462 ESSENTIALS OF ZOOLOGY 



the animals of a climatic region are as distinctive as the plants. The 

 soil, which is the result of interaction between the climate, the plants 

 and animals, and the rock substratum, also is characteristic of each 

 climate. Thus, in the area influenced by each climatic type, there is 

 a definite biotic community or formation, characterized by plants 

 and animals whose relations to their environment are similar or 

 equivalent. 



The Principal Biotic Formations 



A rough classification of the climatic formations of North America 

 includes the following: 



1. The Tundra Formation 



2. The Coniferous (Evergreen) Forest Formation 



3. The Deciduous Forest Formation 



4. The Grassland Formation 



5. The Sagebrush Formation 



6. The Desert Formation 



In addition there are transitional communities difficult of repre- 

 sentation on a map because of their discontinuous distribution or 

 small area. Some of these may be referred to, provisionally, as 

 Woodland, Desert Scrub, Chaparral, and Swamp communities. 



1. The Tundra Fonnation (Sedge-Musk Ox Biome). — This is the 

 community of the arctic ''barren grounds," beyond the northern 

 limit of trees and between it and the polar region of perpetual snow 

 and ice. Included, also, is the region above timber line on high 

 mountains, where climatic conditions are similar to those of the 

 far north. The vegetation of the tundra is composed mainly of 

 grasses, sedges, and lichens (e.g., reindeer moss), although there 

 are also patches of dwarf willows and other woody plants from a 

 fraction of an inch to a few feet in height. Typical animals of the 

 Arctic tundra are the musk ox and the Arctic caribou which feed 

 on the low vegetation. Here, also, are found great hosts of rodents, 

 including lemmings, whose enormous increases in numbers and 

 periodic migrations in Europe have been known for many hundreds 

 of years. Arctic hares and Arctic ptarmigan, with those animals 

 previously mentioned, furnish food for the Arctic fox and the Arctic 

 wolf. Many of these animals, including the Arctic fox, the Arctic 

 hare, the ptarmigan, and the collared lemming, are adapted to the 

 climatic rhythm of their environment by changing in color from 



