Technical Bulletin No. 0934: A Review of Ungulate Nutrition and the Role of Top-Down and Bottom-Up Forces in Woodland Caribou Population Dynamics

Management strategies to conserve populations of woodland caribou Rangifer tarandus caribou frequently emphasize the importance of predator-prey relationships and availability of lichen-rich late-seral forests, yet the importance of summer diet and forage availability to caribou survival is poorly understood. We synthesized published information on the diet and nutritional needs of Rangifer populations to evaluate the importance of vascular forage, as well as potential interactions among forage and climate, disturbance, and predation in affecting caribou survival and reproduction. We also reviewed studies on the population dynamics of woodland caribou in North America to assess the relative importance of top-down and bottom-up factors in the decline of woodland caribou populations. Populations of forest-dwelling woodland caribou that share range with predators and alternate ungulate prey typically occur at low densities and evidence of density-dependent food limitation is not apparent. Predation is generally considered an important proximal factor limiting woodland caribou populations; however, our review suggests that methodological limitations in existing studies prohibit proper evaluation of the mechanism of decline and fail to elucidate potential interactions between top-down and bottom-up effects on populations. Our review of caribou dietary needs highlights the importance of vascular summer forage in the growth and reproduction of caribou. Diet may be seasonally important in relation to snow accumulation, reproduction, and landscape-scale disturbances that change the availability of preferred food items. Based on current knowledge, forest management is expected to have a greater potential to affect the availability of winter forage than summer forage.