Niche partitioning among mule deer, elk, and cattle: Do stable isotopes reflect dietary niche?

Abstract

We examined dietary niches of mule deer (Odocoileus hemionus), North American elk (Cervus elaphus), and free-ranging cattle (Bos taurus) that frequently co-occur in western North America. We tested the hypothesis that those three species would exhibit little overlap in diet and that mule deer, the smallest in body size of the three species, would forage more selectively than either elk or cattle. We determined diet composition from microhistological analysis and used principal components analysis to assess dietary niches. In addition to those conventional methods, we also assessed whether dietary overlap among those three ruminants would be reflected in stable isotope ratios (δ13C and δ15N) from fecal pellets. Principal component 1 represented a foraging axis based on plant classes, whereas principal component 2 represented a continuum from grazing to browsing, which revealed complete separation among those three large herbivores. Similarly, δ13C and δ15N differed significantly among species and indicated differences in moisture regimes within habitats and types of forages used by those three ruminants. Mule deer had the greatest variability in diet and foraged on more xeric forages than did either elk or cattle. Stable isotopes elucidated differences in dietary niche among the three ruminants that were not evident from dietary analysis alone.