Stage 



Apparent 

 mortality 

 per cent 



Real 



mortality 

 per cent 



TABLE 16-7 Evslu 

 mortality (actors 

 different stages in 

 of an insect (Thoi 



itlon of 

 iffectlve at 

 the life cycli 

 ipson 1928). 



The variability of a factor also affects its im- 

 portance. If a factor consistently produces, say. a 60 

 per cent mortality year after year, it will influence 

 the size of the population any particular year less than 

 will another factor that varies from, say, 20 to 30 jier 

 cent. However, a variation of 10 per cent in a factor 

 that averages 60 per cent mortality is more important 

 than the same variation in a factor that usually pro- 

 duces only 20 per cent mortality. An increase in 

 mortality from 60 to 70 per cent reduces the surviving 

 population 25 per cent (40 — 30)/4O, but an increase 

 from 20 to 30 per cent reduces the surviving popula- 

 tion only 12.5 per cent (80 — 70) /80 (Morris 1957). 



Aside from time and variability, the influence of 

 any factor is dependent on the level of population 

 size at which it first comes to exert an effective or 

 critical role. This level represents a threshold oj vul- 

 nerability of the population for that particular factor. 

 The threshold of vulnerability varies between species 

 and within the species, depending on the amount of 

 protective cover that is present, the movements and 

 activities of the species, its protective coloration, and 

 the aggressiveness and capabilities of the predators 

 themselves (Craighead and Craighead 1956). 



There is also an upper limit of vulnerability or 

 escape phase (Votite 1946) above which a factor no 

 longer exerts effective control over a population in- 

 crease. With increase in the number of cocoons of 

 European pine sawfly, predation by small mammals 

 rises to a peak at a density of 2,000,000 cocoons per 

 hectare (800,000 per acre), at which level about 50 

 per cent of the prey are destroyed. W^ith further in- 

 crease in density of cocoons, the percentage of prey 

 taken by mammals decreases in a density-inverse re- 

 lation. The prey has escaped the stabilizing influ- 

 ence of predation or gone beyond the upper limit of 

 vulnerability to this factor (Rolling 1959). 



Competition or fighting between individuals ap- 

 pears at rather low population levels among verte- 

 brates. With crowding, a social hierarchy may be- 

 come established with disadvantages to those in the 

 lower positions, or territories may become seriously 

 compressed in size. When fighting becomes intense, 

 individuals are forced to leave the area (emigrate) ; 

 failing this, their reproductive activities are disturbed. 

 In other species, for instance insects, competition may 



not be sufficient to control the increase in population, 

 so that the population reaches a level at which preda- 

 tion becomes significant. Populations of vertebrates 

 become especially vulnerable to predation when all 

 suitable cover becomes crowded and surplus animals 

 are forced to accept inferior cover or are driven into 

 the open. Once an outbreak surpasses a certain level 

 in spite of predation, predation can on longer take 

 any significant percentage of the species. In fact, bird 

 predation on insects may even become a hindrance at 

 high population levels in that it destroys parasitoids 

 that then become the most effective regulating fac- 

 tor (Strickland 1928, Thompson 1929, Betts 1955). 

 Emigation and epizootics ordinarily do not occur un- 

 less competition and predation fail to hold down the 

 increase in population and very high population 

 thresholds are reached (Severtzoff 1934, MacKenzie 

 1951). Populations occupying inferior habitats usu- 

 ally never reach densities that render them vulnerable 

 to epizootics, and they ordinarily escape the violent 

 fluctuations in size that occur with the species in 

 habitats of higher carrying capacity (Evans 1942). 



These various regulating factors affect species dif- 

 ferently. Song birds with well defined territories sel- 

 dom reach densities at which reduced reproductivity 

 or predation becomes important. They are, however, 

 subject to reduced reproductivity in high populations. 

 Game birds, which do not defend nesting territories, 

 ungulates, and insects, in which group competition is 

 relatively ineffective, commonly have their population 

 levels controlled by predation. If predation, reduced 

 reproductivity, and competition do not curtail popula- 

 tion e-xpansion, lemmings undergo emigrations, and all 

 animals become subject to epizootics or physiological 

 stress. For many species the effect of density- 

 dependent factors is cumulative ; that is, several fac- 

 tors are involved to varying degrees (Milne 1957). 

 For some insects, no density-dependent force is ef- 

 fective : the population is never stabilized, and it con- 

 tinues to increase until there is exhaustion of space 

 or food or curtailment by bad weather. 



A good example of a population that never be- 

 comes stabilized is the rose thrip, that inhabits rose 

 blossoms. Rapid multiplication of the thrip is possible 

 only during a limited period, during spring and early 

 summer. In this period the insect increases rapidly, 



Regulation of population size 23 



