Long-continued studies in the same area, like those of Cooper^'^-^^ 

 in Glacier Bay, Alaska, are needed. 



Summarizing, the stages in the directional change or sequence 

 of communities can be recognized. The changes from the initial 

 establishment of vegetation on an area to the terminal climax 

 community are continuous. However, a given group of species 

 will reach a peak of dominance at a certain stage of the sequence. 

 Then as the dominance of this group decreases, the dominance 

 of another group of species will develop to a maximum. This 

 group is then characteristic as an indicator of its respective stage 

 of the directional change sequence. The change from one stage 

 to the subsequent stage may be especially prominent when there 

 is a change of life-form of the dominant species. There are cer- 

 tain generalizations which can be made concerning the trends in 

 communities when the directional change is a progression. There 

 is usually an increase in productivity per unit of area, in organic 

 mass per unit area because of the presence of the larger life-forms, 

 in complexity and diversity of species and life-forms, and in the 

 relative stability and homogeneity of the populations. The soil 

 and the other aspects of the habitat also undergo progressive 

 development. 



Changes in Space 



The changes heretofore considered have been within commu- 

 nity boundaries. Changes in space resulting in a mosaic of com- 

 munities, on the other hand, are caused by the distribution of 

 environmental gradients. 



As discussed previously, environmental gradients of significant 

 magnitude do occur, they usually persist for a very long period, 

 and there is no known extensive area with a wholly uniform en- 

 vironment. The zone between two communities is considered a 

 reliable indicator of the steepness of the environmental gradient 

 between them. If there is a sharp transition, then the gradient is 

 usually of great magnitude such as that between a sedge-marsh 

 and a dwarf-shrub heath. Bryophytes are especially good indica- 

 tors of environmental conditions in arctic areas. ^^-^^^ However, 



].5<^ • Dynamics of CoznnmvaMmities 



