Delineating seasonal shifts in reindeer habitat and diet selection by integrating GPS telemetry and stable isotope analysis

Seasonal changes shape herbivore behaviour by altering forage availability and habitat conditions; however, few studies integrate diet and habitat selection data across temporal scales. This study uses seasonality as a unifying framework to combine fine-scale GPS-based habitat selection data with broader-scale dietary information from stable isotope analysis (δ13C, δ15N) of hairs in semi-domesticated reindeer Rangifer tarandus tarandus. Thresholds introduced within this framework detect seasonal shifts in habitat and diet selection and classify foraging strategies along a specialist–generalist continuum. Despite individual variability, most individuals exhibited pronounced seasonal changes between spring/early summer (SES) and late summer/autumn (LSA), consistent with generalist foraging strategies. Habitat selection models revealed reduced avoidance of rugged terrain and increased use of mires and bogs from SES to LSA. Concurrently, isotopic enrichment and niche expansion reflected a dietary shift from δ15N- and δ13C-depleted plants (e.g. lichens, shrubs, deciduous vegetation) to more enriched forage types such as sedges, grasses, horsetails, and mushrooms. We assessed whether individual-level shifts in diet and habitat use were linked and found an inverse correlation between the shifts in terrain ruggedness avoidance and dietary change in approximately 67% of individuals, suggesting that behavioural flexibility facilitates seasonal transitions. By integrating spatial and isotopic data, this study overcomes the limitations of single-method approaches and provides a more nuanced understanding of the seasonal foraging dynamics of a keystone Arctic and boreal ungulate. The findings highlight the plasticity of reindeer foraging within a variable environment and suggest a capacity to respond to environmental changes. This framework also offers broader applications for investigating behavioural responses and ecological strategies in other herbivores facing climate-driven habitat shifts.