450 

 400 

 350 



300 

 250 

 200 

 150 

 100 

 50 



I 1930 I 1931 I 1932 | 1933 | 1934 | 1935 | 1936 | 1937 | 1938 | 1939 | I940| 1941 | 1942 | 1943 | 



FIG. 16-1 Seasonal and yearly changes In the population of 

 bobwhite on 1800 hectares (4500 acres) In Wisconsin. The 

 solid line shows the net reproductive Increase each year from 



spring to autumn; the dashed line, the mortality over winter 

 (from Errington 1945). 



terms of mature offspring raised. As a necessary 

 corollary, the mean longevity of a population also 

 varies inversely with its density (Davis 1945). 



DENSITY-STABILIZING FACTORS 



The intrinsic growth rate of a population, 

 is limited to the early stages in the sigmoid growth 

 curve of the population. Very soon, environmental 

 resistance restrains the rate of growth more and more 

 sharply until, at the asymptote, the environmental re- 

 sistance equals the biotic or reproductive potential 

 and the population is stabilized. 



We need now to take a closer look at the proc- 

 esses that produce these efifects and determine the 

 levels at which populations reach their asymptotes. 

 These processes may be conveniently divided into 

 two groups ; those that are density-stabilizing, and 

 those that are density-limiting. The first group of 

 factors are biotic in that they depend on coactions be- 

 tween individuals within the same population or be- 

 tween populations of different species. Limiting fac- 

 tors, which determine the level at which populations 

 become stabilized, are basically physical and vary in 

 intensity because of influences outside or largely in- 

 dependent of the population or community. All fac- 



tors taken together are commonly considered to con- 

 stitute the environmental resistance, a convenient if 

 not entirely accurate term. 



Density-dependent factors (Howard and Fiske 

 1911: p. 107, Nicholson 1933, 1954, Smith 1935, 

 Solomon 1949, Ricker 1954) are those that vary in 

 the intensity of their action with the size or density of 

 the population, but not all density-dependent factors 

 are density-stabilizing. Only if the percentage of a 

 prey species destroyed by predators, for instance, in- 

 creases with the size of the population and decreases 

 as the population declines, is natural control prevent- 

 ing indefinite population expansion, yet preventing 

 extinction, too. This action then tends to stabilise 

 the population size (Table 16-1). If, however, the 

 percentage of prey taken remains approximately the 

 same at all population levels the effect is propor- 

 tional. If the percentage of prey or host affected actu- 

 ally decreases as the population increases, the effect 

 is inverse. This happens occasionally (Tothill 1922). 

 Obviously the proportional or inverse effects of a fac- 

 tor cannot inhibit the continuous expansion of a pop- 

 ulation. Complete quantitative data are required in 

 order to classify and evaluate the effect of any factor. 

 The density-dependent factors that will be considered 

 in respect to their stabilizing effect on population size 

 are competition, reproductivity, predation, emigra- 



220 Ecological processes and dynamics 



