that would lit- (Ifstroycd anyway l)y natural factors. 

 The general trenil is to maintain the population at 

 the carrying ca])acity of the hahitat. Improvement of 

 yield is brought about only by increase in tiie carry- 

 ing capacity in respect to food, cover, and space. 

 These concepts are fundamental not only to an under- 

 standing of population dynamics, but also to wildlife 

 management. 



KKL.VTION TO DISTKIBITJON 



Variations in abundance of a species are 

 closely related to distribution. Three zones of abun- 

 dance may be recognized. There is an inner ro»ir of 

 normal abundance, where climatic and other condi- 

 tions are ordinarily favorable and high populations of 

 the species are characteristic. Surrounding this inner 

 area is a zone of occasional abundance, where cli- 

 matic or other conditions are usually severe enough 

 to hold populations at a low level, but where occa- 

 sional years occur in which high populations may be 

 reached. On the outside is a zone of possible abun- 

 dance, where the normal environment is such that 

 the species cannot maintain a permanent population 

 but where the species may occur during favorable 

 years by emigration from the iimer zones (Cook 

 1929). Populations can become stabilized only in the 

 innermost zone. \Miere climate, suitable space or 

 cover, and food continually vary from year to year, 

 as in the middle and outer zones, stabilization is never 

 attained for any appreciable length of time (Swenk 

 1929). Ordinarily, therefore, one may expect a spe- 

 cies to maintain a stabilized level of abundance only 

 in the center or optimum habitat of its range, and to 

 decline and fluctuate in abundance to an increasing 

 extent tow^ards the limit of its distribution. 



SUMMARY 



The regional density of a species depends 

 on the prevalence of its favored niche, and its habitat 

 density within this niche. Populations become sta- 

 bilized by density-dependent factors whose effects 

 increase in intensity as the population level rises and 

 decrease as the population level declines. The most 

 important density-stabilizing factors are competition, 

 fecundity, survival of young, predation, emigration, 

 and disease and physiological stress. 



The level at which populations become stabilized 



FIG. 16-11 Control of 

 blossoms by density-in 



but 



75 100 125 150 



DAYS 



Hon size of thrips inhabiting rose 

 5nt factors. The total population 

 s indicated by the sigmoid curve, 

 f the available niches because of 

 the onset of summer drought. The dotted lines indicate decline 

 from the maximum population size for each of the years 1932- 

 1937 (Davidson and Andrewartha 1948). 



is determined by such density-limiting factors as 

 space or cover, prevailing weather, and food supply. 

 These factors are largely density-independent, since 

 their magnitude is primarily determined by the physi- 

 cal conditions of the environment. However, their 

 action is responsive to the size of the population as 

 the amount of space, protection from weather, and 

 food available per individual decreases as the popula- 

 tion increases. 



The influence of any factor upon a population is 

 determined by the time in the life-cycle of the or- 

 ganisms at which it is effective, its variability, and 

 its threshold and upper limit of vulnerability for the 

 population. Intercompensations occur so that when 

 one factor becomes ineffective in controlling the 

 density of a population, another factor becomes more 

 effective. 



A species normally attains a stabilized level of 

 density only in the center of its range, where physical 

 conditions are optimum. Towards the periphery of 

 its range, its population density becomes increasingly 

 unstable and fluctuating. 



Regulation of population size 233 



