408 The Community 



Community Dominance 



Two general types of situation exist in regard to the manner in 

 which the community as a whole is controlled. In the first type the 

 presence of one or a few species appears to exert a major effect in 

 determining what other species occurs in the area. In the second 

 type the physical features of the biotope tend to control the occur- 

 rence of most of the species present more directly and independently. 

 No sharp division exists between these two general conditions. A 

 spruce forest will serve as an illustration of the first category. Here 

 the general conditions of soil and climate are such that spruce trees 

 can grow in abundance and the presence of this species secondarily 

 creates a suitable habitat for a large number of other species of plants 

 and animals. Sometimes two or more species, usually of the same 

 general life form, exert a joint major control, as is seen in the oak- 

 hickory forest. This type of dependency may be represented sche- 

 matically. 



Spruce Biocenose Oak-Hickory Biocenose 



In the spruce biocenose the spruce trees are chiefly responsible for 

 the conditions of light and moisture within the forest. They modify 

 the temperature and affect the structure of the soil together with its 

 acidity and chemical composition. The spruce trees thus influence 

 the species of shrubs, herbs, and cryptogams that will grow on and 

 in the forest floor. They provide food and breeding places for spe- 

 cific kinds of birds, insects, and other animals. Grosbeaks and 

 crossbills feed on the seed cones, and woodpeckers devour the grubs 

 that live in the wood and bark. The trees also provide support for 

 pendant moss-like lichens ( Usnea), and these are used as nesting sites 

 by the parula warbler. All these species, and many more, dependent 

 directly or indirectly upon the spruce trees, make up the forest com- 

 munity. 



In the second general type of control over the community the 

 physical features of the environment tend to influence the principal 

 species independently. Situations of this sort are seen with special 

 clarity where the physical environment is particularly severe, as for 



