Seasonal diets, trace minerals, heavy metals, and body condition: nutritional characteristics of barren ground caribou in Southwest Alaska

This is the authors' abstract. We don't add key points for this paper.

Barren‐ground caribou (Rangifer tarandus granti) are a keystone species in tundra ecosystems experiencing rapid climate-driven habitat changes. Synchronous declines and persistent low numbers in many herds across the circumpolar north have raised concerns about the nutritional and demographic health of caribou populations. We sampled 170 animals from two recognized herds—the Mulchatna and Northern Alaska Peninsula herds—and three non-migratory groups in Southwest Alaska inhabiting areas around Goodnews Bay, Togiak, and Kukaklek Lake, analyzing stable isotopes (δ13C and δ15N) and trace elements in tissues (hair, red blood cells, and serum). We also quantified body fat and protein in lactating and non-lactating females from the Mulchatna herd to assess body condition in the region’s largest migratory herd. Using stable isotope mixing models and isotopic niche reconstructions—the first conducted for Southwest Alaska caribou—we quantified spatial and temporal differences in diet composition. We also assessed nutritional status using key markers such as δ15N and trace element concentrations along with body condition metrics. Our findings demonstrate significant spatiotemporal differences in dietary patterns and nutritional markers across groups. The Northern Alaska Peninsula herd exhibited the broadest isotopic niche and the highest consumption of deciduous shrubs (82% of summer-fall diet), while the Kukaklek Lake group displayed the narrowest niche, lowest δ15N levels, and highest levels of lead. Within the Mulchatna herd, the East subgroup showed elevated δ15N values (0.5-1‰ higher), improved mineral nutrition, and lower cadmium levels compared to the West subgroup. Redundancy analysis (RDA) revealed drivers of isotopic variation at two scales: at the regional scale caribou group was the dominant predictor, while within the Mulchatna herd, subgroup identity, lactation status, and several trace elements (barium, chromium, magnesium, lead) contributed additional fine-scale structure. A third RDA on the Mulchatna herd used body condition metrics—body fat and protein—as response variables and showed that only lactation status was a significant predictor, being negatively associated with body fat; subgroup, trace elements, and heavy metals did not explain variation in body condition within the Mulchatna herd. These results suggest that while subregional differences in forage quality and availability drive measurable variation in isotopic and trace elements in Southwest Alaska caribou, the fitness consequences remain unknown. This study highlights the importance of incorporating integrative dietary and physiological data into long-term monitoring to elucidate the nutritional consequences of environmental change in a rapidly shifting northern landscape.