438 Succession and Fluctuation 



seedlings of beech, maple, and other tolerant species that existed as 

 minor members of the community grew into mature trees when they 

 were '"released" by the removal of the pines. White pine stands 

 formed in abandoned fields of New England were therefore not self- 

 perpetuating. Once the mature pines were gone, the species com- 

 position of the forests changed completely, and gave way to a com- 

 munity of mixed hardwoods. Thus, secondary succession originating 

 in old fields of this region produces a transient community of pines 

 but eventually results in the restoration of the typical climax forest. 



Convergence. Since the bare areas in which primary succession 

 can be initiated are extremely diverse, it is not surprising to find a 

 correspondingly great variety in pioneer communities. As succession 

 proceeds, however, the different biotopes tend to be modified toward 

 a more nearly uniform condition that will support similar communi- 

 ties. This convergence is particularly clear in relation to the moisture 

 factor— serai communities cause hydric habitats to become drier, and 

 xeric habitats to become moister, so that a trend toward a more nearly 

 mesic condition is followed from either extreme. As a general prin- 

 cipal we recognize a convergence in succession such that seres orig- 

 inating in diverse habitats within a region of similar climate tend to 

 progress toward similar climax communities. 



An examination of succession in the deciduous forest region of 

 Indiana revealed a tendency to converge that could be traced in seres 

 originating from five kinds of biotopes as shown in the scheme on page 



439 from Clements and Shelford ( 1939 ) . Pioneer communities 

 established on a sand ridge, clay bank, flood plain, shallow pond, or 

 deep pond initiated the successions indicated by the serial numbers 

 with the dominant plants named. The exclusive presence, or relative 

 abundance, of a dominant plant served as a serai index to the stage 

 of succession in each sere. An animal species characteristic of each 

 stage could also be recognized, and the names of these animal serai 

 indices appear in the diagram. Succession in each of the widely 

 different original areas tended to bring about the eventual establish- 

 ment of the same type of climax community in which beech and sugar 

 maple trees and the salamander, PlcfJiodon cinereus, are the indices. 



Succession in Special Habitats. The instances of succession dis- 

 cussed above are typical of broad areas in temperate regions. Other 

 manifestations of succession occur in special habitats— either large or 

 small. A classical example of succession in a microhabitat is the 

 sequence of bacteria, protozoans, and other microorganisms that follow 

 one another in a hay infusion formed by allowing hay to rot in a 

 quantity of water. The seasonal succession of different kinds of 



