Caribou calf production and seasonal range quality during a population decline

To address the importance of winter and summer ranges to the productivity of caribou (Rangifer tarandus), we studied the Southern Alaska Peninsula Caribou Herd (SAPCH). The SAPCH declined by >80% between 1983 and 1993, and concurrently experienced a decline in July calf:cow ratios from 33:100 preceding the population peak in 1983 to 14:100 in recent years. The SAPCH comprises 2 groups that use distinct seasonal ranges, and calf production was consistently higher in i group from 1989 to 1992. We hypothesized that higher productivity in that group was related to greater availability and quality of forage on either its winter or summer ranges, or both. Whereas lichens were scarce throughout the SAPCH range, lichen biomass, cover, and gross energy content (kJ/g) were lower on the winter range of the more productive group. In contrast, herbaceous forage was more abundant on the summer range of the more productive group, where the plant growing season commenced 1 month before calving and lasted nearly 1 month longer compared to the summer range of the less productive group. Finally, during the 1992 calving season, gray wolves (Canis lupus) and brown bears (Ursus arctos) occurred with equal frequency on each range. Thus, low productivity in the SAPCH may have been influenced by scarcity of lichens on winter ranges, but differences in productivity between the groups were apparently related to quality of summer ranges. We suggest that lower productivity in 1 group reflects greater perinatal mortality of weak calves born to females that were unable to make summer gains in body condition before moving onto winter ranges with low lichen availability.