ECOLOGICAL SUCCESSION. 361 



FIG. ii. Showing some relations of the chief animal communities of the forest 

 border region of central North America. The word community or communities 

 is to be understood as following all of the words or phrases appearing in the diagram. 

 Single pointed arrows show the directions of succession, double pointed arrows show 

 similarities of conditions and the occurrence of several or many of the same species 

 in considerable numbers in some of the strata of communities between which such 

 arrows extend. Broken lines indicate less definite relations than the solid lines. 

 Starting with the aquatic communities we note that spring fed and intermittent 

 stream communities converge with physiographic ageing to small permanent swift 

 stream communities and permanent swift stream communities are succeeded by 

 base level stream communities. The characteristic communities of small perma- 

 nent streams and base level streams are indicated above them. Taking up another 

 line we note that the large lake communities are succeeded by the small lake com- 

 munities. Rocky shore communities of the large lake have features in common 

 with those of the rocky rapids of the stream. The sand, gravel and vegetation 

 communities of the baselevel 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 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 substratum; 

 the marsh community may be displaced wholly by a low prairie community, in 

 part by a thicket or forest margin community or wholly by a thicket community 

 which will be succeeded by a forest community. In the savanna or prairie climate 

 the marsh margin thicket may become a climatic thicket or forest margin. In the 

 savanna or prairie climate the communities of all the various soils and the low 

 prairie community may converge to the climate prairie community, or to the forest 

 community as is shown below for the forest climate. In the forest climate and 

 locally in the savanna climate the communities of all the various soils pass through 

 a thicket community stage (T) related to a climatic forest margin. The thicket 

 communities of all the dry soils are related to the forest margin or thicket com- 

 munity of the savanna climate. 



