Landscape Variation in the Diet and Productivity of Reindeer in Alaska Based on Stable Isotope Analyses

Productivity of a managed grazing system is dependent upon both the grazing strategy of ungulates and decisions made by humans. Herds of domestic reindeer (Rangifer tarandus tarandus) graze on discrete ranges of the Seward Peninsula, Alaska with variable production rates. We show that the N-15 natural abundance of reindeer forages vary significantly and the delta N-15 value of collagen deposited in antler bone from spring until ossification was significantly correlated with the delta N-15 signature of the diet. Calf weight in June was related to isotopic signatures in antler and soft tissue of adult females, and was inversely correlated with the absolute differential between summer and winter serum delta N-15 values. This observation suggests that female reindeer with smaller calves had catabolized more body protein during winter than females in adjacent herds. Moreover, in herds with smaller calves, female reindeer consumed proportionately more shrubs in early spring resulting in a strong relationship between delta N-15 signatures of antler collagen deposited in May and calf weight. These data suggest female reindeer catabolizing relatively more body protein during winter may attempt to compensate by increasing consumption of high-protein catkins and leaf buds of shrubs during May. Herders with relatively smaller calves in their herds may be able to improve protein balance of reproductive females and thus increase calf productivity by increasing access to shrub habitats during spring.