312 



ECOLOGY 



communities of small permanent streams and base-level streams are 

 indicated above. Taking up another line, we note that the large-lake 

 communities are succeeded by the small-lake communities. Rocky- 

 shore communities of the large-lake areas have features in common with 

 those of the rocky rapids of the stream. The sand, gravel, and vegeta- 

 tion communities of the base-level stream and the small lake have many 

 things in common, while the silt and humus bottom communities are 

 distinguishing features of the two. Communities of ponds originating 



Sand 



u Climatic 



sl Jr*Moist Forest Margin_*Forest Margin 

 ^ or Thicket for Thicket 



DIAGRAM 9. Showing some relations of the chief animal communities of the 

 forest-border region of Central North America. The word community or communi- 

 ties is to be understood as following all the words appearing in the diagram. For full 

 description see text. 



by very rapid physiographic changes pass through a series of stages 

 comparable to those found in the different parts of the small lake. The 

 lake communities pass to the pond community stage or give rise to a 

 floating-bog marsh community which is displaced by a floating-bog 

 thicket community. Cowles states that this takes place in deep lakes, 

 while the shallow ones become ponds which give rise to marshes with 

 firm substrata. Such a marsh community may be displaced wholly 

 by a low prairie community, in part by a thicket forest margin com- 

 munity, or wholly by a thicket community which will be succeeded by 



