356 



COMMUNITY ECOLOGY: 



to the existing and evolving ecosystem. In general, 

 direction is present in reference to a fairly pre- 

 scribed "path" toward greater complexity in the 

 community as a whole. 



Cyclic changes were mentioned already as reflections 

 of environmental cyclic changes (pp. 335-337). These 

 community cycles are diurnation, aspectation, or 

 any longer periodic cycle. 



Iniercommuniiy Changes. Intercommunity changes 

 involve interworkings among adjacent stands; thus, 

 they have a strong spatial emphasis. 



Noncyclic irregular changes can be due to periodic 

 opening and closing of barriers during geomorphic or 

 climatic cycles, and lead to irregular additions, and 

 perhaps subtractions, of species in the diflTerent com- 

 munities involved. .Noncyclic directional changes may be 

 in the form of cliseral shifts, evolution of new com- 

 munities from the mixing of the old, succession, and 

 regression. Succession (progression in place to a 

 community of stability) will soon be discussed in 

 more detail. Regression is a partial reversal of 

 normal succession, owing to such things as natural 

 catastrophies, human influence, and overgrazing. It 

 is treated under the topic of succession as secondary 

 succession. 



Cyclic changes are similar to regression, but are due 

 to a normal progression from one to another com- 

 munity and then back to the first (Figure 19.3). For 

 example, in certain marshes there is a progression to 

 a marsh community that is destroyed by wind or 

 other normal features of the environment and from 

 this destruction there is development to a marsh 

 community like the original one. 



advanced stage in hydrarch 

 or aquatic succession 



late sere or climax 



SPATIAL CHANGES 



Environmental gradients are the rule in nature. 

 Gradients may display close or remote association in 

 individual factors. The effect of this interplay of 

 gradients is to create either a regular or an irregular 

 continuum; in other words, adjacent communities 

 may be part of a geographic gradient of similar 

 communities or may be part of a geographic mosaic 

 of less closely related communities. Here, the word 

 "community" is used in the broadest sense. A 

 community can range in size from very limited to ex- 

 pansive areas and can even be large enough to in- 

 clude a hierarchy of smaller communities, one within 

 the other. 



Communities through space do not clearly reflect 



wind excavates old depression and 

 water collects so succession starts 



Figure 19.3 A cycle of community progression. 



the gradients in their environment. No organism is so 

 narrowly restricted to a unique set of environmental 

 conditions that it can occur only at one point in an 

 over-all environmental gradient. Rather, each spec- 



