are liigh. Among mammals, cpizDntics liave been ob- 

 served in voles, lemmings, mice, rats, beavers, squir- 

 rels, rabbits, moles, foxes, deer (Hlton 1942), birds, 

 fisbes, and reptiles. Incidence of tbem is often 

 sporadic in tbat they do not always ajjpear with high 

 densities of host populations nor with declines in 

 cyclic species so that their importance as a regulating 

 factor on animal [Xjpulations has been difificult to 

 evaluate (Chitty 1954). 



Phvsiolot/ical stress is not infectious but becomes 

 pathological when extreme and may bring consider- 

 able mortality in the population. States of stress have 

 been produced experimentally by allowing confined 

 populations of albino and house mice and meadow- 

 voles to increase to high levels of abundance, even 

 with a surplus of food and water present. Evidence 

 that a state of stress exists is demonstrable in experi- 

 mental populations by increase in the weight of the 

 adrenals, by decrease in the weight of the body, testes, 

 thymus, preputial glands, and seminal vesicles ; by 

 decrease in the number of circulating eosinophils in 

 the blood ; and by aberrant maternal behavior (Clarke 

 1953, Christian 1956, Louch 1956, 1958). An increase 

 in adrenal weights, especially the adrenocortical tis- 

 sue, as population rises, has also been shown in wild 

 populations of the Norway rat (Christian and Davis 

 1956), and the behavior symptoms usually associated 

 with high physiological stress have been observed in 

 wild populations of European meadow voles (Frank 

 1953). 



It seems probable that shock disease as it occurs 

 in snowshoe rabbits is a manifestation of the stress 

 syndrome (Green and Larsen 1938) and occurs when 

 the liver degenerates leaving inadequate reserves of 

 glycogen available for emergencies. Under these con- 

 ditions, any undue exertion or excitement may cause 

 normal animals to go suddenly into convulsions, sink 

 into a coma, and die. 



Relaxation of density-dependent effects 



With so many decimating factors acting on 

 populations, one wonders why species do not become 

 extinct more often than they do. The explanation is 

 that there is relaxation in the intensity of action of 

 the factors as the affected population becomes re- 

 duced in size. This relaxation is brought about by 

 the heterogeneity of the environment so that at least 

 some individuals escape the full force of the factor; 

 hyperparasitoidism or overcrowding reducing the pop- 

 ulation of parasitoids and predators themselves : de- 

 velopment of immunity to or tolerance of the factors 

 involved : change in behavior so that the decimating 

 factor is avoided : survival of dormant eggs, pupae, 

 or encysted stages after the active stages in the life- 

 cycle perish (Solomon 1949). 



i)K\sri^-i.iMrnN(; factors 



Variations in space or cover, favoraiile 

 weather, and food occur independently of po])ulation 

 densities and may cause drastic changes in the abun- 

 dance of animals. Heavy silting of estuaries along 

 the coast from erosion of the surrounding upland may 

 smother oyster spat and reduce the amount of hard 

 surface available for setting quite independently of 

 the number of oysters already there, or variations in 

 oyster abundance from year to year. The amount of 

 solid surface available also determines the population 

 density reached by sessile rotifers (Edmondson 

 1946). Variations in water level of a stream affects 

 the availability of suitable spawning areas for fish, 

 and consequently their abundance (Starrett 1951). 

 A drought may dry up a marsh, making it unsuitable 

 for muskrats and waterfowl. A severe winter freeze 

 may kill all but a few hardy individuals of any species 

 regardless of the size of the original population. 

 Failure of a food crop, from weather, flooding, or 

 some other physical factor, may deplete populations 

 that depend on it for subsistence. On the other hand, 

 agriculture has provided food, and allowed some 

 species to become abundant that once were scarce ; 

 for instance, many insect pests of crops. 



Although fluctuations in space, weather, and food 

 may directly affect the abundance of animals in an 

 obvious manner, their average or prevailing condi- 

 tion determines the level at which density-stabilizing 

 factors bring populations into equilibrium. With 

 abundant food, cover, and favorable weather, popula- 

 tions will be high. When food is scarce, competition 

 for it becomes acute at a lower population density. It 



FIG. 16-9 The 1932 

 emigration of sharp- 

 failed grouse from northe 

 Ontario and Quebec 

 (Snyder 1935). 



Regulation of population size 229 



