Large-scale trophic interactions:: White-tailed deer growth and forest understory

The role that limiting factors play in population dynamics and the physical condition of animals may be easier to detect at the edge than the centre of a species' range. We took advantage of a density cline at the periphery of distribution range of white-tailed deer to study trophic interactions between this herbivore and understory vegetation. We tested the hypothesis that the availability of summer forage in woodlands for this selective browser determines adult body size through density-dependent forage consumption during growth. We predicted 1) a change of forest composition resulting from persistent selective browsing; and 2) a negative relationship between the size of fully grown deer and the intensity of forage competition in summer. We estimated deer population density, carcass mass at maturity, and standing biomass of vegetation in 13 zones throughout the range of the deer in Quebec, which covers 128,000 km(2). Asymptotic carcass mass was related to body length, and to a lesser extent to percent body fat, and reflected size of fully grown deer. Plant biomass varied with respect to deer density, but the direction and intensity of the relationship depended on deer preference for various plant groups. Occurrence of preferred taxa decreased with increasing deer density. Forage competition in summer was related negatively to deer size, which declined sharply when there was < 10,000 kg of forage per deer. In the trade-off between growth and body reserves, the size of mature deer is likely adjusted to forage intake in order to balance absolute nutrient needs, a response allowing increased foraging efficiency of depleted resources.