The Appalachian region is the home of more species of plants and animals than any other temperate forest on earth. Through the long stretch of geological time, life forms have come and gone. Species such as the woolly mammoth disappeared from the Appalachian region long ago largely as a result of environmental changes. Human activity drove the woodland bison, the eastern elk and other large mammals to extinction in the 19th century. In our own time, the related forces of habitat fragmentation, pollution, climate change, and the global exchange of non-native species has greatly accelerated the process of species extinction. Among the greatest threats to the rich diversity of life in our mountains is the disappearance of the spruce-fir ecosystem dating from the last ice age.