ANIMAL ORGANISMS IN RELATION TO THEIR ENVIRONMENT 507 



in the precise definition of animal communities which zoologists have 

 hitherto been unable to overcome. The seashore affords examples of 

 rather definite and stable animal communities, but such communities are 

 to a lesser degree characteristic of fresh water and are rarely found in 

 terrestrial environments. 



534. Ecological Factors. — The factors in the environment which 

 affect animals most strongly may be enumerated under two heads: 

 physical and biotic. 



Among the physical factors are the presence and composition of water, 

 temperature, light, and molar agents, such as wind and currents. The 

 chemical composition of water involves salts which are more or less 

 ionized (Sec. 18). This results in an increase in alkahnity or acidity. A 

 measure of this ionization is the acidity corresponding to the number of 

 hydrogen ions present in a given unit volume of a solution, indicated by 

 the symbol pH (potential hydrogen) . A pH concentration indicated by 

 7 corresponds to neutrality. Any concentration indicated by a larger 

 number imphes alkahnity; by a smaller number, acidity; and the amount 

 of departure measures the degree of either alkahnity or acidity. It 

 has been found in the case of certain organisms that the pH concentration 

 of water is a very important factor, but in the case of others it seems 

 to have little or no effect. The pH can be determined not only for bodies 

 of water but also for soils containing water and for body fluids. 



The biotic factors in the environment relate to plants and to animals 

 of either the same or other species and affect the organism in such ways 

 as through food supply, competition, mutual help, as in animal com- 

 munities, and the relations of the sexes. 



535. Reactions of the Animal. — In response to these various factors 

 of the environment, reactions take place within the body of the animal 

 which find expression in form, size, and color; in the physiological adjust- 

 ments which the animal makes; in its behavior; in its mode of reproduc- 

 tion ; and in its length of life. Ecology should concern itself fully as much 

 with the reactions within the animal itself as with the conditions of the 

 environment in which it lives and to which it reacts, though up to the 

 present time investigations have dealt largely with the latter aspect. 



536. Commumties. — As has been stated above, animals present them- 

 selves in communities which may vary greatly in extent. A pond or 

 lake is in a sense a great community of aquatic organisms, but within 

 this environment are many lesser environments such as the shore, beds of 

 aquatic vegetation, the surface, and the deeper portions. Each of these 

 smaller parts of the whole has its peculiar animal types. The character 

 of the vegetation, differing in different areas, may directly affect the 

 character of the community of animals found in each. In bodies of 

 water of any considerable depth, as well as in the vegetation in the case 

 of terrestrial forms, there is the phenomenon of stratificaiion. Certain 



