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COMMUNITY ECOLOGY: 



also has wide-ranging species that are more or less 

 common to the entire area, for example, cottontail 

 rabbits, gophers, harvest mice, white-footed mice, 

 gray foxes, racoons, skunks, and deer. 



The Boreal American Element evolved with the Arcto- 

 Tertiary Geoflora; for this reason it contains some 

 species that now are panboreal, that is, distributed 

 across the top of the world. Some of these panboreal 

 forms are of definite Old World origin, but others ap- 

 pear to be of New World origin. In either event, the 

 Bering land bridge allowed the present panboreal 

 distribution. Panboreal forms are hares, beavers, 

 lemmings, red-backed voles, pine mice, meadow mice, 

 weasels, wolves, the arctic fox, and the red fox. New 

 World forms are pigmy shrews, shrew moles. Brew- 

 er's moles, western moles, pine squirrels, flying 

 squirrels, and lemming mice. Other genera definitely 

 developed in the New World but later moved to the 

 Old World and now inhabit both areas. These in- 

 clude western chipmunks, marmots, and jumping 

 mice. 



The Old World Element entered the New World dur- 

 ing the Pleistocene by way of the Bering land bridge. 

 In North America this element is mostly northern in 

 distribution and includes the conies, bears, martens, 

 wolverines, lynxes, elk, moose, caribou, bison, moun- 

 tain sheep, mountain goats, and musk oxen. 



FAUNAS 



As one might expect, most areas of North America 

 have some representatives of the various elements; 

 however, there is a definite tendency for certain areas, 

 or faunas to have more of one element than another. 

 For example, the Tundran Fauna is almost entirely 

 represented by Old World and Boreal Elements; the 

 Tropical North American Fauna is almost devoid of 

 these elements. 



The faunas are geographic units and are static. 

 Each is roughly equivalent to one or more biomes. 

 However, because one can analyze the general nature 

 of each locale of each fauna, the fauna is not as static 

 as the biome. Analysis allows study of locale changes 

 through time, perhaps even study of how a locale can 

 change from one fauna to another. 



Afaunal. Certain North American mammals that 

 occupy a large part of our continent are hard to fit 

 into a particular fauna. This group includes stream 

 species — the otters, minks, and beavers; marsh forms 

 — the muskrats; water border and wet meadow forms 



— jumping mice; burrowing forms — pocket gophers; 

 and certain others such as white-footed mice, elk, 

 Virginia deer, foxes, coyotes, wolves, mountain lions, 

 and bears. These mammals comprise the Afaunal 

 group. 



Tropical Norfb American Fauna. This fauna now oc- 

 cupies tropical and subtropical Central American and 

 Mexican forests. Like any other fauna, its elements 

 vary from locality to locality and element composi- 

 tion defines the general nature of the fauna at any 

 place. For example, at Barro Colorado Island in 

 Panama, 60 per cent of the genera represent the 

 South American Element; 30 per cent, the Tropical 

 North American; and 10 per cent, the other elements. 

 Most of the 10 per cent is represented by the Austral 

 Element. 



Sonoron Fauna. This fauna is most characteristic 

 of the arid southwestern deserts but it extends from 

 the Mexican Uplands and Lower California to south 

 central Canada and eastern Washington state. It 

 occupies areas of Desert Scrub, Shortgrass, Sage- 

 brush, Bunchgrass, Chaparral, and Woodland. It 

 does not include the Tallgrass, which appears to 

 have meadow forms of the Deciduous Forest and 

 Shortgrass forms. In most places, 50 to 60 per cent 

 of the fauna is of Austral North American origin. 



Deciduan Fauna. These mammals are found in the 

 eastern United States. Two subunits can be dis- 

 tinguished: the Deciduous Forest Association and 

 Southern Pine Forest Association. Each faunal as- 

 sociation has a definite and distinctive group of mam- 

 mals. A "faunal association" is not the same as an 

 "association." 



Coniferan Fauna. Found in all Boreal areas except 

 the Tundra, this is the fauna of the coniferous forests. 

 It covers much of Canada, the western mountains, 

 and the area adjacent to the Pacific Coast. This 

 fauna is mostly of Old World and Boreal North 

 American origin. 



Tundran Fauna. The mammals of the far north are 

 almost entirely of Old World and Boreal North 

 American origin. Therefore, this fauna is the most 

 homogeneous insofar as origin is concerned. By way 

 of contrast, the Sonoran, Deciduan, and Coniferan 

 tend to possess representatives from other elements. 



DIFFERENTIATION AREAS 



A subunit, the differentiation area, is an area set 

 aside by barriers that isolate part of a fauna from 



