426 Succession and Fluctuation 



communities tend to succeed one another. The occurrence of a rela- 

 tively definite sequence of communities in an area is known as eco- 

 logical succession. The change in the communities may be due in 

 part to independent physiographic changes such as alteration of drain- 

 age, erosion, or deposition, but more especially it is caused by modi- 

 fications produced by the action of each community on its own en- 

 vironment. The two types of causes are frequently operating to- 

 gether, as is seen, for example, in the replacement of a pond com- 

 munity by a marsh community. The filling of the pond is brought 

 about by the deposition not only of a certain amount of inorganic 

 silt, but also of a large amount of the organic remains of successive 

 communities, and the accumulation of both types of deposit is en- 

 hanced by the presence of the roots and stems of the living com- 

 munity members. 



The extent to which ecological succession is self-induced— as dis- 

 tinct from being caused by changes imposed from without— varies 

 greatly in different situations. Similarly, the predictability of the 

 course and speed of succession is variable. In many instances the 

 presumed course of succession is based on inference derived from 

 studies of surrounding areas so that "space is substituted for time"; 

 but in other instances, some of which are described below, the nature 

 of the succession is substantiated by actual records. Self -induced 

 ecological succession is another outstanding example of the organism 

 and the environment acting as a reciprocating system. 



Living things modify their own habitat so as to cause one com- 

 munity to give way to another in a variety of ways. All species of 

 animals and plants tend to increase in numbers and /or in size. The 

 conditions of the community consequently change because of the 

 growth of the inhabitants even without any change in species com- 

 position. Consider a forest, for example. As the trees increase in 

 size, they provide more shade, higher humidity, and diff^erent condi- 

 tions of food and cover. New types of animals find suitable conditions 

 here; old forms may be eliminated. Wildlife managers have come to 

 realize that the carrying capacity of a forest area for game changes 

 with time because the availability of food and shelter in a stand of 

 saplings is entirely different from that in a stand of mature trees. 



When populations grow in respect to numbers or size of individuals, 

 or both, the total weight of living material in the area tends to become 

 larger. As predators and parasites increase in numbers, they tend to 

 reduce the abundance of their prey, but as food becomes scarcer the 

 consumers in turn are curtailed. At the same time the community 



