
ANIMAL ECOLOGY OF JOHNSON COUNTY © 31 
fact that but few plants have become established in its waters. 
The water is shallow and therefore well lighted to the bottom. 
Each brook has within itself a set of conditions which make 
possible the occurrence of several associations of animals, as I 
have previously pointed out.* There is an association of animals 
in the rapids which are capable of resisting the current, either 
by attachment or seeking shelter, or by strong positive rheo- 
tactic reactions. Within the pools, conditions permit the exist- 
ence of a surface film association of animals, a free-swimming 
association, and a bottom association. These brooks have not 
been indicated upon the maps; they occur, however, at the head 
waters of every branch of the larger streams and are also found 
feeding into the rivers between these streams. A detailed de- 
scription of the forms inhabiting one such brook in Johnson 
County is given in the article just cited. It is sufficient to note 
here that they are mostly insect forms and aquatic snails. 
(2) The creek habitats. The creek is characterized by its 
U-shaped valley, having acquired a narrow flood plain. The 
proportional extent of pools and rapids has been altered into 
more extensive pools separated by occasional rapids. There is, 
of course, an increase in the volume of water which causes a 
greater depth to the pools and does not allow sunlight to reach 
the bottom in all places. There has been a gradual increase in 
the amount of organic matter and coincident with this an in- 
crease in the number of algae in the water. This permits a 
greater profusion of other micro-organisms, which in turn pro- 
vide food for a larger number of individuals and a greater vari- 
ety of species of the invertebrates. The great number of such 
invertebrate forms as the crustaceans and insects, together 
with the increased depth of the water, provide for the ex- 
istence of such small fishes as darters, dace, shiners, and chubs. 
I: have also taken the common ‘‘blue-spotted’’ or ‘‘green”’ 
sunfish from Saunders and Turkey Creeks. As it feeds almost 
exclusively on other fishes, insects, and crayfish, its presence is 
an index to the abundance of such life in the creek. The pools 
permit of such analysis as the pools of the brooks. Rapids are 
frequently of sufficient variety to need classification, as is also 
*Macroscopic fauna of a small brook,’ Iowa Academy of Science, Proceedings; 
22 (1915), pp. 363-374. 
