Animal movement on short and long time scales and the effect on genetic diversity in cold-adapted species

Sammanfattning: The genetic diversity in modern species is strongly affected by contemporary gene flow between populations, which in turn is governed by individual dispersal capacities and barriers in the landscape. However, current patterns of variation have also been shaped by movement over longer time-scales, such as the successive shifts in species distributions that have occurred during past climate changes. This thesis is focused on cold-adapted species, and one parameter that has greatly influenced their current genetic diversity is how they coped with climate warming at the last glacial/interglacial transition, ca 11.7 thousand years ago. I examined this in three different small herbivore taxa; true lemmings (Lemmus), ptarmigan (Lagopus) and hares (Lepus), whose modern distributions stretch from the exposed tundra to the subarctic moorlands and taiga. In the first paper, I investigated contemporary genetic structure in the cyclic Norwegian lemming (Lemmus lemmus) and proposed that mass movements during peak years act as pulses of gene flow between mountain areas, which homogenise the gene pool over surprisingly vast geographic distances. However, when I used ancient DNA to analyse the lemmings’ ability for long-term directional movement, I found that the Ice Age populations that inhabited the former midlatitude European tundra-steppe appear to have been incapable of shifting their distribution northwards following post-glacial climate warming. Instead, the results suggest that the endemic Norwegian lemming descends from an isolated population that survived the last glacial maximum in situ in a restricted ice free refugium. In contrast to the glacial lemmings, as well the majority of previously studied mammals, the ptarmigan (L. lagopus and L. muta) and hare (L. timidus) analyses revealed a long-term genetic continuity in Europe, where the midlatitude populations were able to keep pace with the rapidly changing climate at the last glacial/interglacial transition, enabling them to shift their ranges to northern and high-alpine regions. These different outcomes might be explained by ptarmigans’ flight capability that allows a less restricted dispersal across fragmented landscapes, and that the generalist nature of mountain hares makes them less vulnerable to habitat alterations. Species distribution modelling, however, indicated that continued climate warming will make some isolated regions unsuitable in the future, thereby forcing populations to adapt the new environmental conditions in order to avoid local extinctions.