POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



391 



so depicted that one family of factors con- 

 tributing to population decline is balanced 

 by another family of basically similar ori- 

 gin, either ecologic or genetic, contribut- 

 ing to population increase. Thus, the let- 

 ters without prime superscripts (e.g., A, 

 C, D, E) are associated in one way or an- 

 other with growth, while those with super- 

 scripts (e.g., A', C, D', E') are associated 

 with decline. The arrows may be thought 

 of as "population pressures" that are cumu- 

 lative. For example, A' has a B' and C 

 component; C, a D' and E' component, 

 and so on. 



The figure is drawn to focus attention on 

 population size, which, as discussed earlier 

 (p. 305), is a meaningful end index of 

 population activity. The population is rep- 

 resented as a circle of particular area at 

 any particular moment, whose circumfer- 

 ence enlarges or contracts with changes in 

 the natality-mortality relations symbolized 

 by arrows A and A'. The population may 

 fluctuate between a lowest attainable size 

 below which it would become extinct and 

 a highest attainable size above which its 

 excess members could not be supported by 

 the exploitable potentiaUties of its environ- 

 ment. The size of the circle is fixed for any 

 stated time because of an existing balance 

 between all the factors that operate in its 

 expansion and those that operate in its 

 contraction. 



In the final analysis, if dispersal is ex- 

 cluded, all changes in population size re- 

 sult from the interaction of reproductive 

 performance with mortaUty (see p. 272). 

 Other things being equal, high birth rate 

 and low death rate favor increase. Since 

 these factors are population or group at- 

 tributes, they must be viewed in aggregate 

 terms, as suggested in the diagram. 



The diagram further suggests that ecolog- 

 ical factors capable of affecting population 

 size through their influence on reproductiv- 

 ity and mortality fall into the category of 

 density-independent and density-dependent 

 factors. These, having been discussed in 

 detail, need no further treatment here be- 

 yond pointing out that both sets of factors 

 may be potent, yet different in mode of 

 action. In a broad sense, density-independ- 

 ent factors appear to set the limits of possi- 

 ble abundance for a population in its phys- 

 ical habitat, besides inducing particular 

 changes in density; while density-dependent 

 factors, operating primarily through com- 



petition, appear to regulate population den- 

 sity through time, thereby achieving what- 

 ever condition of equihbrium or "balance" 

 actually obtains. 



There are at least two ways in which 

 genetic factors can influence population 

 size. They may directly affect natality and 

 reproduction, as has been demonstrated for 

 certain species. Also, the genetic composi- 

 tion of a population may influence the 

 mode of response of that population to its 

 particular environment. 



Although Figure 138 stresses the rela- 

 tions of natahty and mortahty to popula- 

 tion growth form, it does not imply that 

 high reproductivity is necessarily associated 

 with low mortahty, or the converse. There 

 are, of course, many possible intergrada- 

 tions, both in theory and in fact, between 

 these factors. However, it is clear that for 

 population growth to obtain, A must ex- 

 ceed A', 



Other formulations from the diagram of 

 change in size can be set forth symbolically 

 as follows: 



EquiUbrium: A essentially equal to A' 

 Fluctuation: A > A' > A > A', and 



so on in time 

 Decline: A' consistently greater 



than A 



The figure shows further that popula- 

 tion size is not explained when one merely 

 discusses natahty and mortahty. These 

 responses, as end products, are intimately 

 under the control of the genetic and ecolog- 

 ical factors that emerge from the recipro- 

 cal interaction between the population and 

 its environment. In the diagram the ar- 

 rows represent pressures or Lines of force 

 that stem from specific operations. Their 

 character, symmetry, and interrelations 

 form the basis of population integration. 



In its treatment and focus on reproduc- 

 tion the pattern proposed in Figure 138 is 

 somewhat modified from that usually pre- 

 sented. The point can be made as follows. 

 In the writings of Chapman (1928), 

 Cause (1934), Smith (1935), as well as in 

 the rationalization underlying the logistic 

 curve, reproduction has been dealt with as 

 a sort of species or group "constant"' that 



" The "h" constant of the logistic equation; 

 "reproductive potential" of Chapman; "poten- 

 tial reproductive capacity" of Smith. A novel 

 method of empirically determining "biotic 

 potential" has been proposed by Birch (1945). 



