TABLE 16-6 Effect of different combinations of temperature and 

 humidity on the levels attained by populations of flour beetles 

 in experimental cultures (Park 1954). 



has been demonstrated experimentally, for instance, 

 that the density attained by the cladoceran Daphnia 

 obtiisa is directly proportional to the food supply 

 (Slobodkin 1954). This principle was known to 

 Thomas R. Malthus back in 1798, and influenced the 

 theory of evolution as developed by Charles Dar- 

 win. If cover is deficient, animals become exposed 

 to predation and bad weather earlier so that the 

 population becomes stabilized at a low level. Like- 

 wise, differences in temperature and humidity afifect 

 the level attained by experimental cultures of flour 

 beetles (Table 16-6). 



Differences between species in their relative de- 

 mands for space, food, and shelter affect the popula- 

 tion levels that they attain. Species of small body size 

 require less space than those of large size. In similar 

 fashion, species that get along in small territories 

 will be more numerous than those requiring large 

 territories. Herbivorous species find more food avail- 

 able in a limited area than do species higher in the 

 food chain and hence will be the most numerous. 

 Hardy species will flourish in climatic areas where 

 less tolerant species are scarce. 



The limiting effects of space, weather, and food 

 are properly considered to be density-independent. 

 However, some actions of these factors are density- 

 responsive. The amount of space available for addi- 

 tional individuals is, of course, inversely proportional 

 to the space already occupied, so that the greater 

 the population density, the less space there is avail- 

 able per individual. If all favorable cover is occupied, 

 additions to the population are forced into inferior 

 cover where they receive less protection in inclement 

 weather and where they become more exposed to the 

 attacks of predators. Voles feeding in grassland may, 

 when abundant, consume so much of the vegetation 

 that they destroy their cover as well as their food 

 supply. When populations of brown lemming are low 

 over winter, they utilize less than one per cent of the 

 annual production of the grasses and sedges which 

 are their favorite food. At peak populations however, 

 they use nearly 100 per cent of the growth, become 

 greatly exposed to predation, and are subsequently 

 forced to shift to the less palatable forage and poorer 

 cover of moss. The lack of adequate winter forage 



and cover, concurrent with a reduction in reproduc- 

 tion and increase in predation, results in a rapid de- 

 cline in population level (Thompson 1955a). 



Outbreaks of spruce budworm do not occur in the 

 coniferous forests of Canada until the succession to 

 white spruce and especially balsam fir develops a 

 sexually mature evergreen canopy overtopping the 

 aspen and birch. Insect larvae newly emerged from 

 hibernation feed on the flowers, especially the male 

 flowers, before the leaf buds open ; then they move 

 down to consume the current foliage, eventually de- 

 foliating and killing the trees. Millions of dollars 

 worth of timber is destroyed. Native parasitoids are 

 incapable of preventing outbreaks. The outbreak dies 

 out in a few years by which time all the mature trees 

 are destroyed and the insects' food supply is ex- 

 hausted. Another outbreak will not occur in the lo- 

 cality until another generation of spruce and balsam 

 matures in the area (Prebble 1954). 



In stored grain infested with insects the heat 

 produced by the insects may raise the temperature 

 beyond their limit of tolerance and prevent further 

 increase in population density (Solomon 1953). In- 

 sects and rodents at high populations tend to have 

 reduced vigor and health and to be affected by 

 weather conditions which at low population levels are 

 easily tolerated (Chitty 1960, Wellington 1960). It 

 is thus apparent that even the climatic environment 

 may be density-responsive in its effects in special 

 situations. However, it is well to distinguish between 

 density-responsive effects that are relatively passive 

 and limiting and density-dependent effects that ^re 

 dynamic and stabilizing. 



INTERCOMPENSATIONS 



The difficulty of evaluating, under natural 

 conditions, the role of any factor in regulating the 

 size of animal populations is in large part a result of 

 the fact that it seldom acts alone. The time in the 

 life-cycle of an organism at which a factor takes effect 

 influences the importance of it. Normally, the earlier 

 in the life of the organism at which a factor is effec- 

 tive, the more nearly its apparent controlling role is 

 a real one. Thus 60 per cent of the mature larvae of 

 an insect may be fatally infested with parasitoids, but 

 if 82.9 per cent of the original output of eggs have 

 already failed to reach this stage for other reasons, 

 the influence of these parasitoids must be evaluated 

 at only 10.2 per cent, 0.60 X (100 — 82.9), instead 

 of the full 60 per cent (Table 16-7). On the other 

 hand, a 10 per cent apparent mortality resulting from 

 egg parasites which comes at an early stage in the 

 life cycle may produce a nearly equal real mortality 

 (9.5) ; but a 10 per cent parasitization of pupae late 

 in the cycle may cause only 0.47 per cent real mor- 

 tality. 



230 Ecological processes and dynamics 



