634 



COLLEGE ZOOLOGY 



Biological (biotic) interrelationships 



The web of life. Although we speak of 

 soHtary, gregarious, and social animals, no 

 animal lives by itself alone; it must live in 

 the same environment with others of its 

 kind and with other species, where it is 

 obligated to compete for space, food, mates, 

 and protection from enemies. 



Plants are also important factors in the 

 biological environment, as are various fac- 

 tors of the physical environment. The com- 



plexity of the "web of life" is shown in Fig. 

 444, which illustrates a typical food chain 

 in a fresh-water lake. Dissolved mineral nu- 

 trients leached from the surrounding earth, 

 coupled with nutrients derived from the 

 decomposition of organic matter, provide 

 food for the growth of the microscopic plant 

 and animal life, such as plankton (Fig. 452) . 

 Use is made of this fact by the fishery biolo- 

 gist who attempts to increase fish production 

 in a pond by use of commercial fertilizers. 



Land predators | 

 (herons, king- ^ 

 fishers, etc.) 



Nutrients from soil • 



Shore line of lake" 



Figure 444. Food chain in a fresh-water lake. Arrows point from the organism eaten to those 

 doing the eating. A food chain starts with the plant where photosynthesis produces the first 

 food; the plants arc eaten by herbivores, which are in turn eaten by a series of carnivores; each 

 animal of the series is usually larger than the one preceding it (see headpiece page 628). 



The plant plankton such as certain algae, 

 together with the higher green plants, pro- 

 vide the chemical energy in such a lake 

 community by their photosynthetic activity. 

 The insects and bottom organisms feeding 



upon the plants and smaller animals are, 

 in turn, fed upon by the fishes which are the 

 primary source of animal food for many ani- 

 mals such as herons, kingfishers, and even 

 man. Under normal conditions, the food 



