BIOLOGICAL BACKGROUNDS FOR POPULATION STUDIES 



285 



nicely the changes in age-composition from 

 one year to another as they might occur in 

 natmal populations of Eshes (see Fig. 84). 



"For purposes of illustration we may con- 

 sider a species with a life span of several years 

 in which the age of individuals can be ac- 

 curately determined and in which adequately 

 large and inclusive samples are obtainable for 

 comparison. Now, assume a highly successful 

 spawning and larval survival in a moderate 



SPECIES 



and of the successful year 1933. As indicated 

 [Fig. 84], the 1930 and 1933 spawning pro- 

 duced 'dominant year classes.' From such com- 

 parative studies of year classes and with a 

 knowledge of the spawning habits and age 

 groups, means are provided for analysis of 

 probable environmental factors that determine 

 the degree of success of spawning or survival of 

 larvae, because the relative number of indi- 

 viduals entering into any year class must de- 

 pend mainly on these critical periods." 



Schislocerco gregor.o 

 (locust) 



Periplaneto omericono 

 (rooch) 



Pieris brossicoe 



Ponolis flammeo 



E phemeridoe 



Pseudoonidio duplex 

 (comphor scole) 



Tenebroides mouretonicus 

 (beetle) 



Trogodermo gronarium 

 (Dermestid) 



Orosophilo me lanogaster 

 Ratlus noruegicus 

 Homo sopiens 



25 



50 



75 



100 



PER CENT OF TIME 

 Fig. 85. Percentage of time spent by various animals in the periods of development, repro- 

 duction, and postreproduction: black, stippled, and plain bars, respectively. 



population of this species in the breeding 

 season of 1930, a very poor spawning season in 

 1931, an average degree of spawning and sur- 

 vival of larvae in 1932, and then another highly 

 successful year in 1933. The 1930 year class 

 will, upon investigation of the whole population 

 in 1931, show up as a disproportionately great 

 number of small, one-year-old individuals in 

 relation to the other age groups in the popula- 

 tion. In the next year ( 1932 ) the two-year-old 

 individuals of the 1930 spawning are still 

 conspicuous in the population, but the smaller 

 number of one-year-old individuals is evidence 

 of a poor spawning or survival for the 1931 re- 

 productive season. Thus, in 1933 and subse- 

 quent years the downward trend of numerical 

 strength of the 1930 and 1931 classes can be 

 traced and compared with other year classes— 

 for example, that of the average year 1932 



AGE DISTRIBUTION AND REPRODUCTION 



Earlier we mentioned that age distribu- 

 tion is significant, not only in relation to 

 mortality, but to natality as well. Within the 

 span of reproductive Hfe the rate and suc- 

 cess of reproduction are usually influenced 

 by the age of the reproducer. Several illus- 

 trations of this fact will suflBce. 



King (1916) followed the sexual history 

 of seventy-six female rats comprising 585 

 litters containing 3955 individuals, of 

 whom 2036 were males and 1919 were fe- 

 males. The mean number of young per ht- 

 ter was 6.7. It was found that fertility, as 

 measured by the total number of litters 

 cast, incieases with the age of the mother 



