12 THE STUDY OF PLANT COMMUNITIES • Chapter I 



ALL LIFE BOUND TO ENVIRONMENT 



An organism without environment is inconceivable, 122 for living 

 things have certain requirements that must be satisfied by their sur- 

 roundings if life is to continue. Their physiological processes, 

 which, to sustain life, must all continue at rates above definite min- 

 ima, are largely controlled by environmental conditions or sub- 

 stances. Most of the processes use water or require its presence; 

 food manufacture is dependent upon carbon dioxide and light con- 

 ditions; the universal process of respiration requires oxygen; and 

 all processes are limited by, or vary with, temperature. 



Since organisms must grow and reproduce to survive, they re- 

 quire energy, which they derive from food by respiration. Food, 

 therefore, becomes a major consideration in explaining the activi- 

 ties of organisms. Green plants must be able to manufacture 

 enough food to grow and reproduce and still leave a surplus for 

 dependent organisms. Among the latter, there are usually several 

 dependent upon each other for food in a relationship called a food- 

 chain. For example, in aquatic environments the food-producing 

 algae are eaten by miscroscopic animals that may in turn be eaten 

 by larger animals upon which small fish feed. Small fish are often 

 eaten by larger fish, and many of these are eaten by man. Any 

 number of things may disrupt such a food-chain, but, under nor- 

 mal conditions, all the organisms are interrelated by their mutual 

 requirement of food, whose ultimate production is dependent 

 upon algal activity in the presence of light. 



Regardless of the environment and the group of organisms 

 adapted to survival in it, similar food-chains and dependencies can 

 be found everywhere. Thus we see that the basic relationship 

 binding all organisms to each other and to the environment is, in- 

 variably, one traceable to energy needs and uses; and, because the 

 ultimate source of energy for both plants and animals is the sun, 

 all organisms are mutually related to each other and to their en- 

 vironment. 



If groups of organisms live together successfully, their demands 

 and effects upon the energy cycle will not disrupt it. All the proc- 

 esses and activities taking place within the group will be in balance 

 with the available supply of energy. A major concern of ecology, 

 therefore, is to learn what that balance is and what controls it. 



