Intercommunity Cycle 



The intercommunity cycle is the kind of change whereby one 

 type of community repeatedly changes to another type and then 

 returns to the first one, such as the invasion of a forest by a bog, 

 and then the return of the bog to a forest condition again — 

 reported as occurring in the Upper Kuskokwim River Region, 

 Alaska. '2 jf thg habitat changes, the vegetation must change. 

 There may be fluctuating climaxes of complementary commu- 

 nities changing in time and space, such as the alternation of steppe 

 and marsh vegetation in a given locality. 2°^'^^'^^ Another ex- 

 ample is a cyclic development of marshes, above the northern 

 limit of conifers in northern Sweden, beginning with the filling 

 of small lakes with marsh vegetation. ^^ Then progressive direc- 

 tional change takes place toward xerophilous heaths rich in 

 lichens, followed by the regeneration of lakes in wind-eroded 

 hollows, and thus the marsh-heath cycle is initiated again. 



It is often difficult to determine the nature of changes. For ex- 

 ample, an intercommunity cyclic change may have several 

 phases comparable to the upgrade and downgrade sequences as 

 described above for microcommunity cycles. Since a community 

 cyclic-change system would first be apparent in replacements on 

 a microcommunity basis, it might be difficult to differentiate 

 cycles on the community and on the microcommunity units of 

 analysis. Replacement change is undoubtedly operative within 

 the phases of an intercommunity cycle, so the change from one 

 phase to another could be confused readily with directional 

 changes. Thus a change in the upgrade series could be confused 

 with succession, and a change in the downgrade series with 

 retrogression. 



Fluctuation Change 



Fluctuation change is a random fluctuation about a norm or 

 average. Any given habitat, even a long-persisting one, is never 

 static, irrespective of the time scale used as a reference. The 

 organisms characteristic of the habitat have adaptability to it 

 and its fluctuation changes, and react to such changes in an in- 



1,4A * Dystamics off ComsitisMvities 



