Ecology of Animals 651 



Biotic Factors. — All these many physical factors affect animals, 

 not individually but in combination. In addition, each animal is sub- 

 jected to biotic factors, making the total complex which is termed the 

 environment. These biotic factors include competition from other ani- 

 mals for available food, for nesting sites, and for mates. 



For convenience of discussing the multitude of biotic pressures on 

 the individual organism and the physical environment, ecologists have 

 adopted the terms coaction and reaction. Coaction refers to the activity 

 of an animal in its relationships to other organisms, while reaction re- 

 fers to the action of the animal in modifying the physical environment. 

 Examples of the first include animals which feed upon other animals or 

 plants, fighting between animals, and parasitism. Some reactions are 

 animals burrowing in soil, beavers building dams to produce mountain 

 lakes, and the carrying in of humus by ants. 



Biomes 



Animals and plants exist together in groups called communities. 

 Each community is characterized by various coactions and reactions of 

 the plants and animals and the physical environment. 



Communities vary greatly in the degree of uniformity which they 

 attain. At one extreme is the compact assemblage of attached animals 

 growing on a rock in the ocean, while at the other extreme are the large 

 diverse communities such as forest and prairie. These largest biotic 

 communities are known as biomes and are recognizable by even the bio- 

 logically uninformed individual. So well known are these larger areas 

 that they have been given common names such as Temperate Deciduous 

 Forest, Desert, and Prairie. 



The major biomes of the world are as follows: 



Northern Tundra 



Northern Coniferous Forest 



Temperate Deciduous Forest 



Montane Tundra 



Prairie 



Desert 



Tropical Rain Forest 



Tropical Deciduous Forest 



Temperate Rain Forest 



Winter Rain Forest 



Northern Tundra. — The northern tundra is one of the more ex- 

 tensive and continuous of the biomes. It is circumpolar, extending from 



