RELATIONS AMONG SPECIES 



385 



Figure 19.23 World Faunal Regions: 1. Neorctic, 2. Neotropical, 

 3. Paleorctic, 4. Ethiopian, 5. Oriental, 6. Australian. 



The six realms are: (1) Neanlic, including all of 

 North America south to the limit of the interior up- 

 lands of Mexico; (2) Neotropical, including the rest of 

 Mexico and extending southward to the southern 

 limit of South America; (3) Palearctic, including all 

 of northern Africa, Europe, and Asia roughly north 

 of the Sahara Desert and Himalaya Mountains; 

 (4) Ethiopian, including the rest of Africa and some of 

 Arabia; (5) Oriental, including the southeastern tip 

 of Arabia, southern Asia, and many islands from Asia 

 toward Australia; and (6) Australian, including the 

 rest of the islands to Australia, Australia, and New 

 Zealand. In addition, the fauna of the islands in the 

 Pacific Ocean, some of it unique, is sometimes used 

 to classify another region, Oceania. 



One cannot deny the value of realms as a point of 

 reference. However, they are so generalized that one 

 gains no appreciation of the details of each area. For 

 example, the entire United States, including per- 

 petual glaciers and some of the most arid places in 

 the world, is in a single region, the Nearctic. This 

 example represents the major criticism of the scheme, 

 a criticism that lead to an early subdivision of faunal 

 regions. In this subdivision, the Nearctic was segre- 

 gated into a Canadian, or Cold, Subregion; a West- 

 ern, or Arid, Subregion; and an Eastern, or Humid, 

 Subregion. However, even these smaller units were 

 too large to be of much use. 



LIFE ZONES 



Inadequacy of distribution schemes led to various, 

 mostly superficial, attempts at meaningful schemes. 

 The first good scheme was presented in detail by 

 C. Hart Merriam in 1892. The scheme resulted from 

 Merriam's study of the San Francisco Mountains of 

 Arizona, an area with relatively clear zonation of 

 plants and animals. Fiom his studies, he concluded 



that temperature was the most important cause of the 

 "life zones'" and that northern limiting factors were 

 different from southern ones. For the north, he used 

 temperature to measure the length of the growing 

 season, the sum of the days with temperatures above 

 6°C. {4°C. is now believed a better approximation) 

 to determine limits; for the south, he used the average 

 temperature during the six hottest weeks of the 

 summer. 



Merriam's data were the basis for designating three 

 major Regions, the northern Boreal, the southern Aus- 

 tral, and the southern tip of Florida's Tropical (Fig- 

 ure 19.24). Within the Boreal and Austral regions, 

 subdivisions called Lije ^ones were made; a Life Zone 

 of transition between the Boreal and Austral Regions, 

 the Transition ^one, was also recognized. Further- 

 more, Merriam knew that plants were the best indica- 

 tors of his Life Zones, but he also listed mammal 

 and bird indicators. 



The Boreal Region contains three Life Zones. The 

 Arctic-Alpine ^one is north of the limit of trees, or in 

 the Tundra, and above timberline in the western 

 mountains. Some of the indicator plants are dwarf 

 willow, arctic poppy, saxifrages, and gentians; indi- 

 cator animals are the arctic fox, arctic hare, lem- 

 ming, snow bunting, snowy owl, and rosy finch. The 

 Hudsoman ^one, named because the Hudson Bay area 

 is typical, consists of the Taiga in Canada and the 

 upper reaches of the Subalpine Forests in the western 

 mountains. Indicator animals are the wolverine, 

 woodland caribou, great gray owl, and pine grosbeak. 

 The Canadian ^one includes the Northern Coniferous 

 Forests in Canada and the lower reaches of the Sub- 

 alpine Forests. Indicator animals are the varying 

 hare, marten, lynx, spruce grouse, Canada jay, and 

 white-throated sparrow. The Boreal Region, then, 

 contains Tundra, Taiga, and most of the Boreal 

 Forests. 



The Transition ^one is a distributional wastebasket, 

 a catch-all of Boreal-Austral overlaps. Being a waste- 

 basket, it is difficult to characterize throughout North 

 America. In the east, it includes areas of vegetational 

 overlap (ecotones) between Boreal and Deciduous 

 Forests; in the west, it includes the Montane and 

 Coast Forests; and in both east and west, it includes 

 certain other Boreal-Austral transitions. Generally 

 recognized animal indicators are the Columbian 

 ground squirrel, the sage grouse, and the sharp-tailed 

 grouse. 



The Austral Regiot> has two Life Zones. The Upper 

 Austral ^one contains two geographic subunits. The 

 eastern, or Carolinian ^one, has its western margin 



