Many high-density populations of wild ungulates have exerted strong negative impacts on their habitat. A decrease in forage quantity and quality may affect individual growth, fecundity, and survival, especially under harsh winter conditions. On Anticosti Island, Québec, Canada, browsing by introduced white-tailed deer (Odocoileus virginianus) negatively affected boreal forest composition. Since the preferred contemporary winter forage of deer, balsam fir (Abies balsamea), has been almost completely extirpated from the island at browse height, deer are forced to increase the proportion of normally avoided forage, such as white spruce (Picea glauca), in their winter diet. We predicted that an increase in the proportion of white spruce in the diet would have detrimental effects on deer body condition and would affect their behavior and life history traits. We experimentally simulated a deterioration of winter forage quality in semi-natural enclosures by increasing the proportion of white spruce in the diet and examined the effects of winter diet quality on 1) forage intake, 2) body condition loss, 3) activity budget, and 4) survival of white-tailed deer fawns. Fawns fed the poor-quality diet maintained a higher forage intake rate throughout the winter than fawns fed the control diet, suggesting a compensatory response to the decrease of forage quality by consuming more forage during winter. Body mass decreased over the winter, but we did not observe any significant effects of diet quality on body mass loss. Diet quality did not influence the activity budget of fawns, but deer decreased activity in cold weather. The main determinant of overwinter survival was individual body mass in early winter. Our study suggests that deer have adapted to the extreme conditions encountered on Anticosti (i.e., harsh and long winters, low-quality browse, and a high-density population). Even though white spruce stands are gradually replacing balsam fir stands, our results suggest that deer on Anticosti could maintain a high-density population by increasing the amount of white spruce in their diet. However, future studies are necessary to address the long-term effects of a diet dominated by white spruce on deer body condition, survival, and reproduction under natural conditions.
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