278
CHARLES J. KREBS AND JUDITH R. MYERS
gathered quantitatively over a three- or €our-year period which fails
to show a population cycle. We conclude that microtine rodent populations
normally undergo population cycles with a period of three to four years
and this density pattern should be assumed to be the normal conjiguration.
Microtine populations that do not fluctuate cyclically are the unusual
situation and if any can be located they would be exceptionally
important to study. We feel that the burden of proof should be shifted
to those who would claim to have a non-cyclic population.
One of the dogmas about population cycles is that-they are more
pronounced in arctic regions (Odum, 1971, p. 193). A corollary of this
dogma is that southern populations of voles should have reduced
amplitudes of cycles and ultimately reach a point of having no cyclic
fluctuations at all. We have been unable to trace the origin of this
dogma; perhaps it was first mentioned by Howell (1923). Dymond
(1947, p. 14) also reports the dogma: “It has long been recognized that
periodic fluctuations in animal populations are virtually confined to the
northern part of the northern hemisphere and are especially characteristic of the Arctic, not only in America but also in Europe and Asia”.
Keith (1963, pp. 67-68) reports that the ten-year cycle is absent from
some southern populations of snowshoe hare (Lepus americanus) and
ruffed grouse (Bonasa umbellus), but the available data are poor and
this conclusion uncertain. We can find no quantitative evidence that
vole and lemming cycles are more pronounced in arctic regions than
they are farther south. Wildhagen (1952) states that lemming fluctuations are more pronounced in northern Norway than in southern
Norway, and Kalela (1962) supports this statement. But no rodent
census data are available and one may be comparing the “visibility” of
peak populations in northern and southern habitats rather than the
cyclic amplitude.
We conclude that vole and lemming populations go through regular
cycles of abundance everywhere they have been studied. We view these
cycles as a special type of population fluctuation, and most of the
discussion to follow is independent of whether -the rodent fluctuations
are regular or irregular. Even if one denies that microtine populations
cycle regularly, one must still explain their fluctuations.
c. S T R U C T U R E O F P O P U L A T I O N F L U C T U A T I O N S I N M I C R O T I N E S
One of the first steps toward understanding a population fluctuation
is to describe it in some detail. Part of the lack of progress in explaining
microtine cycles is due to the fact that emphasis has been on identifying
cycles and determining relative densities. But to understand cycles we
CHARLES J. KREBS AND JUDITH R. MYERS
gathered quantitatively over a three- or €our-year period which fails
to show a population cycle. We conclude that microtine rodent populations
normally undergo population cycles with a period of three to four years
and this density pattern should be assumed to be the normal conjiguration.
Microtine populations that do not fluctuate cyclically are the unusual
situation and if any can be located they would be exceptionally
important to study. We feel that the burden of proof should be shifted
to those who would claim to have a non-cyclic population.
One of the dogmas about population cycles is that-they are more
pronounced in arctic regions (Odum, 1971, p. 193). A corollary of this
dogma is that southern populations of voles should have reduced
amplitudes of cycles and ultimately reach a point of having no cyclic
fluctuations at all. We have been unable to trace the origin of this
dogma; perhaps it was first mentioned by Howell (1923). Dymond
(1947, p. 14) also reports the dogma: “It has long been recognized that
periodic fluctuations in animal populations are virtually confined to the
northern part of the northern hemisphere and are especially characteristic of the Arctic, not only in America but also in Europe and Asia”.
Keith (1963, pp. 67-68) reports that the ten-year cycle is absent from
some southern populations of snowshoe hare (Lepus americanus) and
ruffed grouse (Bonasa umbellus), but the available data are poor and
this conclusion uncertain. We can find no quantitative evidence that
vole and lemming cycles are more pronounced in arctic regions than
they are farther south. Wildhagen (1952) states that lemming fluctuations are more pronounced in northern Norway than in southern
Norway, and Kalela (1962) supports this statement. But no rodent
census data are available and one may be comparing the “visibility” of
peak populations in northern and southern habitats rather than the
cyclic amplitude.
We conclude that vole and lemming populations go through regular
cycles of abundance everywhere they have been studied. We view these
cycles as a special type of population fluctuation, and most of the
discussion to follow is independent of whether -the rodent fluctuations
are regular or irregular. Even if one denies that microtine populations
cycle regularly, one must still explain their fluctuations.
c. S T R U C T U R E O F P O P U L A T I O N F L U C T U A T I O N S I N M I C R O T I N E S
One of the first steps toward understanding a population fluctuation
is to describe it in some detail. Part of the lack of progress in explaining
microtine cycles is due to the fact that emphasis has been on identifying
cycles and determining relative densities. But to understand cycles we