Dynamic selection for forage quality and quantity in response to phenology and insects in Arctic caribou

Heather E. Johnson, Trevor S. Golden and Layne G. Adams, Alaska Science Center, US Geological Survey, Anchorage, AK, David Gustine, National Park Service, Elizabeth A. Lenart, Alaska Department of Fish and Game, Fairbanks, AK, Perry S. Barboza, Texas A & M University, College Station, TX

Background/Question/Methods
Spatiotemporal variation in forage is a primary driver of ungulate behavior, yet little is known about the nutritional components they select and how selection varies across the growing season with changes in forage quality and quantity. We addressed these uncertainties in barren-ground caribou (Rangifer tarandus), which experience their most important foraging opportunities during the short Arctic summer. Recent declines in Arctic populations have raised concerns about the influence of climate change on summer foraging opportunities, given shifting vegetation conditions and insect harassment, and their potential effects on caribou body condition and demography. We examined the summer foraging behavior of Arctic caribou by pairing locations from females in the Central Arctic Herd of Alaska with spatiotemporal predictions of biomass, digestible nitrogen (DN), and digestible energy (DE) to 1) assess selection for these nutritional components across the growing season at landscape and patch scales, and 2) determine whether foraging opportunities were constrained by insect harassment.
Results/Conclusions
During early summer, at the landscape scale, caribou selected intermediate biomass and high DN and DE, following expectations of the forage maturation hypothesis. At the patch scale, however, caribou selected high values of all forage components, particularly DN, suggesting that protein may be limiting. Later in the summer, after DN declined below the threshold for protein gain, caribou exhibited a switch at both spatial scales, selecting for higher biomass, and to a lesser extent DE, likely enabling fat deposition. Mosquito activity strongly altered caribou forage selection and increased their energy expenditure, while oestrid fly activity had little influence. Our results demonstrate that early and late summer periods afford Arctic caribou distinct foraging opportunities, as they prioritize quality earlier in the summer and quantity later. Climate change may further constrain caribou access to DN as earlier, warmer Arctic summers may be associated with reduced DN and increased mosquito harassment.