CHAPTER VIII 



BIOLOGICAL FACTORS 



Associated organisms having mutual relationships to each other 

 and to their environment are recognized as a community. Many, 

 if not all, of the organisms in a community are thus not only a part 

 of the community but also a part of the environment of every 

 other organism there. The dominants obviously compete with 

 each other and with subordinate individuals. At the same time, 

 they provide conditions that permit the survival of lesser organ- 

 isms, which, though quite inconspicuous, may yet markedly affect 

 the permanence of the community as a whole. Both plants and 

 animals are factors of the environment of any community, and 

 man is not the least of these factors. 



PLANTS AS FACTORS 



Competition.— It has been shown that, within a community, 

 competition occurs between individuals of the same species, or 

 between different species, whenever some requirement of the 

 organisms is available in amounts insufficient to supply all demands 

 adequately. Each organism involved in competition is a factor in 

 the environment of all other organisms so involved. The effects of 

 competing organisms upon each other are more apt to result from 

 their influence upon physical or physiological conditions of the 

 environment (such as available water or nutrients, light, tempera- 

 ture, humidity, and air movement) than they are from direct ac- 

 tion. An extreme example of direct competition as a factor is that 

 of the strangling fig, a liana of tropical forests, which climbs to the 

 tops of the dominant trees that support it. Eventually the tree is 

 killed as the pressure of the vine about its trunk increases. When 

 the tree falls, the vine may pull down numerous other trees over 

 whose tops it has sprawled. The community, however, is only 

 locally disturbed and soon readjusts itself, for the forest is climax 

 and these giant lianas are a part of it. 



The introduction of new species into a community, by man or 



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