274 



POPULATIONS 



2. "Ecological longevity is the empirical 

 average longevity of the individuals of a 

 population under given conditions." 



These two concepts would become most 

 meaningful if we could examine a popula- 

 tion under conditions approaching physio- 

 logical longevity and compare it with an- 

 other otherwise similar population of the 

 same species under known conditions of 

 ecological longevity. The diflFerence in size 

 between the two (the latter population is 

 always smaller) would give an expression 

 of the intensity of the mortality that can be 

 ascribed to the environmental vicissitudes. 



sources of ecological mortality as well as, 

 frequently, of mortality itself. There is an 

 emergent ecological principle here of some 

 validity: The greater the ecological mor- 

 tahty, the greater the reproductive potential 

 of the species. The converse statement, that 

 with decreased ecological mortality the re- 

 productive potential also decreases, is 

 equally true. 



Several workers, notably Rubner, Pearl, 

 and Bodenheimer, have concluded that the 

 curve of physiological longevity (number 

 still alive plotted against age) is quite 

 similar in its form for all species. It is de- 

 parture from the curve owing to ecological 



-100 



MOO 



+ 200 



PERCENTAGE DEVIATION FROM MEAN 



LIFE DURATION 

 Fig. 73. Survivorship curves for (1) starved, wild-type Drosophila melanogaster; (2), 

 starved, vestigial type Drosophila; (3), fed, wild- type Drosophila; (4) fed, vestigial type 

 Drosophila. (From Bodenheimer.) 



Knowing this intensity, we then become in- 

 terested in those ecological factors operating 

 to produce it such as unfavorable climate, 

 excessive crowding, infection, disease and 

 parasitization, predation pressure, accidents, 

 and so on. It stands to reason that there are 

 great differences between species in terms 

 of ecological mortality, some regularly hav- 

 ing many deaths from many causes, grading 

 imperceptibly into others with lower death 

 rates from fewer causes. An example of the 

 first would be locust populations living in 

 relatively unprotected situations in the 

 grasslands; an example of the second, 

 honeybees, before they become fieldbees, 

 which live in a highly buflFered microniche 

 with a well-organized societal structure. 

 Bodenheimer points out that one of the 

 common attributes of a social population is 

 its reduction, through group activity, of the 



mortality that is significant in the popula- 

 tion sense. These writers suggest that each 

 species-population attains a maximum life- 

 span when living under "optimum" ecologi- 

 cal conditions and having a genotype that is 

 "healthy" and homogeneous for all the com- 

 ponents. A survivorship curve (a curve that 

 graphs age on the abscissa against numbers 

 surviving on the ordinate; see page 296) 

 plotted for a population under these condi- 

 tions would approach a right angle. Here it 

 is self-evident that the group's members live 

 practically without death until they reach 

 a ripe old age, whereupon they quickly die. 

 In short, they live the life span character- 

 istic of their species without the complica- 

 tions of deleterious heredity or environment. 

 This concept is essentially academic, since 

 its chance of occurrence in natural popula- 

 tions is negligible, although, as we pointed 



