Resource Partitioning and Competition Among Cervids in the Northern Rocky Mountains

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Abstract
(1) Patterns of spatial distribution, habitat preferences, forage-class selection, and niche overlap were investigated for sympatric white-tailed deer (Odocoileus virginianus ochrourus Rafinesque), Rocky Mountains wapiti (Cervus elaphus nelsoni Bailey) and Shiras moose (Alces alces shirasi Linnaeus) during consecutive severe and mild winters in north-western Montana, U.S.A. The objectives were to determine mechanisms of resource partitioning, the role of interspecific competition in shaping niche relationships, and the current potential for interspecific competition. (2) Fourteen discrete plant communities were identified on the basis of previous community classifications. Additionally, two primary gradients of vegetative structure were described using principal components analysis: (i) a successional gradient of overstorey complexity, and (ii) a gradient of tall-shrub biomass. (3) White-tailed deer, wapiti, and moose were distributed unevenly among three segments of the study area that partitioned elevational and snow-depth gradients, and among vegetation communities. Habitat preferences were predictable on the basis of body size, energy expenditures, and nutritional requirements. Cervids were ranked white-tailed deer-wapiti-moose, in order of increasing body size and affinities for open canopy, deep snow, and dense shrubs. Wapiti also favoured grass-dominated communities when snow did not limit foraging options. (4) White-tailed deer and wapiti fed on dwarf evergreen shrubs and grasses, respectively, although deer snow restricted diet selection during the severe winter. Moose were browse specialists in both years and favoured deciduous shrubs more and coniferous browse less during a mild than a harsh winter. (5) Interpsecific and annual comparisons of niche overlap provided little evidence that resource partitioning resulted from interspecific competition: (i) trophic and ecological overlap between species was greatest during nutritionally restrictive winters; (ii) overlap was greater between similar-sized cervids than between species of dissimilar body size; (iii) species pairs with the greatest trophic overlap did not exhibit compensatory spatial divergence. Large overlap in resource use, which increased during severe winter periods, suggested possible temporary interspecific competition between white-tailed deer and wapiti and between moose and wapiti.