POPULATION CYCLES IN SMALL MAMMALS
329
and a 70% decrease in the seed fall of preferred grasses in grazed
areas.
Kalela (1962) suggested that microtine density fluctuations were
coupled to variation in plant production resulting from weather
conditions. The emphasis on the climate as the underlying variable
leading to microtine cycles arises from the geographically widespread
synchrony observed in small mammal density fluctuations. Rodent
cycles are viewed by Kalela as being the result of random oscillations of
weather conditions which influence the flowering frequency of food
plants and variations in their nutritional state, as well as inherent
rhythms of the food plants and rodent populations. To investigate this
association Tast and Kalela (1971) accumulated density data on four
species of microtines (Lemmus lemmus, Microtus agrestis, M . oeconomus,
Clethrionomys rufocanus) and two of their preferred food plants,
narrow leaved cotton grass, Eriophorum angustifolium, and golden-rod,
Solidago virgaurea.
The microtine populations, estimated by two trappings a year with
snap-traps, showed peak densities in the fall of 1964 and again in the
fall of 1969 (Fig. 32). While the number of shoots of the cotton grass
was highest in 1964 in synchrony with the peak microtine density, in
1969, the year of the second microtine peak population, the number of
cotton grass shoots was no higher than it had been during the intervening years of low microtine densities. The number of shoots of
Solidago showed little change between 1965, when quantification of this
plant began, and 1969, but the year after the microtine peak there was
a small increase in the number of shoots of this plant.
Exclosures were used by Tast end Kalela t o show that microtines
had no influence on the number of flowering Eriophorum plants, and
through the study there was a decrease in the number of flowering
shoots of Eriophorum because of human disturbance. The microtines
still reached peak densities in 1969 even though the number of flowering
shoots of Eriophorum was about one-Hth that of the 1963-1964 peak.
The number of flowering shoots of Solidago was the highest in 1966, L
year of very low microtine density, and 1969, a year of peak microtine
density. While Tast and Kalela (1971) report positive correlations
between food conditions and rodent population fluctuations, none of
the correlations were significant.
A more recent paper (Tast, 1972) has related mean winter weights
of Microtus OeconOmuS to the food supply as indicated by the density
of sterile shoots of Eriophorum (Fig. 34). A single point gives the
impression that there might be a relation between these factors. The
sample size on which this point is based is four animals. More data are
obviously required.
329
and a 70% decrease in the seed fall of preferred grasses in grazed
areas.
Kalela (1962) suggested that microtine density fluctuations were
coupled to variation in plant production resulting from weather
conditions. The emphasis on the climate as the underlying variable
leading to microtine cycles arises from the geographically widespread
synchrony observed in small mammal density fluctuations. Rodent
cycles are viewed by Kalela as being the result of random oscillations of
weather conditions which influence the flowering frequency of food
plants and variations in their nutritional state, as well as inherent
rhythms of the food plants and rodent populations. To investigate this
association Tast and Kalela (1971) accumulated density data on four
species of microtines (Lemmus lemmus, Microtus agrestis, M . oeconomus,
Clethrionomys rufocanus) and two of their preferred food plants,
narrow leaved cotton grass, Eriophorum angustifolium, and golden-rod,
Solidago virgaurea.
The microtine populations, estimated by two trappings a year with
snap-traps, showed peak densities in the fall of 1964 and again in the
fall of 1969 (Fig. 32). While the number of shoots of the cotton grass
was highest in 1964 in synchrony with the peak microtine density, in
1969, the year of the second microtine peak population, the number of
cotton grass shoots was no higher than it had been during the intervening years of low microtine densities. The number of shoots of
Solidago showed little change between 1965, when quantification of this
plant began, and 1969, but the year after the microtine peak there was
a small increase in the number of shoots of this plant.
Exclosures were used by Tast end Kalela t o show that microtines
had no influence on the number of flowering Eriophorum plants, and
through the study there was a decrease in the number of flowering
shoots of Eriophorum because of human disturbance. The microtines
still reached peak densities in 1969 even though the number of flowering
shoots of Eriophorum was about one-Hth that of the 1963-1964 peak.
The number of flowering shoots of Solidago was the highest in 1966, L
year of very low microtine density, and 1969, a year of peak microtine
density. While Tast and Kalela (1971) report positive correlations
between food conditions and rodent population fluctuations, none of
the correlations were significant.
A more recent paper (Tast, 1972) has related mean winter weights
of Microtus OeconOmuS to the food supply as indicated by the density
of sterile shoots of Eriophorum (Fig. 34). A single point gives the
impression that there might be a relation between these factors. The
sample size on which this point is based is four animals. More data are
obviously required.