142 Adams, Ecological Succesision of Birds. [tndi 



greater number of birds can live in a given area, but also that they 

 can live closer together, if they vary in kind. Then again, within 

 the association there are marked differences in habitat preference. 

 Thus in the forest there are those birds which nest in the trunks 

 or among the topmost branches of the trees, or even upon the 

 ground ; and these are differences largely distinct from the spacing 

 of the pairs of the same species. These influences must be recog- 

 nized among the dominant influences within the association, and 

 upon which much emphasis must be placed. 



4. Environmental Factors. Then in addition to these internal 

 factors, there are the dominant physical factors. In the following 

 discussion primary em})hasis will be placed upon succession as 

 found in the Northeastern Biotic or Conifer Center, because succes- 

 sions at other centers Avith different biotic components and other 

 dominant physical conditions nmst possess a certain amount of 

 individuality, in addition to those features common to succession in 

 general. The dominant biotic tendency or d\Tiamic trend of this 

 center, as a resultant of all internal and environmental influences, 

 is for the conifer biotic association to encroach iipon all other socie- 

 ties and habitats and to become the dominant or universally distributed 

 association. Thus, in general, all habitats produced by local in- 

 fluences tend to become transformed into the dominant biotic 

 association or formation. In general also, small bodies of water 

 are rapidly encroached upon by inwash, vegetation or drainage, and 

 tend to become extinct and forested. All other openings, as the 

 rocky ledges and ridges or burns, are encroached upon as soil 

 accumulates or fires are prevented, and the forest biotic association 

 spreads over the entire area. 



From such relations it will be seen that our knoAvledge of the 

 causes and conditions of succession must largely result from the 

 study of these local environments or habitats and their biotic succes- 

 sion, because, where dominance is established the succession is 

 almost completely obliterated. Each minor habitat and society is 

 to be looked upon as simply a stage, more or less temporary, in the 

 onward wave toward the dominant or climax association. Thus in 

 the marshes, birch or aspen woods, rock openings and ponds may be 

 "orginal" condition* which are becoming cumulatively transformed 

 in the direction of the final dominance of the climax biotic type. 



