geologic erosion, block uplift, glacial retreat, and climatic change. 

 Such changes are at so slow a rate, that usually no significant 

 alteration occurs during the period of field study conducted by 

 one investigator. The same pattern of communities usually per- 

 sists, even though the spatial disposition of each changes. 



Rate of Change 



The rate of any type of change is important. The analysis of 

 changes within a community and between communities is facil- 

 itated by determining the rate of each, preferably in absolute, but 

 at least in relative, terms. 



An indication of the rate of development or succession of a 

 hchen community may be found in the fact that lichens of the 

 family Umbelicariaceae require several hundred years to attain 

 full growth, ''^ so these lichens are, of course, absent where soU- 

 fluction occurs. A good example of a study showing the rate of 

 succession following the retreat of glaciers is that of Cooper, at 

 Glacier Bay, Alaska.^'*^^ The duration of stages in secondary 

 succession on old fields and on forest lands has also been given 

 by several writers. ■^^•^^•^^^ 



The permanence of many communities in arctic and alpine 

 areas indicates that changes in the habitat are not significant or 

 that the species present in rapidly changing habitats such as 

 solifluction areas, are adapted to them either by special structural 

 features or by wide ranges in ecological amplitude. The duration 

 and stability in Scandinavia of arctic and alpine plant commu- 

 nities not influenced by man, even of the half-open grass-heath 

 communities such as those o{ Kobresia myosuroides and Festuca ovina, 

 have been stressed.^^^ The habitats and vegetation of polygons on 

 Akpatok Island have probably changed very little in 1000 to 

 2000 years.'" 



The numerous arctic and alpine species which survived glacia- 

 tion in refuges, as indicated by biogeographic evidence, have had 

 a long time in which to form communities of great stability. 

 Evidence for the great age of the arctic and alpine flora in 

 Scandinavia is seen particularly in the relatively large number 

 of endemic species and the presence of a large west arctic element 



158 • Dynamics of ComBimunities 



