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Figure 5-3. Replacement of jack pines (Pinus banksiana) 

 in this 70-year-old stand by the balsam fir {Abies balsamea) 

 which is capable of growing in the shade of the pines. July, 

 Minnesota. (U. S. Forest Service.) 



following: diversity, stability, productivity, self-maintenance, 

 uniformity within and between stands, and soil maturity. This is 

 considered as a positive directional change or a progression. 

 Each community or ecosystem is in its respective state of 

 flux.^^^ Within a nonstable community, development takes place; 

 there are biotic interactions. This gradual development of the 

 vegetation, such as the migration of Sitka spruce {Picea sitchensis) 

 westward on Kodiak Island, Alaska, results in a gradual change 

 in the habitat. The habitat is of course also subject to any phys- 

 ical changes which are taking place and which are independent 

 of the biotic influences. The gradual change in habitat is such 

 that the conditions become less suitable for the contemporary 

 species, which are then replaced by species better suited to the 

 new habitat, and thus a new type of community develops (auto- 

 genic succession). There is usually a change of species composi- 

 tion, but in some cases the change is in the populations or quan- 



150 • Dyzmamnics of Coanmnvaimities 



