Research on large predator-prey interactions are often limited to the predators' primaryprey, with the potential for prey switching in systems with multiple ungulate species rarelyinvestigated. We evaluated wolf (Canis lupus) prey selection at two different spatial scales,i.e., inter- and intra-territorial, using data from 409 ungulate wolf-kills in an expanding wolfpopulation in Scandinavia. This expansion includes a change from a one-prey into a twopreysystem with variable densities of one large-sized ungulate; moose (Alces alces) andone small-sized ungulate; roe deer (Capreolus capreolus). Among wolf territories, the proportionof roe deer in wolf kills was related to both pack size and roe deer density, but not tomoose density. Pairs of wolves killed a higher proportion of roe deer than did packs, andwolves switched to kill more roe deer as their density increased above a 1:1 ratio in relationto the availability of the two species. At the intra-territorial level, wolves again responded tochanges in roe deer density in their prey selection whereas we found no effect of snowdepth, time during winter, or other predator-related factors on the wolves' choice to killmoose or roe deer. Moose population density was only weakly related to intra-territorial preyselection. Our results show that the functional response of wolves on moose, the specieshitherto considered as the main prey, was strongly dependent on the density of a smaller,alternative, ungulate prey. The impact of wolf predation on the prey species community istherefore likely to change with the composition of the multi-prey species community alongwith the geographical expansion of the wolf population.
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