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Analytical Zoogeography of North American Mammals

John W. Wilson, III
Evolution
Vol. 28, No. 1 (Mar., 1974), pp. 124-140
DOI: 10.2307/2407244
Stable URL: http://www.jstor.org/stable/2407244
Page Count: 17
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Analytical Zoogeography of North American Mammals
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Abstract

The latitudinal increase of mammal species per unit area from the temperate zone to the tropics is not characteristic of the class Mammalia as a whole, but is primarily due to the order Chiroptera. Bats have evolved a way of life that is relatively predation-free and have filled most of the new niches which are associated with the canopy and terminal branches of trees, where productivity is highest. North of the tropics, where year round fruit, flowers, and active insects constitute typically tropical resources, mammals do not suffer any decrease in density due to increasingly severe winters until they reach the severity found in Canada (north of the 50th parallel). Mammals have evolved homeothermy to avoid consequences of being poikilothermic in an environment with temperature fluctuations. The ratio of species per genus is generally lower in tropical faunas when compared with temperate ones. Temperate mammals are relative generalists because often they cannot depend on the same year round food source. Tropical mammals are relative specialists, living in a region with year round fruit, flowers, and insects. These resources are more predictable and allow greater specialization but, except for those utilized by bats, appear to be no more abundant. This leads to greater 'taxonomic distance' between species in the tropics. Part of this may be due to a greater tendency of mammal taxonomists to split tropical taxa, but at least some portion of the drop in species per genus in the tropics is a real reflection of the difference in selective environment. The latitudinal trends in present terrestrial communities may be due to the coevolution of the angiosperms and insects in the absence of a cold season in lower latitudes. The birds and mammals have radiated into this more diverse biotic environment and are often used by the plants for pollination of flowers and dispersal of fruit and seeds. A measure of hierarchical taxonomic diversity was devised to study the distribution of species among higher taxa in faunas. As faunas increase in size and higher taxa become more numerous, the equitability of the distribution of species among those higher taxa decreases.

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