Use of stable isotopes to identify dietary differences across subpopulations and sex for a free-ranging generalist herbivore

Carbon and nitrogen isotopes in tissues can be used to understand plants consumed by various taxa, but can they provide additional information about consumers? Values of δ¹³C and δ¹⁵N were assessed from tissue of free-ranging elk (Cervus elaphus) occupying disparate habitats of mixed prairie-oak savannah that contained C₃ agricultural crops in a C₄-dominated landscape and in key plants consumed by elk. Muscle and hoof samples were collected from female and male elk in two subpopulations (forested land and grassland) from private land and one subpopulation from the Wichita Mountains Wildlife Refuge (refuge) in 2001-2006. Previous research identified differences between mean muscle δ¹³C and δ¹⁵N and mean hoof δ¹³C and δ¹⁵N indicating that isotopes differed between tissues of varying metabolic activity. Mean δ¹³C in hoof of elk on forested land and grassland were lower than hoof δ¹³C from elk in the refuge indicating greater long-term consumption of C₃ plants by elk on forested land and grassland subpopulations. The δ¹⁵N in hoof was greater for elk outside the refuge than that for elk in the refuge. Interaction of sex and subpopulation only occurred for hoof δ¹⁵N suggesting that factors such as tissue type, sex, and habitat need to be considered to understand free-ranging ecology of generalist herbivores using stable isotopes. Availability of C₃ agricultural crops high in percent nitrogen on a nearly annual basis in a C₄-dominated landscape was likely driving differences in tissue δ¹³C and δ¹⁵N among subpopulations of free-ranging elk. An increase in tissue δ¹⁵N resulted from an increase in the consumption of higher δ¹⁵N in forage for sexes and subpopulations of a free-ranging ungulate in North America but δ¹⁵N should be further evaluated as an index of nutrition for subpopulations of generalist herbivores.