127 



[j. Animal Association of a Temporary Stream] 



The prairie animal communities were arranged in an order to aid 

 in looking upon the prairie habitats as so many different degrees or 

 stages in the progress of drainage development, this being a dominant 

 physical environmental factor upon the prairie. Similarly, the forest 

 communities are easily arranged in a developmental sequence depend- 

 . ent upon the combined influence of the progress of erosion and drain- 

 j age and the advance of forest upon the prairie. Thus the prairie and 

 forest are given an orderly sequence, and the only remaining important 

 habitat, in the region examined, is that of the stream series. 



Very little time was devoted to the study of the stream animals, 

 and mention of it is made here mainly because of this opportunity to 

 I show the harmony and continuity of treatment which it is possible to 

 give to all the habitats and communities of a limited forested region. 

 This small temporary stream formed the southern boundary of the 

 area which was studied in the Bates woods. It formed Station IV, e, 

 and is an early stage in stream development. To understand just what 

 this means it is necessary to consider the processes which have been in 

 operation and which have reached the present stage of stream develop- 

 ment. This stream flows in a steep-sided ravine cut in the unconsoli- 

 dated glacial deposits which form the sides of the Embarras valley, a 

 ravine between 75 and 100 feet deep when it enters the valley, which 

 narrows rapidly, turns to the northwest, and soon ascends to the sur- 

 , face of the upland oak-hickory forest. The upper parts and head end 

 ' of the ravine are dry, except during rains and soon after ; but the lower 

 1 part may retain water in the basins for a number of days after rains. 

 The same conditions which we now find at the head of this ravine 

 once existed at the edge of the valley. That is, at one time there was 

 no ravine in this region. As the rainfall from the uplands flowed over 

 the edge of the valley it started a small gully; this, once formed, be- 

 came the trail for waters of other rains, each shower tending to cut 

 the ravine deeper and wider and to advance it into the upland. This 

 process has continued until now the head of the ravine has cut back 

 about one half of a mile. The unconsolidated debris is not composed 

 of homogeneous materials, and has therefore been washed away more 

 rapidly at some places than at others. In this manner pools have 

 formed where less resistant materials were, and between these pools, 

 over more resistant gravel or stone, miniature cascades or rapids have 

 been formed, the tendency thus being towards an alternation of pools 

 and cascades. In these pools Mr. T. L. Hankinson took a number of 

 vertebrates, and upon the surface of the pools were many water-stri- 

 ders, Gerris remigis. From the burrows along the margin of the 



