The sika deer (Cervus nippon) population on Nakanoshima Island (5.2 km²), Hokkaido showed repeated irruption which imposed irreversible changes on the vegetation. The sika deer reached to the first peak (52.5deer/km²) in 1984 and the second peak (83.5 deer/km²) in 2001 that was 1.6 times higher than the first peak. The population relatively remained stable at 45-59 deer/km² during 2008-2012. Under heavy grazing and browsing, deer eliminated dwarf bamboo during the first irruption and plum yew during the second irruption, thereafter fallen leaves were an important food in all season. The deer herd exhibited density dependent changes in life history trait, such as delayed sexual maturity, lower calf recruitment, and lower body mass. The body mass of adult female (≥3 yrs old) was related to pregnancy rate, which decreased as body mass decrease. The sika deer herd can tolerate long resource limitation due to the ability to utilize alternative resources and change life history trait of body mass and reproduction. This phenotypic plasticity is the main factor that the deer population continues to sustain high deer density under severe resource limitation.