588 - Heredity and Evolution 



grades accumulations of organic matter, re- 

 storing the abiotic base of inorganic nu- 

 trients. In short, each ecosystem represents a 

 microcosm, governed by the same inexorable 

 laws that determine the economy of life upon 

 our terrestrial planet as a whole (Chap. 10). 

 A pond (Fig. 30-13) will serve to exemplify 

 the economy of a small but definitive eco- 

 system. Such a system may endure for only a 



century or two, because an invasion of rooted 

 plants (cattails, water lilies, etc.) will gradu- 

 ally tend to fill the pond, converting it to 

 marshland and finally to "terra firma." But 

 while the pond endures, interactions within 

 the communities of inhabitants can be ana- 

 lyzed with some precision. 



The abiotic base in a small pond ecosys- 

 tem is provided partly by the nitrates, phos- 



DEC0MP0SERS 



ABIOTIC BASE MATERIAL 

 WATER, SALTS, ORGANIC 

 DETRITUS, ETC. 



Fig. 30-13. Ecosystem of a small pond. The system operates upon solar energy and the producers are a com- 

 munity of green plants of which the free-floating phytoplankton is of greatest importance. Abiotic base 

 material, for supporting the producers, is provided by the water, salts, and carbon dioxide of the pond 

 and by the organic detritus that sediments to the bottom. Primary consumers are small crustaceans and 

 other herbivores that feed directly upon the producers; the secondary and tertiary consumers, respectively, 

 are the smaller and larger carnivorous species. The decomposers, which prevent an unduly large accumula- 

 tion of organic matter, are constituted by the community of fungi— bacteria and molds— that mainly inhabit 

 the bottom sediment. (After E. P. Odum.) 



