THE BIOTIC ENVIRONMENT OF ORGANISMS 



573 



In every riffle, we find a definite assemblage of organisms that are 

 adapted — by their needs and reactions to current, hard surfaces and 

 oxygen concentration — to live in riffles, and by these same factors are 

 prevented from establishing themselves in pools. All the species in these 

 communities have reactions to physical factors that are sufficiently 

 similar to keep them confined to riffles, but they do not all have closely 

 similar biotic relationships. Some of the members of the riffle community 

 are various species of algae that are carrying on photosynthesis; some are 



Phytoplankton 



Animal Plankton 



Fingerling Bass 



Fingerling Sunfishes 



Gizzard Shad 



Sunlight 



Larger Vegetation 



Detritus 



Amphipods 



Herbivorous 



Insects 



Annelid Worms 



Midges 



Burrowing Mayflies 



Young Bass 

 Sunfishes 



Predaceous 

 Insects 



Crayfish 

 and Crabs 



Legal - Size Bass 



Fig. 33.7. Diagram of certain food-chain relationships in the St. Johns River, Fla. 



animals that feed upon the algae; others are food strainers; and still 

 others are somewhat larger animals (chiefly insect larvae) that feed upon 

 the algae eaters and the food strainers. 



Other primary communities occupy the pools; and in like manner 

 all types of aquatic and terrestrial situations — roadside ditches, the 

 shores, margins, bottoms, and open waters of lakes, the soil, leaf-mold 

 and surface strata of forests, swamps, and grasslands — have their char- 

 acteristic primary communities. These, in turn, are interrelated to form 

 larger and more complex communities. We can recognize stream com- 

 munities, composed of the members of pool and riffle communities and of 

 some additional organisms that are not restricted to either pool or riffle 



