CLIMAX FRESH-WATER COMMUNITIES 



309 



(Krecker and Lancaster, 1933; cf. Needham, 1901; Needham and 

 Christenson, 1927). 



Although the development of this community has been observed but 

 little, it is evident that it takes place very rapidly in denuded areas 

 through invasion (a) by individuals that have lost attachment farther 

 upstream, (6) by the deposition of eggs (aquatic insects) or (c) by 

 movement upstream, as in the case of fishes. Invasions by the third 



Fig. 72. — The constituents of a rapids community. Figs. A-H, general form of 

 rapids animals. Drawn on the same scale; all about natural size; seen from 

 the side slightly above. A, the rainbow darter {Oligocephalus coeruleus Stor.) ; 

 B, a third-grown hog-sucker {Catostomus nigricans Le S.) ; C, snail (Goniobasis 

 livescens Mks.) ; D, caddis worm {Hydro-psyche) ; E, damsel-fly nymph {Argia 

 sp.) ; F, stone-fly nymph (Perla sp.) ; G, may-fly nymph {Heptageninae) ; H, 

 water penny {Psephenus sp.). 



method have been especially noticeable following floods in small 

 tributaries ordinarily intermittent. The rapids in these acquire a 

 large part of the prevalcnts of the swift-water community during two 

 seasons of continuous flow in a rainy period. However, these preva- 

 lents do not have the characters of dominants for the following 

 reasons : 



1. They do not control the habitat, but modify it only slightly. 



2. The control is primarily by physiographic forces. 



3. The different species are not dependent upon one another 

 since they neither render conditions more suitable for other or- 



