Stewart et al. examined the forage used, and its contents of isotopes of nitrogen (N) and carbon (C) by cattle, elk and mule deer on the Starkey Experimental Forest and Range in northeastern Oregon. Mule deer had the widest variety in diet, which consisted mainly of sedges, forbs and browse, while cattle were primarily grazers. Elk concentrated on forbs, while grasses and browse made up a significant part of their diet. Stewart el al. found a depletion of N-15 and an enrichment of C-13 in the feces of mule deer. The most obvious reasoning the authors could find for this is that this association is common in plants growing a xeric habitat, and mule deer diets contained a high amount of xeric forages. An earlier study of these three species on Starkey showed that their was a large overlap in habitat use by all three species. This study showed almost no overlap between the species, concerning dietary strategy and site selection. The authors urged for more studies to concentrate on the relationship of habitat selection, feeding sites and isotopes to assess actual niches for large herbivores.
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