112 



ANIMAL COMMUNITIES OF STREAMS 



bed and became hypothetical stage C of the diagram, the fish community 

 of stage i was succeeded by a fish community ecologically similar to the 

 fish communities at the localities marked 2 in Fig. 67. The fish now eco- 

 logically representing this community are the horned dace and the red- 

 bellied dace. The community of the single species, the horned dace, 

 had at such a period moved inland to the point where line 1-1 (Fig. 69) 

 crosses the curved line representing the profile of hypothetical stage C. 

 As erosion continued, the fish community ecologically represented by 

 the horned dace and red-bellied dace moved gradually inland and was 

 succeeded by a fish community occupying the mouth of hypothetical 



H G F E D C B 



\i£jj 



Fig. 68. — A diagram showing the successive stages in the profile ("general shape 

 of the bottom) of a very young stream, curved lines, A-B, A-C, A-D, A-E, A-F, 

 A-G, A-H representing the successive profiles. The uppermost horizontal line 

 represents the surface of the land into which the stream is eroding. The horizontal 

 line with the arrowheads indicates the migration of the source of the stream and 

 accordingly of similar stream conditions. The vertical line with arrowheads when 

 followed downward passes through a succession of stream conditions and represents 

 physiographic succession at the locality B. The point A is the mouth of the stream. 

 Opposite this are shown three successive sizes of the stream, and therefore succession 

 at that point. 



stage D, ecologically similar to that now found at the point 3. This is 

 represented by the three daces and the Johnny darter. 



As the hypothetical stage D eroded its bed and became stage E, 

 which is represented by County Line Creek, fish community 3 was then 

 succeeded by a fish community ecologically similar to the fish community 

 now present at point 4. This is ecologically represented by the three 

 daces, the Johnny darter, ancj. the young of the common sucker. The 

 fish communities designated as 1, 2, 3 have meanwhile moved inland and 

 are arranged in the order which their ecological constitution requires. 



The continuation of the process resulted in displacing a fish com- 

 munity ecologically similar to the fish community 4 by a fish community 



