Ecology of Animals 663 



In the fall of the year, the cooling of the surface waters and the 

 wind action again result in a complete turnover of the lake. This lasts 

 until the chilling effects of winter bring about stagnation. 



Running- Water Communities. — Flowing-water communities 

 differ in many respects from the standing-water ones. They are not 

 isolated, usually being connected in a network. Thus the fauna and 

 flora tend to become more or less uniform within any system. Unless 

 pollution has depleted the oxygen supply, there is ordinarily an abun- 

 dance of this vital gas. Also there is no thermal stratification, and plank- 

 ton is scarce. 



Two distinct types of stream communities may be recognized : the 

 pool and rapids habitats. 



The pool community is characterized by the slow flow of the water 

 and usually has a mud or sand}' bottom. In such a community there 

 are many fish such as the sunfish and catfish, numerous minnows, 

 crayfish, and many insect larvae. Within the rapids community, the 

 water flows swiftly over rocks. The swiftness of the water flow varies 

 from the comparatively small, slow streams to the turbulent rapids of 

 the mountains. Under these latter conditions, there is a large amount 

 of dissolved oxygen. Here live fish such as the trout and darters with 

 their high oxygen demand. Many of the insect larvae have developed 

 sucker devices for clinging to the stones or special nets for entangling 

 passing organic debris. 



The development of streams is in the direction of the so-called 

 "base level" type which is similar to the pool community. The base 

 level stream may be considered to be the climax type. This type of 

 stream is slow flowing and often muddy. Within it conditions are 

 quite stable until altered by geologic changes or by man. 



OCEANIC COMMUNITIES 



The ocean covers the greater portion of the earth's surface. So ex- 

 tensive is it that innumerable physical conditions vary in extreme from 

 one habitat to another. These variations include such fundamental 

 factors as temperature, pressure, salinity, wind movements, and tidal 

 movements. The temperature varies from that of the frigid arctic and 

 antarctic to that of the warm tropical seas; the depths range from the 

 shallow shores to those of many miles in the great depths. This results 

 in pressures varying from one atmosphere at the surface to several 



