(Dvnioiul 1947), ami Mack iTapjHcs in the Illinois 

 River (Thompson UMl ). hnt these explanations have 

 limited a])plication. 



A persistent theory of the general cause of cycles 

 hypothesizes that animal iwpiilations build up to a 

 peak, at which time an epidemic disease occurs so 

 reducing numbers that the disease can no longer 

 spread. The cycle then starts over again. Epizootics 

 observed in cyclic species are those caused by the 

 blood-sucking stomach worm ObcUscoidcs ciiniciili 

 in the snowshoe rabbit : the blood protozoan Lctico- 

 cytocoon botiacac in the ruffed grouse; protozoan in- 

 fection of the brain caused by Toxol^lasiiia in rodents ; 

 and so forth. However, these diseases have been en- 

 countered in some cyclic declines but not in others 

 and offer no ex])lanation of the regular recurrence 

 of the cycles. Toxoplasma, for instance, was re- 

 jwrted in three early jjopulation declines of rodents, 

 l)ut was not demonstrably present in more recent 

 declines (Elton 1942). 



It is generally agreed that the basic cycle is that 

 of the herbivores : rodents, grouse, snowshoe rabbit, 

 and the like. With a rise in number of herbivores, 

 predator populations may eventually increase suffi- 

 ciently by reproduction and immigration to reduce the 

 herbivores, but the predator population cycle depends 

 fundamentally upon the herbivore cycle. The herbi- 

 vore cycle may in turn be deijendent on interrelations 

 with its plant food .supply. This suggests the follow- 

 ing explanation. 



Xorthern plants are often unable, in the short 

 season available, to make a luxuriant vegetative 

 growth and produce seed every year. During lem- 

 ming peaks, the animals deplete their usual food 

 plants and are forced to turn to emergency species 

 which are not self-sustaining. A period of two or 

 more years may be required, following an irruption 

 in the lemming population, for full recovery of the 

 vegetation both quantitatively and qualitatively. The 

 relation between herbivores and plants is very similar 

 to that between predator and prey. The difficulty 

 with this hypothesis is the lack of evidence that lem- 

 ming mortality during population declines is actually 

 a result of starvation. Species depending on seeds for 

 food would be similarly affected by the interval be- 

 tween abundant seed crops (Lack 1954a, Thompson 

 1955a, Watson 1956, Lauckhart 1957). 



Variations in mineral salts within plants may be 

 involved in the cycles of herbivorous species. Vari- 

 ations in climate may affect the bacterial flora of the 

 soil and consequently the availability of calcium to 

 plants. In Bavaria, it has been possible to correlate 

 the gradual decrease in calcium content of hay with 

 the development of "licking disease" in cattle. Other 

 elements, although needed only in minute quantities 

 in animal metabolism, sometimes lead, if absent, to 

 an upset in the acid-base balance of the body and the 



di'Mlopment of acidosis and ketosis or other effects 

 (Hraestrnp 1940, VHl). 



lnadc(|uate nutrition, either quantitatively or 

 qualitatively, is well known to affect the rate of re- 

 production in animals (Hammond 1955). The size 

 of egg clutches and the vigor of the hatched young 

 in grouse and other tetraonids in Finland seem to 

 (lei)cnd on whether the females are able to get new 

 green vegetation for food in the critical period just 

 before the start of egg-laying. That tiiis vegetation 

 become available requires that temperature be suffi- 

 cient to melt the snow cover and to initiate plant 

 growth early in the year (Siivonen 1957). 



Clianges in reproductive vigor and health may 

 also depend on the vitamin content of the food con- 

 sumed (Mason 1939) and on the extent of the ani- 

 mal's exposure to solar radiation. The vitamin con- 

 tent of animal food is known to vary quantitatively 

 from time to time (Lehmann 1953). 



It is obvious that at times of high population den- 

 sities animals are subject to increased stresses of vari- 

 ous sorts in their search for food and cover and 

 escaping predators. They may have to go longer 

 distances to find the essentials for existence and to 

 fight with other animals for possession of them. All 

 of this puts an extra drain on their energy resources 

 at the same time that they may be compelled to 

 subsist on inferior food or tolerate nutritional defi- 

 ciencies of one sort or another. The body adapts 

 ]jhysiologicalIy to these stresses under the stimulus 

 of increased hormone secretion from the adrenal and 

 pituitary glands, but when the stresses for existence 

 and reproduction become too great, death results. It 

 has been postulated that the die-off at the end of a 

 cycle is due directly to such an exhaustion of the 

 adrenopituitary system- rather than to such external 

 factors as lack of food, disease, or predators (Chris- 

 tian 1950). "Shock disease" is a manifestation of 

 this stress syndrome. It has been repeatedly ob- 

 served in Minnesota during the decline of the snow- 

 shoe rabbit cycle. Symptoms of the exhaustion phase 

 of the stress syndrome have also been observed in 

 wild populations of European meadow voles (Frank 

 1953). The continuous decline of populations for 

 three to five years may be due to constitutional de- 

 fects resulting from the stress of overcrowding be- 

 ing transmitted to following generations (Chitty 

 1952), but direct evidence for this is meager. 



The difficulty in finding a satisfactory explanation 

 of cycles as they occur under natural conditions is 

 that they may be only one manifestation of an under- 

 lying, more fundamental, cycle in physiological vigor 

 which is not easily detected. Changes in population 

 size are not always correlated with changes in physi- 

 ological vigor, since extremes in weather may at 

 times produce catastrophes even in healthy and vigor- 

 ous populations. But changes in physiological vigor 



rruptions, catastrophes, and cycles 241 



