The biosphere is subdivided into distinct geographical regions called biomes. Newfoundland and Labrador are divided into two biomes: tundra and taiga. Northern Labrador is part of the tundra, while southern Labrador is part of the taiga. Newfoundland is not typical of either biome, as it lacks much of the plant and animal life that are characteristic to these biomes. During the last ice age the island of Newfoundland was completely covered by glaciers and swept clean of life. Only those species which were able to recolonize the island after the glaciers retreated about 18,000 years ago are considered "native". Similarly, only freshwater-fish capable of surviving seawater swam to the island. Labrador has 42 native mammals, Newfoundland is home only to 14, with no snakes, raccoons, skunks or porcupines. Large herds of Woodland caribou can be found in the barren interior regions of the island. Two animals formerly resident in Newfoundland have been declared extinct: the Great auk, a flightless seabird, and the Newfoundland wolf, a subspecies of the Gray wolf. The Labrador duck, believed to have nested in Labrador, was one of the first North American bird species to be recorded as becoming extinct.