ENVIRONMENTAL RELATIONSHIPS 



tion, the increase becomes more rapid; near the inflection point new in- 

 dividuals are being added at the maximum rate. But this rate of increase 

 cannot be sustained, for now the population begins to approach the maximum 

 number of individuals which the environment will support. Various environ- 

 mental factors operate to increase the mortality rate; among these are 

 (1) limitations of the food supply, (2) intensified predation and destruction 

 by natural enemies, and (3) increased mortality from disease and parasitic 

 infestation. Although the addition of new individuals (crudely, the "birth 

 rate") may continue at a high level, the population does not grow larger 

 indefinitely because of the concomitant increase in the mortality rate. Even- 

 tually, there is established an equilibrium between the "birth rate" and 

 the "death rate," maintaining the population at levels which, averaged over 

 long periods, approximate the maximum allowed by environmental condi- 

 tions. We shall return later to consider the nature of this equilibrium. 



The difference between the curve of geometric increase and the logistic 

 curve represents the limiting action of environmental factors, the "checks 

 against unlimited increase" which, as we shall see (p. 649), Charles Darwin 

 embodied in his theory of evolution through natural selection. In addition 

 to its tendency to increase in abundance, every species tends to increase its 

 range and to spread more widely into new environments. Just as the environ- 

 ment sets limits on the abundance of a species within its range, so also 

 environmental factors limit the distribution of animals. In a manner of 

 speaking, checks against increase counteract the reproductive powers of the 

 species, and its powers of dispersal are counteracted by other environmental 

 factors. This control of distributional tendencies is another aspect of the 

 equilibrium between animals and their environments. 



In a natural community, then, populations of a species do not exist in 

 isolation and do not find unlimited opportunity for increase and spread. 

 Rather, they are involved in complex interactions with other organisms in 

 the community, and an equilibrium is maintained among interacting species. 

 It should not be supposed, however, that this equilibrium is a stable, un- 

 changing balance between fixed numbers of organisms. For evidence of the 

 dynamic, unstable nature of conditions temporarily in equilibrium, we may 

 turn again to the fresh-water pond community analyzed in Figure 19.4, and 

 quote Shelford, the originator of this figure: 



Any marked fluctuation of conditions is sufficient to disturb the balance of an 

 animal community. Let us assume that because of some unfavorable conditions in 

 a pond during their breeding period the black bass decreased markedly- The 

 pickerel, which devour young bass, must feed more exclusively upon insects. The 

 decreased number of bass would relieve the drain upon the crayfishes, which are 

 eaten by bass; crayfishes would accordingly increase and prey more heavily upon 

 the aquatic insects. This combined attack of pickerel and crayfishes would cause 

 insects to decrease and the number of pickerel would fall away because of the 

 decreased food supply. Meanwhile the bullheads, which are general feeders and 

 which devour aquatic insects, might feed more extensively upon mollusks because 

 of the decrease of the former, but would probably decrease also because of the 



611 



