644 



COLLEGE ZOOLOGY 



series of succession is a climax community. 

 Ponds at the same stage in their develop- 

 ment tend to contain the same kinds of 

 animals; lakes of the same size and depth 

 have similar faunas; and so on for each type 

 of community. 



Ecologic succession can also be observed 

 in a jar of water containing a source of 

 organic material and bacteria. For example, 

 if a few pieces of hay are placed in a beaker 

 of water, countless bacteria will appear in 

 the water in a day or two. Soon minute 

 flagellated protozoans make their appear- 

 ance, feeding on the bacteria and on the 

 products of bacterial decomposition. Next, 

 ciliated protozoans (often Colpoda) be- 

 come numerous; these eat bacteria but are 

 themselves eaten by larger ciliates which 

 soon become dominant. After all the smaller 

 organisms are eaten, the large ciliates die 

 unless green plants are established in the 

 community. If this is done the plants and 

 animals become adjusted to one another 

 and a comparatively stable condition is 

 established which we call a balanced aqua- 

 rium (microcosm); this is essentially a 

 climax community. In this community we 

 find a few representatives of each preceding 

 stage, but the greater numbers of green 

 algae, rotifers, and crustaceans represent 

 more recent stages. 



Ecologic dominance 



In many communities certain organisms 

 will be more important than others because 

 of their size and numbers, and these will 

 have much to do with the determination of 

 its character; they can be said to control or 

 rule the community. Such organisms are 

 called dominants. In land communities 

 these are usually plants. For example, in a 

 beech-maple community, these tall trees cast 

 shade that limits the shorter plants to cer- 

 tain types, and these, in turn, limit the 

 types of animal species that can live 

 there. 



Stratification 



Stratification is the tendency for a com- 

 munity to arrange itself in vertical layers. In 

 aquatic communities this is usually due to 

 a stratification of physical factors such as 

 light, temperature, or oxygen content of the 

 water. Thus, the organisms that live on the 

 bottom of a deep lake will not be the same 

 as those near the surface. Similar divisions 

 into layers occur in animal communities, 

 but it is usually based upon the var)dng 

 heights of vegetation such as grass, shrubs, 

 and trees. Stratification often exhibits sea- 

 sonal and daily changes. In a forest, certain 

 animal species that are associated with the 

 tree tops at night may live underground 

 during the day. In fresh-water lakes there is 

 often a seasonal reversal in temperature lay- 

 ers of the water and certain animal species 

 will move with the shifting strata of tem- 

 perature. 



Periodicity 



Periodicity is the tendency for communi- 

 ties to exhibit rhythms or cycles. These in- 

 volve recurring changes in the activities or 

 movements of organisms. Thus certain spe- 

 cies may be active only during the day 

 while others are active only at night. 



In aquatic communities the zooplankton 

 organisms generally move toward the sur- 

 face at night and return to deeper waters 

 during the day. These are called daily 

 rhythms. There are also seasonal rhythms. 



Types of communities 



Fresh-water communities 



As noted earlier, the major habitats are 

 fresh-water, salt-water, and terrestrial. As an 

 example of the subdivision of a major habi- 

 tat into communities we may select fresh 

 water. This type of habitat is less extensive 

 than any of the others, but it is probably 

 better known to most of us. First we divide 



