In many areas of North America and Europe, population densities of large herbivores are increasing and strongly affecting species composition and structure of plant communities. Although reduced resources associated with increasing density affect life history traits of large herbivores, their effects on foraging behaviour have received little attention. We experimentally controlled population density in large enclosures to assess how increasing density affected white-tailed deer (Odocoileus virginianus) space use in relation to forage biomass and cover at a fine scale. We quantified space use in 3 blocks, each with 2 enclosures, one containing deer at a density of 7.5 deer·km-2 (low density) and the other containing deer at a density of 15 deer·km-2 (high density). We interpolated forage biomass, lateral cover, and canopy cover in space by kriging and divided deer observations (radiolocations) into 3 diel-periods: dawn/dusk, day, and night. Deer space use was positively related to forage biomass and negatively related to lateral cover at both densities, but it was not affected by the diel-period. Deer increased the use of areas with dense canopy cover at low density, but not at high density. Population density thus modified deer resource use by constraining deer at high density to forage where canopy cover is lower but forage biomass higher. Our results provide evidence of density dependence in foraging decisions, as deer space use patterns appeared to be based more strongly on forage biomass than on cover, particularly when population density was high.
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