344 Relations within the Species 



tion continued to overshoot and undershoot a possible equihbrium 

 level. When this experiment was repeated at a lower temperature, 

 the population changed more slowly, with the result that after one or 

 two oscillations an equilibrium value was reached and maintained. 

 Delay in the manifestation of excessive natality and prolongation of 

 the effects of overcrowding similarly act on natural populations and 

 add their influence to that of external factors in causing fluctuations. 



The relative abundance of individuals of various ages in the popu- 

 lation is known as the age distribution of the population. Differences 

 in age distribution depend upon the species and whether the popula- 

 tion is changing in size or stationary (Petrides, 1950). A rapidly 

 growing population usually contains an especially large number of 

 young individuals, whereas a declining population includes a rela- 

 tively high proportion of old individuals. In a stationary population 

 the distribution of ages is more uniform and tends to approach a 

 stable pattern. Since natality and mortality vary with the age of the 

 individuals, the age distribution of the population influences the birth 

 and death rates for the population as a whole. Species that produce 

 large numbers of young generally suffer high mortality during the 

 young stages. A complete description of the mortality of a popula- 

 tion is furnished by a life table such as has been constructed for man 

 and has long been used by life insurance companies. Life tables and 

 survivorship curves have now been worked out for a number of 

 natural populations and show characteristic differences in mortality 

 patterns among various species (Deevey, 1947 and 1950). For ex- 

 ample, mortality in the oyster is extremely high during the larval stage 

 and becomes much lower later in life, in hydra mortality is nearly 

 constant at all ages, and in man mortality tends to be low for a long 

 period during youth and to become high rather abruptly in old age. 



Optimal Yield 



The foregoing analysis of population development is of interest 

 not only in relation to theoretical considerations but also in connec- 

 tion with practical applications. In the exploitation of natural popu- 

 lations—either plant or animal— a harvest is desired of the largest 

 number of individuals per unit of time that is possible without per- 

 manently impairing the breeding stock. In other words, we wish to as- 

 certain the size of the largest sustained yield that can be obtained and 

 learn at what level the population should be maintained to produce 

 this yield. An answer to this question is the formation of the theory 

 of the optimal yield. The basic idea underlying this theory is that in 



