98 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



quite dissimilar, though exhibiting various intergrades through mature 

 rivers and "cutoff" lakes. 



Apart from man, the beaver is the one animal that produces reac- 

 tions in streams on a large scale, though the damming and flooding 

 are an outcome of a coaction. At one time this was a widespread 

 effect of much importance, favorable streams often being converted 

 into a succession of ponds wdth adjacent swamps, but today the reac- 

 tion is practically confined to remote regions. 



Reactions in Swift Water. It is rare that either plants or animals 

 produce reactions of any significance in rapid streams, though algae, 

 aquatic mosses, and amphibious flowering plants may slacken or deflect 

 the current locally. A number of fish bury their eggs in the loose 

 gravel of moderately swift water, but with little or no significant 

 effect. A characteristic minor reaction but likewise without impor- 

 tant effect is the habit of larvae of caddis flies in binding together 

 tiny pebbles and attaching the cases to the bottom. This may result 

 in increasing the stable area to which other organisms may attach 

 themselves, but it is quite transitory in nature. 



Reactions in Sluggish Water. As reactors, both plants and animals 

 are of more importance in sluggish streams, the former sometimes 

 affecting the slow current materially and both having some slight 

 effect at least upon the gases and solutes of the medium. Their reac- 

 tion upon the bottom becomes significant only where the current 

 slackens to the point of permitting accumulation, but even in such 

 places, except in baselevel streams, floods continually recur to sweep 

 away the effects. The movements of mussels and snails tend to mix 

 organic matter with the earthy bottom, and the nest-building fishes 

 bury similar materials in the outer rim of the concave nests. Bottom- 

 feeding fishes tend to stir the bottom materials and increase turbidity 

 (see Chapter 9). These are often numerous along the margins of 

 mature rivers, sluggish creeks, and ponds. Crayfish also burrow in 

 such bottoms, bringing up terrigenous material and burying organic 

 matter and other detritus. 



The reaction of animals which remove plants from the land and 

 increase erosion is evidenced in streams and sometimes in lakes as 

 increased turbidity. The clearing of the land and attendant increase 

 in stream-borne silt has doubtless shifted the fish constituents of 

 stream communities from the trout-bass type toward the buffalo- 

 sucker type with attendant changes in the invertebrates. 



