Figure 5-2. The invasion and aggregation of Poa macrantha 

 by means of long stolons among the clumps of Ammophila 

 arenaria produces an increase in complexity as succession 

 progresses on the sand dune. Oregon. (U.S.D.A. Soil Con- 

 servation Service.) 



succession occurs in an area where the habitat conditions have 

 been altered by the plants themselves, it is autogenic; when 

 altered by agents external to the plants, it is allogenic. Direc- 

 tional change must rank as a change of major consequence. This 

 change occurs where the vegetation has not reached a steady- 

 state and the progression culminates in a steady-state, climax 

 community. Directional changes, that is, the stages and processes 

 in succession, have been described in much detail by many 

 writers 26.47.41.58.198 



Succession is commonly and preferably considered as a pro- 

 gressive development from a simple to a more complex commu- 

 nity (Figure 5-2) which usually involves one or more of the 



Habitat Patterns, CHanges, and CliBna: 



149 



