CHAPTER XXXIII 



THE BIOTIC ENVIRONMENT OF ORGANISMS 



We have seen something of the diverse physical media and factors that 

 the organism encounters, and by which much of its activity and existence 

 is controlled. However, no organism "liveth unto itself." It has a variety 

 of inescapable relationships with the other organisms about it, which 

 involve various types of feeding-upon, being-fed-upon-by, competition- 

 with, and cooperation-with activities, as well as many less obvious and 

 less direct interdependencies. The "other organisms" include individuals 

 both of its own species and of other species that may belong to different 

 genera, families, orders, or phyla and often to the other organic kingdom. 

 Some of the more conspicuous of these biotic relationships are outlined 

 below. 



The biotic potential. One of the very important considerations in 

 fixing the organism's role in the plant and animal society in which it lives 

 is its own potential reproductive rate, or biotic potential, which is, theo- 

 retically at least, a constant for a given species, dependent upon that 

 species' inherited sex ratio, number of young per female, and number of 

 generations per unit of time. It may be stated as a formula: 



PZ n (R n ~ l ) = biotic potential (for a given unit of time) 



where P = number of females started with. 



n = number of generations in a given unit of time. 

 R = proportion of females in each generation ( = 0.5 when sexes 

 are equal; 1 when all females reproduce by parthenogenesis). 

 Z = number of young produced by each female. 

 Since n and Z are theoretical maxima, they could obtain only under 

 optimum conditions for all the factors that affect the species in question 

 and are, at best, difficult to determine. However, sufficiently accurate 

 approximations can be obtained to show the almost incredibly great 

 potential reproductive ability of even slow-breeding organisms and the 

 tremendous differences of biotic potential that exist among different 

 kinds and groups of organisms. 



Biotic potential versus environmental resistance. When one cal- 

 culates some of the approximate values for biotic potential for various 



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