308 AQUATIC CLIMAX COMMUNITIES 



The different habitats in a small-river system flowing through an 

 area without rock outcrops, such as characterize many of those in the 

 eastern portion of the Mississippi Valley, comprise the following, in 

 the order of importance: 



1. Mud-bottomed pools. 



2. Shifting sand-bottomed pools. 



3. Semi-pools with gravel bottoms. 



4. Coarse-gravel and rocky rapids. 



Farther west on the Great Plains, sand bottoms predominate 

 (Jewell, 1927), while in hilly and mountainous regions, rapids with 

 larger rock and gravel-bottomed semi-pools are typical. Obviously, 

 the character of the underlying rock determines the quality of the bot- 

 tom material and the degree of benching. This results in the alter- 

 nation of deeper and shallower water with corresponding differences 

 in rate of flow. 



In dealing with the community history of a stream, its develop- 

 ment as discussed by Adams (1901) and by Shelford (1911, a-e) 

 is a source of confusion rather than a help toward clarification of 

 biotic succession. There are, however, two community types that are 

 recognizable as distinct, namely, those of mud-bottomed pools and 

 swift water with rock bottom. The well-known distinctive taxonomic 

 composition, as well as its brief development, makes the swift-water 

 community stand apart sharply from the climax on mud bottoms. 



Swift-water Community. The rapids community of streams in the 

 Mississippi Valley occurs regularly on rocks, usually covered by 

 Cladophora glomerata (Fig. 72). Hydropsyche (caddis fly) larvae 

 are commonly of outstanding abundance; they spin Cladophora 

 threads together, making a mat over the rocks or forming the small 

 stones into cases. Among the stones are darters of the family Etheo- 

 stomidae (one or more species, such as Nanostoma zonule [Cope], 

 Oligocephalus coeruleus [Storer], and Catonotus flabellaris [Raf.], or 

 the like, and sucker-mouthed minnows such as Phenacobius mira- 

 bilis [Gir.]). Usually crayfishes of the genus Cambarus are present, 

 though the species differ from place to place. Mayfly numphs (Hep- 

 tageninae) are very common, clinging under stones; less abundant in 

 the same position are damsel-fly nymphs (Argia), stone-fly nymphs 

 (Perla), and also dobson larvae (Corydalis), while a snail (Gonio- 

 basis) is often abundant on rocks. The rapids of the major streams 

 contain some of the larger fish, such as the hog-sucker {Cato8tomus 

 nigricans Le S.), and a few insects likewise. The rock and gravel 

 shores of large lakes may have somewhat similar communities 



