The respiratory equipment of pond-inhabiting 

 animals permits them not only to live in habitats with 

 lower oxygen tensions but also to survive longer at 

 high water temperatures. Often, these animals dis- 

 play relatively low rates of general body metabolism 

 and oxygen requirement. Such relations between 

 riffle and pond animals have been observed for may- 

 fly naiads, caddisfly larvae, isopods, crayfish, and 

 fishes (Allee 1912-13, Wells, 1918, Fox et al. 1935. 

 Clausen 1936, Whitney 1939, Bovbjerg 1952), and 

 to some extent for limpet snails (Berg 1951). 



RESPONSES TO STREAM SIZE 



Of the species of clams indigenous to Mich- 

 igan, the 3 commonest are largely limited to creeks, 

 14 others to medium-sized rivers, and 5 to large rivers 

 (Van der Schalie 1941 ) . In central Illinois, the num- 

 ber of species of fish per collection increased from 

 about 4.5 in streams draining 4 sq km to 15.5 in 

 streams draining 500 sq km of upland. At the same 

 time the number of fish decreased from 9 to 2.5 per 

 sq m of water surface (Thompson and Hunt 1930). 

 Large species of fish can occur only in stream with 

 sufficient volume of water to permit freedom of move- 

 ment ; small fish may find orientation difficult in 

 large rivers. The preference of fish for streams of 

 specific size is evident in the tendency for some spe- 

 cies to travel upstream in times of flood and down- 

 stream in times of drought. 



An increased number of species downstream cor- 

 relates with greater variety of available niches and 

 moderate environmental conditions. In many in- 

 stances the correlation between distribution of species 

 and stream size, or volume, is not direct but depend- 



FIG. 5-9 Clam tracks 

 1956). 



sandy pool (courfesy R.E. Rundus 



ent on associated changes in temperature, type of 

 bottom, fertility, silting, pollution, and other factors. 



Headwaters 



The headwaters of drainage streams present a 

 highly variable habitat. During dry periods, pools 

 shrink and may disappear ; temperature may be very 

 high in summer and the water largely converted to 

 ice in winter ; there may be a lack of oxygen, an ex- 

 cess of carbon dioxide, and a high acidity ; fishes and 

 other organisms may become greatly overcrowded. 

 In times of heavy rain, on the other hand, the stream 

 is swollen, there is considerable erosion of materials 

 into the stream, and animals are washed downstream. 

 At all times food is likely to be scarce. 



Only the hardiest species can exist under these 

 conditions. The creek chub is a remarkably hardy 

 fish ; it may be found in large numbers in shrunken 

 pools, stirring up the water with tail action and gap- 

 ing for air at the water surface. Crayfish burrow 

 into the bottom when the pool dries up. Small snails 

 may survive desiccation of habitat by crawling un- 

 der rocks or into crevices, secreting a mucous mem- 

 brane across the aperture of their shells, and remain- 

 ing dormant until water returns. The occurrence of 

 insect larvae and naiads is hazardous, for if the 

 aquatic stages of their life cycles are characteristically 

 prolonged, they perish at times of low water or 

 drought. 



Temperature and altitude 



In drainage streams the temperature of the 

 headwaters is variable, but as the water volume in- 

 creases downstream and becomes more constant, the 

 range of temperature variation decreases. The head- 

 waters of spring-fed streams, or of streams arising 

 at high elevations, usually have a progressive increase 

 in temperature downstream. 



Some species of stonefly and mayfly naiads and 

 caddisfly larvae are absent from the headwaters of 

 Ontario streams because the temperature never gets 

 high enough to permit them to complete their life 

 cycle. More species are present downstream, and the 

 headwaters species tend to emerge earlier and earlier 

 in the summer while the waters are still cold. Still 

 further downstream, the headwaters species disap- 

 pear altogether. Species that are limited to the lower 

 portions of the stream emerge late in the season, 

 when the waters are the warmest. Closely related 

 species are thus segregated to different positions in 

 the stream by temperature tolerances. Headwaters 

 species have generally a northerly distribution over 

 the continent and the downstream species a southerly 



52 Habitats, communities, succession 



