THE BIOTIC ENVIRONMENT OF ORGANISMS 



571 



even thousands of individuals. Many or most of these groups are pri- 

 marily family aggregations, resulting from the long-continued association 

 of parents and offspring. Others have their nucleus in a family group that 

 has been joined by unattached individuals or smaller groups in the same 



Fig. 33.5. A small swift trout stream in northern Michigan, showing a pool in the foreground 

 and a riffle just above the bend in the stream. (Photo by Dr. Justin W . Leonard.) 



area. In all instances the group tends to confer some type of advantage on 

 its members that is not equally available to the isolated individual. 



BIOTIC COMMUNITIES AND THEIR SUCCESSIONS 



As a result of the many sorts of interrelationships that obtain between 

 organisms and their physical and biotic environments, the plants and 

 animals of any region tend to group themselves into a series of "biotic 

 communities." 



The simplest of these are the large number of 'primary communities 

 that are based upon the similarity of responses shown by their members 

 to certain intensities of various physical factors. For instance, in most 

 creeks the processes of stream erosion and the nature of the eroded mate- 

 rial have resulted in an almost invariable sequence of swift, shallow riffles 

 and deep, quiet pools (Fig. 33.5). In the former, the water is well aerated 

 and flows over a clean pebble and coarse sand bottom; in the latter, the 

 water moves slowly past silty margins and over a mud- and silt-covered 

 bottom. 



