HO ANIMAL COMMUNITIES OF STREAMS 



deeper water at their sources (due to springs) and in which the graded 

 series of conditions found in the North Shore streams is wanting. 



d) The swift-water fishes begin markedly at the head of erosion in 

 Hickory Creek. 



e) The fish communities differ as to species where the conditions 

 are very similar, for example, in Thorn-Butterfield and Hickory creeks. 

 The general habits of the fishes are the same. 



/) Larger fishes are found in the larger water course and in the down- 

 stream portions of the smaller streams. 



g) Fish, when entering a stream, go upstream to a point suited to 

 their physiological constitution, regardless of its physiographic mode of 

 origin. 



4. GENETIC ECOLOGY OF STREAMS 



Several years ago Adams (103) pointed out that the dispersal of 

 aquatic animals is determined by the shifting backward of the head- 

 waters and other conditions in streams as erosion proceeds. The forms 

 that are in the young streams are moved back as the headwaters are 

 moved back and as the river system spreads out into the usual fan shape, 

 the animals that belonged in or near the headwaters move backward as 

 the conditions migrate backward. In a broad geographic way this is 

 unquestioned but details may be studied in the small streams of the 

 bluff between Glencoe and the Wisconsin state line. 



Fish are the only strictly aquatic forms in these streams that might 

 not have entered by some other method than through the mouth of the 

 stream. We have made a study of the fish of these streams for the pur- 

 pose of determining whether the fish in the headwaters of the large 

 streams are the same as the fish that are found in streams that are just 

 large enough to have a single fish species, and the relation of the animals 

 to stream development. The changes in animal communities which 

 take place at one point are called succession. 



a) Ecological succession. — Ecological succession is the succession 

 of ecological types (physiological types, modes of life) over a given point 

 or locality, due to changes of environmental conditions at that point. 

 From this point of view we have nothing to do with species, except that names 

 are necessary. However, we may speak of the succession in terms of 

 species whenever their life habits {mores) are not easily modifiable. 



Succession always involves all the animals of a community but it is 

 often easier to discuss the changes which take place with respect to one 

 group, such as the fishes. It is always to be understood that with changes 

 in the fish communities there are similar changes in the communities of 



