FIG. 5-1 Diagrammatic arrangement of streai 



of different physiographic age on the 



south shore of Lalte Michigan. Each number 



shows the location of that pool nearest 



a headwaters which first contains these 



fish: (I) creek chub; (2) redbelly dace; 



(3) blacknose dace; (4) suckers; minnows; 



(5) grass pickerel; bluntnose minnow; 



(6) sunfish, bass; (7) northern pike, lake 

 chubsucker, and others (after Shelford 1913). 



fish commonly found in streams. Some salamander 

 species occur only in fast mountain streams ; other 

 species are more typical of pond-like pools. The 

 belted kingfisher feeds on stream fishes, and nests in 

 adjacent clay banks. In the western mountains, the 

 water ouzel feeds under water on the insect larvae 

 and naiads of the riffles. Muskrats make their bur- 

 rows in the stream banks and feed on vegetation and 

 clams. Mink patrol the streams for the muskrats and 

 fish that serve them as food. The once-abundant otter 

 is now absent from most localities. Beaver dam 

 streams to enlarge the pools in which they build their 

 lodges and find shelter. Beaver feed on the bark and 

 cambium of aspen, willow, and other trees and 

 shrubs occurring on the shores of the stream. 



ADJUSTMENTS TO CURRENT 



Probably the characteristic of a stream 

 most critical to the life therein is the current. All 

 organisms that occur in streams must adjust to it to 

 maintain constant position. Torrential floods scour 



FIG. 5-2 Apparatus for collec 

 quantitative samples of botton 

 in streams: above, swift-water net, 



;0.l m' (Surber 1936); right, 

 sampling cylinder for use in pools, covers 

 0.2 m', has sharpened lower edge. 



the stream bed, move rocks and sand, cut new chan- 

 nels, and destroy entire populations. Recovery after 

 such catastrophes, however, may take place within a 

 few weeks or months, especially by those species pos- 

 sessing short life cycles (Moffett 1936, Surber 1936). 

 Position is ordinarily maintained by clinging to the 

 substratum, avoidance of the current, or vigorous 

 swimming, and requires a good development of ori- 

 entation behavior. 



Clinging mechanisms 



The growth form of fresh-water sponges is af- 

 fected by a number of factors, but in riffles sponges 

 are usually simple encrustations. In quieter water, 

 long, slender, finger-like processes may form. The 

 distribution of species depends both on current and 

 organic content of the water (Jewell 1935). 



Plumatella is a common bryozoan that forms an 

 encrusting, plant-like, branching colony on tiie under- 

 side of rocks or fallen trees in swift water. Pectina- 

 tella. on the other hand, forms a gelatinous spherical 

 ball, and is more commonly found in ponds or slow- 

 flowing portions of streams. 



Turbellarians, such as Planaria, and swift-water 

 snails, such as Goniobasis and Pleurocera, and the 

 limpet Ferrissia. cling to the substratum by means 

 of flat, slimy, adherent body or foot surfaces, and are 

 most common on the protected lower surfaces of 

 rocks. 



Mayfly naiads have efficient adaptations which 

 enable some of them to tolerate currents up to 300 

 cm-sec (Dodds and Hisaw 1924). The animals cling 

 to the smooth undersurfaces of the rocks, keeping 

 their heads toward the current and their bodies par- 

 allel with it as they move sideways, forward, and 

 back. The head is flattened, and when pressed firmly 

 against the substratum the water current exerts a 

 downward pressure which helps to hold the animal 

 in position. Compared with forms found in quieter 



44 Habitats, communities, succession 



