310 AQUATIC CLIMAX COMMUNITIES 



ganisms nor maintain conditions suitable for themselves. They are 

 neither climax nor subclimax dominants. They are comparable 

 to lichens on granite rock, though far less effective. 



Comparison of Aquatic Communities. The habit ratios based upon 

 species of animals outstanding in importance for the rapids and for 

 the muddy pool community in a stream in central Illinois are shown 

 in Table 12. 



TABLE 12 



It is difficult to experiment with the animals of most communities, 

 but the swift-water community lends itself to such treatment. Shel- 

 ford (1914, a) found that it is characterized by two common reac- 

 tions: (a) marked positive response to a comparatively strong water 

 current, (t>) positive response to rock as opposed to sand or other bot- 

 tom materials. This was so striking as to constitute a life-habit char- 

 acteristic of the community, which was consequently termed rheotac- 

 tic. This characteristic is easily illustrated by experiment on the 

 common species listed above as belonging to the rapid-water com- 

 munity, such as Etheostomids, Hydropsyche, Heptageninae, Perla, 

 and Corydalis. These forms practically always show an 80 to 100 per 

 cent positive response to a fairly strong water current and a similar 

 one to large stones as opposed to sand. In the study of current, it is 

 necessary to consider only those individuals actually in it, and this 

 may be accomplished by omitting all those resting in contact with ob- 

 jects against which the current strikes at right angles. Many will 



