COMMUNITIES AS INDICATORS. 73 



The basic correlations of communities may be illustrated by the following 

 diagram (fig. 2.) : 



CuuAX Formation. 

 society - consociation - association - ecotone - association - consociation - society 



1 I. . . 



Bocies - consociea - associes - subclimax - associes - consocies - socies 



(Prisere) associes. associes. (Subsere) 



colony. 



t 



family. 

 Fia. 2. Diagram of the climax and serai communities of the formation. 



Community structures. In addition to the units themselves, associal and 

 consocial communities show general structural features, such as zones, alternes, 

 layers, and aspects. These are due primarily to the grouping or appearance 

 of the subordinate communities with reference to a particular factor or factor- 

 complex, and are of the greatest indicator value. The well-known zonation 

 of the hydrosere in and about ponds is the best example of this. Each zone 

 not only marks the general factor limits for its proper community, but also a 

 distinctive step in the decrease of water-content and the increase of soil-air 

 from the extreme conditions in the center. Such a series actually shows on 

 the ground the " before-and-after " correlation of each stage typical of succession. 

 Serai zones may be formed by consocies or associes; in their fullest expression 

 the major zones are marked by associes within which occur minor zones con- 

 stituted by the consocies in the order of their requirements. The zones of 

 high mountains are essentially similar, though they have to do with climax 

 associations and consociations. The same zonal structure recurs universally, 

 wherever climax or serai communities are grouped about a center of excess or 

 deficiency of some factor or group of factors. Zonation is sometimes obscured, 

 especially in the dense vegetation of prairies (Plant Succession, 133), but 

 it is rarely altogether absent, except in initial communities. 



Alternes. Alternes are due to the interruption of zonation through any 

 cause whatsoever (Clements, 1916 : 115), but they are especially typical 

 where disturbed or other successional areas are found. They are frequent 



