Identification of subpopulations of North American elk (Cervus elaphus L.) using multiple lines of evidence: habitat use, dietary choice, and fecal stable isotopes

We used multiple lines of evidence to assess habitat selection, dietary choice, and nutritional outcomes for a population of North American elk (Cervus elaphus), confined to a relatively small and isolated landscape of public and private land in south-central Great Plains, USA. The area of suitable elk habitat was a topographically diverse matrix of mature oak savannah, C₄-dominated grasslands, and C₃-dominated agricultural fields surrounded by unsuitable lowlands fragmented by anthropogenic activities. We hypothesized that such disparity in habitat availability and quality resulted in subpopulation differences in the overall elk population. We used 3 methods to evaluate this premise: radiotelemetry to determine home range and habitat use, microhistology of plant fragments in feces to determine dietary selection, and fecal nitrogen (N) and stable isotope ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C) to assess nutritional outcomes of habitat use and dietary choice. By comparing these 3 approaches, we wanted to determine if fecal indices alone could efficiently and accurately identify subpopulation structuring. Compositional analyses from radiotelemetry observations of 21 female elk identified 2 subpopulations that occupied relatively disjunct areas and showed differential preferences for forested and cultivated fields in summer but comparable preferences for cultivated fields in winter. A third unmarked subpopulation of elk was known to be largely confined to an adjacent wildlife refuge. Microhistological analyses of feces collected in all 3 areas highlighted distinct diets, outcomes of habitat occupation by the 3 subpopulations. Increased use of cultivated forages in winter was evident for 2 of the subpopulations, but the extent of use by elk was dependent on availability of cultivated forages in areas they occupied. The refuge subpopulation had no access to cultivated forage. Fecal N, fecal δ¹³C, and fecal δ¹⁵N supported the premise that the subpopulation with the greatest access to cultivated forages was on a higher nutritional plane than the other 2 subpopulations. Changes in fecal N, fecal δ¹³C, and fecal δ¹⁵N paralleled percentages of cultivated forages in the diets highlighting the utility of such fecal indices as supplemental to or surrogates for traditional methods of habitat use and dietary selection in free-ranging ungulates.