POPULATION CYCLES IN SMALL MAMMALS
337
grasslands in California, England or New York (Haynes and Thompson,
1965). If we are seeking generalities of microtine cycles, as pointed out
earlier in this article, we must not overlook the uniqueness of the
tundra and we must avoid overemphasis of characteristics specific to
this situation.
To summarize, microtine rodents take only a small fraction of the net
primary production. Microtine grazing affects the plant species composition of the habitat, and at least in tundra ecosystems also affects
the nutrient composition of the forage. No one seems to suggest that
voles or lemmings are limited by the amount of food available, and
attention has turned to the quality of the food available. The nutrient
threshold hypothesis has been suggested as an explanation for lemming
cycles, but we can find no evidence that nutrient levels cause any of
the characteristic features of cyclic lemming populations.
B. P R E D A T I O N
“Foxes also hunt them, and the wild ferrets in particular destroy
them; but they make no way against the prolific qualities of the animal
and the rapidity of its breeding.’’ These are the words of Aristotle
quoted by Elton (1942, p. 3) and the animal to which he was referring
was the field mouse. However, even though this opinion was expressed
in the early stages of the history of small mammal cycles, the question
whether predation might be the driving force behind population
fluctuations was by no means put to rest.
Two aspects must be separated in discussing predator-prey questions:
(1) How do the predators respond to variations in prey abundance? and
(2) Are these responses sufficient to explain population changes in the
prey? As with the food hypothesis, one of these two elements may be
present without the other.
Early writers were most taken by the correspondence between
fluctuations in populations of herbivores such as lemmings, voles and
rabbits, and of carnivores, such as foxes, weasels and raptors. Data
revealing oscillations of predators and their cycling prey was reviewed
by Elton (1942) and Lack (1954, pp. 204-226). Shelford (1943) considered the cycle of the varying lemming as being typical of predatorprey oscillations described by Lotka and Volterra. The prey species
increases in density providing more food for the predator, which responds with increased reproduction and a build-up in its own population to a level larger than can be supported by the prey population. A
decline in the prey population ensues, followed by a fall in the predator
population through death or emigration.
An extensive literature exists documenting the numerical response
337
grasslands in California, England or New York (Haynes and Thompson,
1965). If we are seeking generalities of microtine cycles, as pointed out
earlier in this article, we must not overlook the uniqueness of the
tundra and we must avoid overemphasis of characteristics specific to
this situation.
To summarize, microtine rodents take only a small fraction of the net
primary production. Microtine grazing affects the plant species composition of the habitat, and at least in tundra ecosystems also affects
the nutrient composition of the forage. No one seems to suggest that
voles or lemmings are limited by the amount of food available, and
attention has turned to the quality of the food available. The nutrient
threshold hypothesis has been suggested as an explanation for lemming
cycles, but we can find no evidence that nutrient levels cause any of
the characteristic features of cyclic lemming populations.
B. P R E D A T I O N
“Foxes also hunt them, and the wild ferrets in particular destroy
them; but they make no way against the prolific qualities of the animal
and the rapidity of its breeding.’’ These are the words of Aristotle
quoted by Elton (1942, p. 3) and the animal to which he was referring
was the field mouse. However, even though this opinion was expressed
in the early stages of the history of small mammal cycles, the question
whether predation might be the driving force behind population
fluctuations was by no means put to rest.
Two aspects must be separated in discussing predator-prey questions:
(1) How do the predators respond to variations in prey abundance? and
(2) Are these responses sufficient to explain population changes in the
prey? As with the food hypothesis, one of these two elements may be
present without the other.
Early writers were most taken by the correspondence between
fluctuations in populations of herbivores such as lemmings, voles and
rabbits, and of carnivores, such as foxes, weasels and raptors. Data
revealing oscillations of predators and their cycling prey was reviewed
by Elton (1942) and Lack (1954, pp. 204-226). Shelford (1943) considered the cycle of the varying lemming as being typical of predatorprey oscillations described by Lotka and Volterra. The prey species
increases in density providing more food for the predator, which responds with increased reproduction and a build-up in its own population to a level larger than can be supported by the prey population. A
decline in the prey population ensues, followed by a fall in the predator
population through death or emigration.
An extensive literature exists documenting the numerical response