OTHER COMMUNITIES 311 



show the clinging reaction as an expression of their choice of large 



rough rock surfaces. 



OTHER COMMUNITIES 



Intermediate between the swift-water communities and the pool 

 climax are various others, chiefly on sand and gravel bottoms. Those 

 on gravel partake of the general life form, life-habit characters, and, 

 to some degree, of the taxonomic composition of those of the rapids. 

 Sand communities resemble the pool climax. A type of community 

 noted by Gersbacher (1937) on sand is characterized by a predomi- 

 nance of various sphaerids and fresh-water mussels. It lacks sharp 

 distinction from the pool community and its developmental stages, 

 and more study is required to determine its rank. Obviously, animals 

 do not dominate the sand habitat in the way they do in the baselevel 

 mud-bottomed pools (cf. Reighard, 1908). 



The relation of stream to lake communities is made evident in 

 glaciated areas such as northern Illinois and Wisconsin. Here the 

 very sluggish rivers connect lakes which are in several cases merely 

 broad and sometimes irregular expansions of the river itself. These 

 rivers sometimes contain the climax community, but vegetation grows 

 in them and often covers much of the bottom, which contains much 

 organic matter. Dr. D. H. Thompson (Illinois National History Sur- 

 vey) has observed that during the recent drought period vegetation 

 appeared in the Rock River, which includes areas of climax. He 

 believes this is due to lower turbidity resulting in better light condi- 

 tions. In the Fox River (at Gary, Illinois), vegetation occurs along 

 the margins and the climax at points between the vegetation and the 

 center of the stream. In other words, there is a tension between land 

 and water climaxes which manifests itself in these wide sluggish 

 waters. 



Streams are in constant state of change from season to season or 

 from year to year, except in the physiographically stable old age or 

 baselevel condition or in other conditions approaching this, due to 

 retarded flow. 



The small unstable streams likewise show communities in which 

 development may be traced for a time, but which are soon destroyed 

 by flooding and thus rendered difficult to study. The literature deal- 

 ing with the stable or climax communities is cited in connection with 

 the description of them on pages 305 to 307 (see Gole, A. E., 1932). 



Fresh-water climates and climaxes have only very recently been 

 recognized. One of the earliest ecological classifications of com- 

 munities divided them into edaphic (including water) and climatic. 

 The local character of the edaphic communities and the very exten- 



