Partial migration as a response to ground icing events in a high arctic ungulate
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- Master's theses (INA) 
Partial migration – a phenomenon where only part of a population performs annual migrations, is common in ungulates. Despite partial migration being well documented, little is known about the annual partial migration frequency in ungulates and if individuals perform the same strategy (migratory or stationary) every year. Additionally, few studies have investigated if variations in weather conditions have an impact on the degree of partial migration. The endemic Svalbard reindeer (Rangifer tarandus platyrhynchus) live under extreme climatic conditions on the island of Svalbard in the Arctic. During winter, temperatures may rise above 0° C and precipitation fall as rain, an occurrence known as rain-on-snow events (ROS). Such warm spells lead to the formation of ground ice, creating locked pastures, inhibiting foraging thus potentially leading to starvation and death for Svalbard reindeer. In this thesis I analyzed patterns of partial migration in Svalbard reindeer. I used GPS data from 36 females of a Svalbard reindeer population in the period 2009-2013. The GPS data of Svalbard reindeer were related to field validated data of ground ice, snow depth, ground temperature and air temperature. I found that winter conditions and amount of ground ice clearly differed between years and valleys. Years were thus distinguished as icing years (two years) and non-icing years (two years) in this thesis. Warm spells followed by ground icing affected migratory behavior of Svalbard reindeer by increasing the probability of migration and increasing migration distance. I also show that the warm spells corresponded with the timing of migration from summer to winter range, as well as causing most migrants to seek ranges with less ground ice nearer the coast. Furthermore, 41% of the Svalbard reindeer monitored for at least two years changed annual movement strategy when facing differing ground ice conditions, i.e. adopted a migratory strategy in icing years and stationary strategy in non-icing years. To my knowledge, this has rarely been documented for any ungulate. This study provides rare evidence of what drives partial migration in ungulates by showing how a partially migratory population responds to climate and extreme weather events, in turn affecting the annual portion of migrants in a wild ungulate population.