Arctic lemmings include true lemmings, of the genus Lemmus, and collared lemmings, of the genus Dicrostonyx. Although both have a circumpolar – though not identical – distribution, the two genera differ in many respects. Dicrostonyx is much more resistant to extreme low temperatures than Lemmus. Consequently, its distribution extends farther north: collared lemmings are found on the northernmost islands of the Canadian Arctic and in northern Greenland, whereas the range of true lemmings reaches down to the boreal zone and does not include Greenland or the northern Canadian islands. Lemmus lemmings eat mosses, supplemented by grasses and sedges. Dicrostonyx lemmings prefer forbs and shrubs like avens and willows. This distinction is reflected in habitat use. In the Arctic tundra, Lemmus is usually found on wet lowlands or moist patches. Dicrostonyx lives almost exclusively on dry and sandy hills and ridges.

Lemmings are well-known for their violent population fluctuations, the impacts of which are reflected in various ways. High numbers of lemmings have a strong impact on the vegetation and affect the nutrient cycle. Shallow permafrost slows down soil processes, but when lemmings at peak densities graze away much of the vegetation cover, the summer thaw can penetrate deeper into the soil, making more nutrients available. Lemmings also have a strong impact on the biomass and species composition of the vegetation.

The numbers of both mammalian and avian predators depend on the lemming fluctuations. In low lemming years, resident mammalian predators, including the arctic fox, ermine, and least weasel, hardly breed. During peak years, arctic foxes can have litters of up to 20 kits, and ermine and weasel numbers increase rapidly. Many species of birds of prey move nomadically, or briefly visit their breeding grounds in the Arctic in the search of lemming peaks. Snowy owls and jaegers (or skuas) are particularly well-known for their dependence on lemmings, breeding only in years when lemmings are abundant.

The causes of these population cycles have long been a topic of research and often heated discussion. Food, predation, and intrinsic physiological factors are possible explanations, each of which has some scientific support. It may well be that in different areas different factors play the critical role. In the Canadian Arctic, for example, the longterm low density of Dicrostonyx is explained by heavy predation. In Fennoscandia, on the other hand, lemming crashes in alpine areas seem to be caused by shortage of food, but when lemmings migrate into the boreal forests, they are regulated by predation.

In Northern Fennoscandia, the famous migrations of the Norwegian lemming ( Lemmus lemmus) can temporarily extend its range some 200 kilometers into the boreal forest, though they only occur three times a century. Norwegian lemmings have both spring and fall movements. Spring movements are caused by snow melt and last only two or three weeks, whereas fall migrations are density-dependent, and may last two to three months. Such remarkable movements in the fall have not been reported for other lemming species.

Heikki Henttonen, Vantaa Research Centre, Finnish Forest Research Institute, Finland. From the CAFF publication, Arctic Flora and Fauna, www.caff.is