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COMMUNITY ECOLOGY: 



tion of communities rather than taxa. Biogeograph- 

 ical succession is very similar to biotic succession but 

 is of much longer duration, treating the development 

 of all past floras and faunas through space and time 

 rather than biotic climaxes under a relatively stable 

 space-and-time segment. It is particularly difficult to 

 relate to a serai concept because a single biogeograph- 

 ical succession {bwgeographical sere) encompasses the 

 evolution of all past communities. Therefore, a bio- 

 geographical sere includes all of the complex com- 

 munities between the first living organisms and the 

 present biosphere in the development of all life on 

 our planet. 



The various kinds of succession are phases of a 

 single phenomenon, changes of life and its environ- 

 ment through time. Moreover, each phase is closely 

 related to all others. Biotic succession is of shortest 

 duration. Biotic succession is related to climatic 

 succession, and climatic succession might be due to 

 physiographic changes. Finally, geologic and bio- 

 geographic succession present long-term views of the 

 consequences of the other types of succession. 



TERRESTRIAL SUCCESSION 



In terrestrial environments, including both land 

 and fresh water, the entire concept of biotic succes- 

 sion is most closely applied to plants. This is the 

 consequence of plants' usually showing greater 

 ecological restrictions than animals. The ecological 

 relationships of animals usually can be associated 

 with those of plants. 



Climaxes on land are largely due to climate. How- 

 ever, if climate is the only criterion of the climax, 

 only mature to old land forms would allow time for 

 full development of soil and the rest of the environ- 

 ment necessary for a single climax in one locality. 

 Limitation of climaxes to the climatic type alone also 

 would ignore many habitats that are in dynamic equi- 

 librium (the fundamental criterion of a climax) be- 

 cause of environmental factors other than climate. 

 Actually these other factors are never the sole reason 

 for any climax; the so-called nonclimatic climaxes 

 really are affected by climate. For example, certain 

 localized areas are in apparent equilibrium, but these 

 communities differ from climax communities of the 

 climatic type, in that their organisms are more 

 closely afifected by factors of soil, topography, and 

 recurring fires. Many ecologists now would consider 

 these localized communities to be climax (edaphic, 



topographic, and fire climaxes) because it seems that 

 the environments of such communities are relatively 

 fixed and the various communities display dynamic 

 equilibrium in their particular environments. 



In addition to primary and secondary climaxes, 

 climax patterns are now being recognized in which 

 vegetation is conceived as a pattern of populations, 

 variously relating to one another, and corresponding 

 to the pattern of environmental gradients (a con- 

 tinuum concept). Therefore, one should realize that 

 a climax is a homogeneous thing only in a restricted 

 space and that one climax habitat often grades into 

 another climax. In fact, all vegetation might be 

 considered part of a single, world-wide continuum. 



SUCCESSION AND GEOMORPHOLOGY 



Plant succession bears a relationship to geology 

 and, in a broad sense, to the constructive phase of a 

 geomorphic cycle. The destructive phase resem- 

 blance (regression) would be caused by fire, vol- 

 canism, disease, erosion, and any other phenomenon 

 that can bring about biologically sterile or disturbed 

 areas. On land the sterile areas can be of many types; 

 however, the types approach two extremes, bare rock 

 and water. Study of succession, starting from 

 biologically sterile bare rock and water, will illustrate 

 the various types of succession. 



XERARCH SUCCESSION 



Succession on land normally starting from bare 

 rock is called xerarch succession. Many successional 

 stages are possible (Figure 19.8). The primary stage 

 usually consists of combinations of algae and fungi 

 called lichens but may be blue-green algae. In se- 

 quence, the remaining possible stages usually are 

 moss stage, herb stage, shrub stage, tree stage, and 

 climax. However, many deviations from this se- 

 quence are possible. First, particular intermediate 

 stages — the moss stage, for example — may be absent. 

 Second, more than one stage of the same general type 

 may occur. For example, more than one herb stage 

 might exist. Third, terminal or near-terminal stages, 

 especially the shrub and tree stages, do not necessar- 

 ily precede the stage of climax. A shrub climax, for 

 example, may form prior to the invasion of any tree 

 stage. Fourth, there can be a reversal of stages on the 

 way to the climax. Such a condition could come 

 about in an area having a stream. For example, the 



