Feeding-crater selection by high-arctic reindeer facing ice-blocked pastures

Increased frequency of ground-icing events is likely to influence population dynamics in arctic ungulates, but their behavioural responses remain unexplored. During a record-mild winter with heavy rainfall, we analysed snow and ice characteristics and foraging trade-offs by Svalbard reindeer ( Rangifer tarandus platyrhynchus Vrolik, 1829) on a semi-isolated, recently occupied range. Snow depths were well within thresholds for cratering, but >90% of low altitudes was covered by a thick ice coat on the ground (median thickness 9 cm). Different strategies to cope with these conditions appeared. Part of the population sought mountainous habitat with very sparse vegetation. Individuals remaining at lower altitudes either used sparsely vegetated, wind-blown ridges partially covered with ice, or apparently applied olfactory senses to locate vegetation in ice-free microhabitat beneath the snowpack. No feeding craters were covered by ground ice, compared with most nearby controls. Following ground-ice avoidance, vegetation rather than snowpack properties determined fine-scale crater selection. Even under such poor conditions, the presence of medium- to high-quality forage (dwarf willow ( Salix polaris Wahlenb.) and fruticose lichens) rather than low-digestible, high-biomass forage (mosses) influenced cratering decisions. Behavioural plasticity combined with a gradually depleted lichen resource can partly buffer the reindeer against predicted climate change, at least in the short-term.