Recent Geological History and Biological Diversity

Because much of the Arctic has only recently become deglaciated and there have been recent land connections ("bridges") from Arctic Europe, through Siberia, to Alaska and the rest of North America, there is a common terrestrial, circumpolar, flora and fauna. The number of species that are special or restricted (i.e., endemic) to various geographical parts of the Arctic is small. Specializations or adaptations according to habitat are, however, well known. The distributions and abundances of terrestrial animals parallel the veg-etational patterns described above. Thus, the diversity and abundances of insects, other terrestrial invertebrates, mammals, and birds increase from the High Arctic polar desert to the treeline.

Despite the recent history of Pleistocene ice ages (last glacial maximum about 18,000 years ago), there are regions of the Arctic that may never have been glaciated. These areas may have supported life during the short summers that may have prevailed. The mountains of northwestern North America, including northern Alaska, would have been a major barrier to moist Pacific air reaching the Arctic beyond. With scant precipitation, summer sunshine may have been enough to keep inland tracts of land ice-free and seasonally snow-free, allowing hardy plants and invertebrates to persist. Refugia have been suggested to have existed in various places, from northern Greenland to the southwest and into Alaska and the Yukon. Much of Siberia was not glaciated, but treeless Arctic conditions may have extended further to the south than they do today.

There are interesting correlations between the extent of glaciation, progress of ice retreat, and postglacial processes that all relate to biogeographical patterns. Although, as noted above, many Arctic species are circumpolar in distribution, biological diversity across northern North America decreases from west to east, with apparent stepwise declines noted from the western to the eastern side of Hudson Bay, and perhaps across the Mackenzie Valley. Water and forest may be thought of, respectively, as the biogeographic barriers to the spread of plants and invertebrates.

After deglaciation, the land of much of Arctic Canada has slowly risen by isostatic rebound following relief from the mass of ice. This phenomenon is increasing the land area of Canada annually as Hudson Bay becomes slowly more shallow and smaller, and many Arctic islands rise above sea level. Raised beaches, reflecting the seashore's past positions, are a common sight along Canada's Arctic coastline. Walking up a series of raised beaches, one can see increased soil development, more and more varied vegetation, and often enough remains of beach activities (sea mammal hunting) and houses of people past.

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