THE ORGANISM AND ITS ENVIRONMENT 379 



edge of the water outward to deeper portions of the lake or ocean 

 one finds distinct temperature gradients, pressure gradients, Hght 

 gradients, and gradients in oxygen concentration. Animals as a 

 result of their behavior to external stimuli therefore tend to such 

 regions as are optimum for their life processes, provided, of course, 

 that the regions contain a food supply. 



Similar facts apply to animal life of land areas. Thus the slope 

 from the valley level to the crest of a mountain range, or divide, 

 may represent a number of gradients, of temperature, rainfall, salt 

 content of the soil, and so on. If the slope is steep the gradients may 

 be correspondingly steep and the changes in fauna quite abrupt as 

 one proceeds from the valley level upward. 



Within areas thus established by the major environmental factors 

 of temperature, pressure, moisture, and other conditions are to be 

 found rather definitely outlined small communities. Thus a pond, 

 a swamp, a hill, or during the summer season at least, even a single 

 tree has its local fauna. A survey of a tree discloses that a variety of 

 animals find food and shelter there; small mammals, slugs, birds, 

 tree frogs, adult and larval insects compose a close community. The 

 single and all-important connecting link in such communities is the 

 interdependence of all members for subsistence. Ecologists speak of 

 this linkage as the food chain. 



Any food chain begins with the metabolism of green plants; a 

 moment's consideration will direct our attention to the fact that 

 such a linkage is in reality a detailed expression of the relations of 

 life forms imposed by the nitrogen cycle (p. 43). In a fresh water 

 pond, for instance, alg^e and other water plants are present. Upon 

 them feed insects and insect larvae, snails and amphibian larvse. 

 Decaying plants aiiford food for bacteria, which in turn are eaten by 

 Protozoa. Hydra are present and feed upon Protozoa and small 

 Arthropoda. Annelida and flatworms feed upon bits of plant ma- 

 terial and injured insects and other small animals. Turtles and 

 crustaceans feed upon snails; small fish devour insects and insect 



