A classification of North American biotic communities by David Earl Brown, Frank Reichenbacher, Susan E. Franson

By David Earl Brown, Frank Reichenbacher, Susan E. Franson

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Each life zone, he concluded, was determined by temperature, with humidity, topography, and other factors being of lesser importance. To provide a scientific foundation for his life-zone theories, Merriam conducted an ex post facto analysis of his distribution data and postulated the effects of various temperature parameters. The northward distribution of animals and plants, he reasoned, was restricted by the total quantity of heat during the growing season when temperatures exceeded 6°C. Conversely, he concluded that the southward distribution of species was determined by the mean temperatures experienced during the hottest six weeks of the year.

The biotic equivalent of a plant formation, in a concept conceived by Clements and Shelford (1939), was the biome. Modifying earlier vegetation maps, Clements and Shelford mapped eleven biomes in North America to show the applicability of such an approach. They were not disappointed. Biologists such as Pitelka (1941), Miller (1951), and Aldrich (1967) readily adopted and adapted the biome concept in their studies, some of them substituting the term "biotic Page 12 community" for biome. Although Shelford (1963) considered the biotic community designation redundant, he had used it himself (Shelford 1932b), and the two terms have become almost synonymous.

1994). To correct this omission, Gill proposed the addition of six marine realms determined primarily on water temperaturesan Arctic Realm, a Paractalian or North Temperate Realm, a Tropical Realm, a Notalian or South Temperate Realm, an Antarctican Realm, and a Deep Sea or Bassalian Realm. Botanists meanwhile were hard at work developing their own phytogeographic classifications. As early as 1859, J. G. Cooper had divided North America into twenty-seven "natural provinces" and regional subdivisions, giving each one a geographic or Native American name to emphasize its geographic center.

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