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 
<a 
Silt > Climatic 4 Gravel 
aS 
ce. 
rs Rock 
ae Bottom ; 
d Climatic 
’ ies Sees ioe a Margin—Forest Margin 
ateseond te ee rho, ea, icket for Thicket 
: Rs _ suscarn Base Level Stream—p 9, 4 
x * ¥ oe Silt Bottom ~%y, a ' 
ee oe ¢ Thicketwe-=- geThicket i Rock 
Spring Fed eae. a 
‘ 
ge : 
< AVP 
ae Silt @T—» Climaticg Grave} 
yw Forest ; 
mA a 
Ke prog t 
Clay Sand 
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 
