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ANIMALS AND THEIR ENVIRONMENT 



compare the survival curves for organisms with very cHfferent total life 

 spans. Civilized man has improved his average life expectancy greatly 

 by modern medical practices, and the curve for human survival ap- 

 proaches the curve for minimum mortality. From such curves one can 

 determine at what stage in the life cycle a particular species is most 

 vulnerable. Reducing or increasing the mortality in this vulnerable 

 period will have the greatest effect on the future size of the population. 

 Since the death rate is more variable and more affected by environ- 

 mental factors than the birth rate, it has a primary role in population 

 control. 



It is quite obvious that populations that differ in the relative num- 

 bers of young and old will have quite different characteristics, different 

 birth and death rates, and different prospects. Death rates typically vary 

 with age, and birth rates are usually proportional to the number of indi- 

 viduals able to reproduce. Three ages can be distinguished in a popula- 

 tion in this respect: prereproductive, reproductive and postreproductive. 

 A. }. Lotka has shown from theoretical considerations that a population 

 will tend to become stable and have a constant proportion of individuals 

 of these three ages. Censuses of the ages of plant or animal populations 

 thus are valuable in predicting population trends. Rapidly growing 

 populations have a high proportion of young forms. The age of fishes 

 can be estimated from the growth rings on their scales, and studies of 



1000 



800 



Number 



of 

 survivors 



per 

 thousand 



600- 



400- 



200- 



Percenf of total life span 



Figure 37.7. Survival curves of four different animals, plotted as number of sur- 

 vivors left at each fraction of the total life span of the species. The total life span for 

 man is about 100 years; the solid curve indicates that about 10 per cent of the babies 

 born die during the first few years of life. Only a small fraction of the human popu- 

 lation dies between ages 5 and 45 but after 45 the number of survivors decreases 

 rapidly. Starved fruit flies live only about five days, but almost the entire population 

 lives the same length of time and dies at once. The vast majority of oyster larvae die 

 but the few that become attached to the proper sort of rock or to an old oyster shell 

 survive. The survival curve of hydras is one typical of most animals and plants, in 

 which a relatively constant fraction of the population dies off in each successive time 

 period. (Villee: Biology.) 



