Sea or summit? Wild reindeer spatial responses to changing high-arctic winters

Abstract

Because of climate change, wildlife is facing altered environments, including profound shifts in temperature and precipitation regimes. In snow-dominated ecosystems, winter warming and resulting changes in snowpack properties impact forage accessibility for ungulates—often for the worse. The potential of individuals and populations to buffer negative fitness effects of harsh winters with “basal ice” (i.e., ice on the ground) and/or a harder or deeper snowpack depends on their ability to adjust behaviorally through changes in diet, dispersal, or small-scale habitat use. Here, we use ten years of late winter snowpack monitoring and population census data from three neighboring, semi-isolated coastal populations of high-arctic wild Svalbard reindeer to explore small-scale space use responses to annual variation in late winter-foraging conditions. Based on location data from the population censuses, we roughly classified individuals' spatial foraging strategy (i.e., habitat use) during late winter into “tundra” (foraging on tundra plains), “mountain” (foraging at high elevations, with low plant biomass but less snow and ice), or “shore” (foraging along the seashore, subsidizing terrestrial food with kelp and seaweed). Using multinomial logistic regression, we modeled the probability of reindeer adopting either of these strategies as a function of density-dependent winter severity. Our results suggest that effects of winter severity on habitat use are density-dependent and that snowpack depth and hardness (excluding basal ice, measured as “integrated ram hardness,” IRH) have stronger influence on reindeer foraging behavior than basal ice, at least at such spatial scales. Particularly, high IRH increased the probability of reindeer seeking high-elevation and steep terrain instead of tundra lowlands, but not at low population density, that is, when competition for food is negligible. This strategy was most pronounced among adult males, possibly reflecting their risk-prone behavior and/or reduced competitiveness related to lack of antlers during winter. This study demonstrates important patterns of temporal, spatial, and demographic variation in reindeer's winter-foraging strategies, adding novel, nuanced insights into how climate change affects spatial processes in isolated ungulate populations. The results add to the impression of considerable behavioral flexibility, which may aid buffering the negative fitness effects of complex changes in foraging conditions due to environmental changes.