The Ecological Region

North America, which is separated from the Old World by major oceans (apart from the Bering Strait, now a mere 85 kilometers wide), encompasses an area of just over 19 million square kilometers. It stretches across 136° of longitude, from 52° west at Cape Spear in Newfoundland to 172° east longitude at Attu Island in the western extremity of the Aleutians, but its latitudinal extent of 64°, from 83° north at Ellesmere Island to the Florida Keys, is all north of the Tropic of Cancer. Nevertheless, tropical influence is not unknown in the region, as the continent is open to climatic incursions along the Gulf coast penetrating across the interior Mississippian lowlands northward to the Great Lakes. In essence, this means that extratropical cyclones develop along the jet stream and migrate from west to east and, in so doing, draw up hot humid air in summer, and lead to incursions of freezing air masses as far south as Florida in winter.

Throughout the eastern and central sections then, seasonal contrasts are marked, especially when compared to those of northwestern Europe. The mountains of the West modify these conditions, giving rise to wet Pacific coastlands in the north and semiarid rainshadow valleys on the eastern lee with sub-humid areas extending over more and more extensive areas toward the southwest. The natural vegetation mirrors the climatic conditions. It ranges from luxuriant forests of the Northwest through scrub and grasses of the semiarid plains and the evergreen boreal forests of the northlands, to the deciduous forests of the East fringed on the southeast coast by marshlands, with each presenting its own niches for life forms from buffalo to mosquito.

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