Vegetation in the Arctic, limited to regions having a tundra climate, flourishes during the short spring and summer seasons. The tundra's restrictive environment for plant life increases northward, with dwarf trees giving way to grasses (mainly mosses, lichen, sedges, and some flowering plants), the ground coverage of which becomes widely scattered toward the permanent snow line. There are about 20 species of land animals in the Arctic, including the squirrel, wolf, fox, moose, caribou, reindeer, polar bear, musk ox, and about six species of aquatic mammals such as the walrus, seal, and whale. Most of the species are year-round inhabitants of the Arctic, migrating to the southern margins as winter approaches. Although generally of large numbers, some of the species, especially the fur-bearing ones, are in danger of extinction. A variety of fish is found in arctic seas, rivers, and lakes. The Arctic's bird population increases tremendously each spring with the arrival of migratory birds (see migration of animals). During the short warm season, a large number of insects breed in the marshlands of the tundra.