a limited amount of dwarf bamboo in winter, which was preserved under the snow. The carrying capacityCarrying capacity differences between summer and winter and its combination with winter severity might generate oscillations while maintaining a high-quality population on CS. “Surplus” forages shape the initial irruption, and shifting to alternative resources, combined with snow accumulation generates various post-crash behaviors.