508 GENERAL CONSIDERATIONS 



animals in forests are restricted to the ground; others, to the lower 

 vegetation; others, to shrubs; while still others reach the tops of the 

 tallest trees. These may be considered as separate communities. Thus 

 the number of possible community relations becomes great and an animal 

 may belong to a small community unit and at the same time be a member 

 of several larger units differing in numbers and areas of distribution. 

 The precise hmitation of such communities, however, is made difficult 

 by the freedom with which animals migrate from one place to another, 

 since some animals may be members of different communities at different 

 times of the day or year, under different weather conditions, or during the 

 different periods of their lifetimes. 



In any given animal community there is a food cycle or a food chain. 

 We find a starting point in such a chain or cycle in the plant life upon 

 which herbivorous animals feed. These in turn are devoured by car- 

 nivorous types, and through the metabohsm of the animal or its ultimate 

 death the materials of the body are returned to the environment, again to 

 be used by plants. It is rarely, however, that food chains can be expressed 

 in such simple terms. Thus in a pond the decaying organic matter is 

 utilized by bacteria. These in turn are eaten by certain small protozoans 

 which form the food of larger and more complex ones. The protozoans 

 may be eaten by rotifers, crustaceans, and other animals, which in turn 

 form the food of aquatic insects. The smaller fish feed upon these insects 

 as well as upon other forms, and they in turn are eaten by the larger 

 predatory fish. From these fish the food cycle may, within the body of 

 water, return to the stage of decomposition and decay; but since these 

 larger fishes are eaten by a variety of animals outside the water, the 

 cycle may not be completed within the aquatic environment. 



In nature a balance is often developed within a given environment 

 which results in a very stable condition. Such a balance, however, does 

 not long remain. Changes ensue which cause it to be disturbed, and so 

 constant readjustments are necessary. Readjustment may result in a 

 balance at a new level, but usually such changes are progressive, and thus 

 the successive states of balance are not permanent but merely steps in a 

 process which forms an orderly sequence and leads to a definite end. 



537. Succession. — The series of readjustments which have just been 

 referred to result in what has been termed hiotic succession, which is a 

 term applied to the progressive changes in the composition of a fauna 

 and flora that ensue because of modifications in the environment. For 

 instance, if an area of land is denuded of all vegetation certain pioneering 

 plants will appear first in the process of restoration. These in turn will 

 give place to others, and this will continue through a series of vegetation 

 changes. The process ultimately leads to the establishment of a per- 

 manent grassland or forest. With the vegetation changes which have 

 occurred there are corresponding changes in the character of the animal 



