BIOLOGICAL BACKGROUNDS FOR POPULATION STUDIES 



273 



tive potential is as high as earher." Expo- 

 nentially, for the same period, the protozoa 

 would have increased from 4096 to 32,768. 

 For organisms with exclusively biparental 

 inheritance the principles are similar, al- 

 though the details may diflFer. 



Realized reproductive performance is the 

 observed population birth rate. This is the 

 amount of reproduction that actually occurs 

 over a defined time interval. Thus, a popu- 

 lation of 2000 organisms of the same spe- 

 cies might have a potential of 12,000 off- 

 spring per year, but a birth rate of, say 

 2000. t The birth rate is influenced by the 

 potential reproductive capacity, which in 

 turn is affected by both genetic and ecologi- 

 cal factors. In addition, mortality of the re- 



group, is antithetic to natahty. It can be 

 defined loosely as the "force" of total popu- 

 lation deaths. The biologist is interested 

 both in why organisms die and why they 

 die at a given age, an interest shared by 

 the population student. The first aspect is 

 significant for us whenever the causes of 

 death can be ascribed to the ecological envi- 

 ronment. The second aspect is significant 

 because of the obvious relation between age 

 of death and the birth rate. Under mortal- 

 ity we discuss physiological fife expecta- 

 tion, ecological life expectation, and age 

 distribution in populations and its impor- 

 tance. We shall return to certain other con- 

 siderations of mortality in the chapter on 

 Demography. 



Absolute species potential 



Departures from optimum 



Genetic and ecologic effects 

 Mortality of reproducers 



Partial potential 



Birth rate 

 Fig. 72. Schematic relation between species potential and birth rate. 



producing members of the population af- 

 'ects (usually lowers) the birth rate. Some 

 of these relations are styfized in Figure 72. 



MORTALITY 



Mortahty is the population-decline factor 

 and thus, in terms of its effect on the 



" This, an assumption commonly made on 

 mathematical grounds, seems dubious from the 

 biological point of view. See discussion on page 

 392. 



•[■Imagine that 1000 of the 2000 were 

 females, each physiologically capable of pro- 

 ducing twelve young per year, or one a month. 

 The population potential for twelve months 

 then would be 1000 X 12 = 12,000 under the 

 assumed conditions. Actually, each female dur- 

 ing that year might give rise on the average to 

 only two young. Thus the birth rate would be 

 1000 X 2 = 2000. Whether the population as a 

 whole would grow, remain stationary, or de- 

 cline under these conditions would depend on 

 the mortality and dispersal relations and could 

 not be answered by knowledge of natality taken 

 alone. 



Physiological and Ecological Longevity 



The population student recognizes two 

 types of longevity— physiological and ecolog- 

 ical. The former represents the capacitici 

 of the individuals of a species to live out 

 their life span (or their capacities to resist 

 death), while the latter depicts the ob- 

 served life duration of the members of the 

 population. There is an analogy here with 

 the concepts of potential natahty in contra 

 distinction to realized reproduction. 



Bodenheimer (1938), on whom we lean 

 considerably in this section, has given a 

 workman-like discussion of this question, 

 and his definitions follow: 



1. "Physiological longevity is the average 

 longevity of individuals of a population living 

 under optimal conditions and of genetically 

 liomogeneous stock.** 



• This definition seems to imply that all 

 mortality is environmentally produced. 



