642 



COLLEGE ZOOLOGY 



Alpine Plateau 

 I — 7^ — '• 



Q> 



O 



Mountain 



Lowland 



Continental shelf 



Ocean 



Figure 450. An ideal section at the margin of a continent to indicate the location of various 

 types of ecologic environments available to animals. The diflFerent ecologic areas shown make 

 possible different biotic communities. 



ment exists, are likely to contain similar 

 types of plants and animals. For example, 

 fresh-water ponds are never exactly alike, 

 but the environment in them is similar, and 

 we look for the same kinds of plants and 

 animals in them. Pond animals may be 

 permanent or temporary residents, or merely 

 visitors. Among the permanent residents are 

 protozoans, some crustaceans, a variety of 

 snails, and other small animals. Frogs, toads, 

 and some salamanders are temporary resi- 

 dents; and water birds such as ducks and 

 herons visit the pond to feed on the animals 

 in it. Hundreds of different types of com- 

 munities have been described and classified 

 by ecologists. 



There are a number of ecologic principles 

 that are best understood in terms of com- 

 munities; among these are succession, domi- 

 nance, stratification, and periodicity. 



Ecologic succession 



When a community has reached the com- 

 parative stability of dynamic equilibrium, it 

 is called a climax community. But every 

 climax community has had a history of 

 changes; such changes constitute ecologic 



succession. A new habitat, as, for example, 

 a pond resulting from the overflow of a 

 river, may contain certain types of fish, in- 

 cluding black bass and sunfish. Later the 

 sides of the pond become overgrown with 

 vegetation and the clean bottom becomes 

 covered with deposits; the black bass and 

 sunfish disappear because such an environ- 

 ment is not suited to them, but catfish may 

 persist. Still later, just before the pond be- 

 comes a swamp, the mud minnow may re- 

 place the catfish. Finally, conditions become 

 such that no fish are able to live in the 

 habitat. Here are several stages, each differ- 

 ing in the type of fishes and other animals 

 present. The final stage (Fig. 451) in the 



Figure 451. Facing page, ecologic succession, 

 simplified. Pond succession, from the practically bare 

 bottom (pioneer) stage, which is gradually replaced 

 by sequential stages, each more mature than the 

 preceding one, until a climax is reached where the 

 community becomes relatively stable. The stages 

 shown here are based on studies in the Middle West. 

 The succession in ponds with a different climate will 

 not be the same in detail; and the climax will be 

 different. Several intermediate stages are omitted. 

 (Redrawn with modifications from Basic Ecology by 

 R. and M. Buchsbaum, Boxwood Press, Pittsburgh.) 



