BIOLOGICAL BACKGROUNDS FOR POPULATION STUDIES 



281 



noted, is a most obvious but valid principle 

 of ecology. 



AGE DISTRIBUTION IN POPULATIONS 



A consequence of ecological mortality in 

 a population is its effect on the age distri- 

 bution within that population. This, in turn, 

 is significant, since the age of the compo- 

 nents is characteristically related to their 

 reproductive performance as well as to their 

 morbidity and mortaUty. It is a valid prin- 

 ciple of ecology and demography that a 

 rapidly growing, vigorous population has a 

 preponderance of young organisms; a sta- 

 tionary population, an intermediate number 



tion than in a stationary or declining group. 

 The curves come close together for the age 

 interval thirty to forty years— the class of 

 males most consistently represented in all 

 three populations. After forty years there 

 is an increasing divergence until age 

 seventy is attained. Thereafter, among the 

 few really old members, this divergence de- 

 creases until at 100 years of age the three 

 populations are essentially similar. Boden- 

 heimer describes such changes by three 

 geometric figures: (1) a pyramid for the 

 growing population suggesting by its broad 

 base many young and few old components; 

 (2) a bell- shaped structure for a stationary 



AGE DISTRIBUTION 

 IN STATIONARY POPULATION 



Per 100,000 living at all ages 

 Compored with increasing and decreasing population 

 Based on mortality among wtiite males 

 in United Stotes, exclusive of Texos, 1930 



I 6.B4G_ 



, I 6 192 

 ~l 



5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 



AGE 

 Fig. 81. Age distributions in human populations that are stationary, increasing, and decreasing. 



(From Dublin and Lotka.) 



of young, middle aged and old organisms; 

 and a declining population, a preponder- 

 ance of the older age groups. Figure 81 ex- 

 presses this principle. The chart is based 

 on the mortality of United States males (ex- 

 clusive of Texas) in 1930. It contrasts a 

 "stationary population," in which the rate 

 of natural increase is zero per 1000 per 

 year, with an "increasing population" hav- 

 ing a rate of natural increase of five per 

 1000 per year, with a population decreasing 

 at a rate of five per 1000 per year. 



The three curves show clearly the age 

 differences. They start far apart at the zero 

 to ten year age class. This indicates a higher 

 proportion of children in a growing popula- 



population suggesting about equal numbers 

 of young and middle-age classes; and (3) 

 an urn-shaped structure for a contracting 

 population depicting the increase in middle- 

 aged and old organisms relative to the 

 young. 



These ideas can be expressed in another 

 way, using actual census data, as Dublin 

 and Lotka (1936) did for the United States 

 population from 1850 to 1930 (observed) 

 and from 1940 to 1980 (estimated). Table 

 19 condenses their data and summarizes for 

 certain years the percentage composition of 

 the population by age classes. 



Thus, in 1850 the small but vigorous 

 United States population yields, on random 



