Populations of large mammals in the Arctic tend to be small. Those that occur in large numbers, such as caribou, are migratory. Populations of small mammals tend to fluctuate, sometimes with some predictability (4—7-year cycles). Lemmings, and to a lesser extent voles, are notorious in that regard (see Microtines Lemmings, Voles). The Norwegian lemming's mass-suicidal plunges into the sea are the stuff of myth; nevertheless, many small mammals' populations undergo cyclical population explosions and crashes. The reasons appear to be a combination of benign conditions for population growth, followed by overpopulation, social strife, starvation, environmental stress, for example, from harsh winters, and melting snow that kill most of the population. Populations of their predators, such as jaegers, owls, weasels, and fox, also fluctuate, lagging some time interval behind the population fluctuations of their prey. The populations of larger mammals, such as muskox and barren ground caribou, also seem to fluctuate, but in accordance with periodicities in weather patterns. In Greenland, short-term fluctuations of the wind regime force strong interannual climate variations. Continental climatic periods are characterized by dry, cold westerly winds, but oceanic periods are associated with moist and warmer easterly winds with wetter snow that freezes and crusts over. The latter conditions make feeding difficult for grazing ungulates.

At present, populations of snow geese are high in the Arctic. A combination of factors, mostly to do with agriculture in the south where they winter and feed, allow healthy, fat geese to migrate north and reproduce. In some places, such as parts of the Hudson Bay lowland, the populations are so high that shoreline plant life is being adversely affected.

Populations of many marine mammals and of polar bear are now protected from commercial hunting by federal or state legislation. Whale, walrus, and bear populations are growing, as are populations of unprotected species of seal.

Some animals always seem to be in superabundance in the Arctic. Mosquitoes are notorious, and blackflies in the Low Arctic. Even in the High Arctic where mosquitoes are rarely pestiferous and blackflies cannot live, nonbiting midges occur annually in huge clouds. Their aquatic and semiaquatic larvae are important food for migratory birds that fly north every year to reproduce.

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