Isotope ecology detects fine-scale variation in Svalbard reindeer diet: implications for monitoring herbivory in the changing Arctic

The Arctic is experiencing rapid climatic and environmental changes, which could alter diets of nonmigratory herbivores both within and between populations. Here, we used carbon and nitrogen stable isotope analysis (δ13C and δ15N) in hair to infer summer dietary differences of Rangifer tarandus platyrhynchus (nonmigratory reindeer) in Nordenskioldland, Svalbard and used regression models to predict how reindeer dietary intake depend on climatic variables. In linear regression models, current-year and last-year July temperatures positively correlated with both the δ13C and δ15N of reindeer hair, and were stronger predictors of isotopic signatures than July precipitation. δ13C and δ15N analysis indicated robust dietary differences between populations separated by less than 50 km and within these valley systems in the proportions of nonmycorrhizal vascular plants and mosses. Remote sensing (NDVI) and isotopic data together suggested an early-season switch from moss to vascular plant dependence in valleys closer to the sea, suggesting both high site fidelity and localized differences in forage availability related to the onset of the growing season. 13C photosynthetic discrimination increased with the rising atmospheric pCO2, indicating that Svalbard plants are already responding physiologically to rising CO2 concentrations. Calves were higher in δ13C and δ15N than adults, consistent with a weak trophic effect. We also determined how hair C:N correlated with δ13C and δ15N patterns. Overall, these data provide evidence that δ13C and δ15N measurements can detect dietary shifts over short time periods and fine spatial scales, indicating that isotopic studies can help monitor herbivory in the changing Arctic.