Species 



Intrinsic 

 growth rate 

 per day (r) 



Average 

 longevity 

 in days T 



Net produc- 

 tion rate 



young and niiddle-aged classes are more or less equal 

 in numbers, the decline in size occurring progressively 

 throughout life. 



ADAPT.\TION TO NICHE 



In the stabilized population of any species, 

 whatever the number of eggs or young produced per 

 pair of adults, the number of offspring reaching re- 

 productive status can never be greater than two in 

 sexual forms, which is the number required to replace 

 the parents on their death. With each new generation 

 there is, therefore, a population turnover, with newly 

 born individuals replacing the adults that die. In a 

 stabilized population, the rate of increase of a popu- 

 lation through the course of several reproductive 

 cycles must equal the death-rate, so that the value 

 of one factor is also a measure of the other. Either 

 factor is indicative both of the rate of population 

 turnover and of the intensity of environmental 

 resistance. 



The intrinsic growth rates for populations of sev- 

 eral species under optimum conditions is given in 

 Table 1 S-7 by the factor r. which represents the mean 

 rate of increase per individual per day. There is a 

 general inverse relation between growth rate and 

 longevity, T. However, if growth rate were depend- 

 ent only on the longevity of the species, then rT 

 would equal a constant. Obviously this is not true. 

 It appears that difTerent growth rates may correlate 

 with various intensities of environmental resistance in 

 the different habitats occupied by different species. 

 If there were a habitat offering no environmental re- 

 sistance, and all offspring therefore survived, then a 

 female would need to produce only one female off- 

 spring to replace herself when she died. This would 

 be the net production rate of Table 13-7. Actually, 

 the number of female offspring that must be produced 

 to offset mortality caused by the environment is al- 

 ways more than one, attesting the rigor of the natural 

 environment in spite of the species' adaptations for 

 life in it. The method used in calculating the net pro- 

 duction rate, S'x ' »ij-. the sum of number alive at age 

 X, I, times rate of reproduction at age x, m, for all 

 age groups, is e.xplained by Evans and Smith (1952). 

 It is interesting that as a result of long evolutionary 

 processes, the low net reproduction rates for the 



TABLE 15 7 Comparison of 

 intrinfie growth rates and other 

 data on the populations of 

 different species (compiled fro 

 various sources by Evans and 

 Smith 1952). 



herbivorous vole, the omnivorous rat, and the para- 

 sitic Iniman louse indicate that they are in much bet- 

 ter balance with what to them are optimum environ- 

 ments than are the graminivorous flour beetle and rice 

 weevil. The vole and rat are viviparous ; the other 

 three, oviparous. It would be very interesting to have 

 similar data on other species to show the degree to 

 which adjustments to particular environments have 

 become perfected and the reproductive strain imposed 

 upon related species for occupying different habitats. 



SUMMARY 



Reproductivity is the rate at which a spe- 

 cies reproduces. The number of offspring raised to 

 maturity per unit of time is generally characteristic 

 of a species, and varies with fecundity and survival 

 of the young. Fecundity depends upon the morpho- 



FIG. 15-5 Monthly changes In the density and age-structure of 

 a population of prairie deer mice in Michigan. As one popula- 

 tlon dies out a new one takes its place, and there is a popula- 

 tion turnover (Howard 1949). 



Reproductivity and structure 217 



