360 



COMMUNITY ECOLOGY: 



tions must be a dynamic one. Also, the fact of dy- 

 namic equilibrium necessitates a synergistic response 

 to the environment by the climax. Otherwise, the 

 climax could not partially control its environment to 

 the extent of preventing extremes in the physical en- 

 vironment from destroying some of its organisms. 

 This is another way of stating that climaxes have 

 some power to control their environments and serai 

 stages do not. 



OCCURRENCE 



Succession is not a unique phenomenon that takes 

 place in a single fragment of the biosphere. It is 

 found in most, if not all, habitats. For example, 

 succession is found among the microscopic animals 

 in water. It can occur in as temporary a situation as 

 a puddle of water. Also, a fallen tree may go 

 through a sequence of serai stages that are as- 

 sociated with the progress of complete decay. Al- 

 though such microsuccessions are of definite interest, 

 they are beyond the scope of this book. Present 

 treatment is limited to the two general patterns of 

 large-scale land succession, that starting on a dry 

 substrate and that starting in fresh water. 



Succession starting in either dry or water areas 

 can be of two types, primary or secondary. Primary 

 succession starts on bare rocks or in water where vege- 

 tation has not grown before. Secondary succession is 

 the consequence of disturbance of previous vegeta- 

 tion, serai or climax, and begins after such things as 

 fire, abnormal weather, overgrazing, or cultivation. 



Succession is an orderly phenomenon; however, 

 one can predict the likely stages only in primary suc- 

 cession. For this reason, only primary succession 

 from rock and from water will be examined in detail. 

 Secondary succession resembles primary succession 

 if the disturbance of previous organisms was not too 

 great. On the other hand, severe disturbance might 

 create a unique set of serai stages. This might even 

 be true in the case of mild disturbance. For example, 

 if some of the climax vegetation is removed from an 

 area, secondary succession might consist of one or 

 more peculiar serai stages in which both climax and 

 serai plants are present. 



COMMUNITIES AND SUCCESSION 



A convenient classification of communities is 

 made on the basis of successional status. Serai com- 

 munities are those of a temporary nature that will be 



replaced by other communities. Climax communities 

 are in a self-regenerating state of equilibrium 

 and are not replaced by another community unless 

 some outside disturbance affects their environments. 

 Climax communities are also called associations. 



Climaxes and the associations they represent can 

 be of two major types (Figure 19.6). Primary climaxes 

 are those that tend to reflect major environmental 

 features. They include the climatic climaxes that are 

 found on gentle topography and loamy soils; edaphic 

 climaxes that are in unique soil conditions; topographic 

 climaxes that are in slope, exposure, or some other 

 unique landscape feature; and lopoedaphic climaxes 

 that are in places with unique interrelations of 

 topography and soil. 



The second major type of climaxes, secondary 

 climaxes or disclimaxes, reflect disturbance. Dis- 

 climaxes include those produced by animals, zootic 

 climaxes, and by fire, fire climaxes. Although all pri- 

 mary and secondary climaxes are recognized here as 

 true climaxes, many ecologists treat only climatic 

 climaxes as true climaxes. 



CAUSES OF SUCCESSION 



It might seem that most of the landscape should 

 consist of climax communities. However, this is often 

 far from the case. There are portions of the earth 

 where most of the communities are stable and in 

 equilibrium, but widespread areas are composed of 

 serai stages alone. Various things contribute to the 

 presence of serai stages. They may come from pro- 

 nounced environmental changes that are sufficient 

 either to remove a climax or to cause the old climax 

 to be unstable. Possible sources of new environ- 

 ments can be geology, weather, or life itself. Geo- 

 morphic cycles, such as those involving glaciers, vol- 

 canoes, earthquakes, tidal waves, landslides, and 

 shifting streambeds, can remove vast areas of climax. 

 Weather catastrophies, such as floods, tornadoes, 

 hurricanes, or rare extremes of temperature and 

 moisture also can alter succession. Biotic factors, 

 including great increases in plant-eating animals, 

 overgrazing, and disease, can denude or modify a 

 habitat. In addition, fire, no matter what its source, 

 may be extremely destructive. 



KINDS OF SUCCESSION 



In biotic succession, the kind treated here, both 

 plants and animals are related in bioseres (often 



