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 Québec, 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. Le rôle que les facteurs limitatifs jouent sur la dynamique de population et la condition physique des animaux peut être plus facile à détecter à la frange de l'aire de répartition d'une espèce. Nous avons profité d'un gradient de densité à la limite de l'aire de répartition du cerf de Virginie pour étudier les interactions trophiques entre cet herbivore et la végétation du sous-bois. Nous avons testé l'hypothèse selon laquelle la disponibilité de nourriture estivale dans les boisés détermine la taille des cerfs adultes par le truchement d'une prise alimentaire influencée par la densité des cerfs pendant la période de croissance. Nous avons prédit : i) un changement de la composition forestière résultant du broutement sélectif répété et ii) une relation négative entre la taille des cerfs pleinement développés et l'intensité de la