POPULATION CYCLES IN SMALL MAMMALS
347
numbers and then holding numbers low so as to delay the next cyclic
build-up. In some declines only a small fraction of the loss can be
attributed to predation, and the evidence suggests that predation is not
necessary to cause the decline phase. Predation may contribute to the
rate of decline of a population, and this seems to be its major role in
many populations. Whether predators hold prey numbers down in the
phase of low numbers is an interesting question on which few data can
be cited. Experiments manipulating predator numbers will be necessary
t o answer this question.
C . W E A T H E R A N D S Y N C H R O N Y
Weather can affect microtine populations, and Fuller (1967, 1969)
has suggested that weather effects are one explanation of microtine
cycles. Because Fuller’s work has been concerned with high latitude
microtines, he was particularly interested in winter weather conditions,
the critical periods being at the time of the fall freeze and the spring
thaw (Fuller, 1967). A study of Clethrionomys gapperi, C. rutilus and
the cricetine Peromyscus maniculatus in the vicinity of Great Slave
Lake, N.W.T. was undertaken by Fuller (1969) to compare the demographic characteristics of these three species living in the same habitat
and under the same general weather conditions. Populations of all
three rodent species were high in the summer of 1966. The spring of
1967 was the coldest and wettest, and was followed by low summer
population densities of C. gapperi and P. maniculatus. Because data
were not collected during the winter we do not know exactly when the
populations declined. However, C. rutilus was undaunted by the severe
winter and late spring in 1967 and remained at peak densities during the
summer of 1967. Fuller proposed that the difference in the reaction of
the three species to the “hard” winter of 1966-1967 was due to greater
cold-tolerance of C. rutilus.
Another study of Clethrionomys gapperi was undertaken by Elliott
(1969) in the vicinity of Edmonton, Alberta. This study covered the
years 1965-1968. The winter of 1967-1968 was judged most severe by
Elliott because of its thin and unstable snow cover and the greatest
amounts of rain during weeks with freezing temperatures. C. gapperi
densities were the lowest observed in the spring of 1968 for any of the
four years of the study and there was almost no recovery of the population during the summer of 1968. Thus a severe winter was clearly
associated with a population decline.
Fuller andElliott couldonly conjecture what was happening to thevoles
during the winter because their data consisted only of density estimates
in fall and in spring, and survival estimates from animals marked in the
347
numbers and then holding numbers low so as to delay the next cyclic
build-up. In some declines only a small fraction of the loss can be
attributed to predation, and the evidence suggests that predation is not
necessary to cause the decline phase. Predation may contribute to the
rate of decline of a population, and this seems to be its major role in
many populations. Whether predators hold prey numbers down in the
phase of low numbers is an interesting question on which few data can
be cited. Experiments manipulating predator numbers will be necessary
t o answer this question.
C . W E A T H E R A N D S Y N C H R O N Y
Weather can affect microtine populations, and Fuller (1967, 1969)
has suggested that weather effects are one explanation of microtine
cycles. Because Fuller’s work has been concerned with high latitude
microtines, he was particularly interested in winter weather conditions,
the critical periods being at the time of the fall freeze and the spring
thaw (Fuller, 1967). A study of Clethrionomys gapperi, C. rutilus and
the cricetine Peromyscus maniculatus in the vicinity of Great Slave
Lake, N.W.T. was undertaken by Fuller (1969) to compare the demographic characteristics of these three species living in the same habitat
and under the same general weather conditions. Populations of all
three rodent species were high in the summer of 1966. The spring of
1967 was the coldest and wettest, and was followed by low summer
population densities of C. gapperi and P. maniculatus. Because data
were not collected during the winter we do not know exactly when the
populations declined. However, C. rutilus was undaunted by the severe
winter and late spring in 1967 and remained at peak densities during the
summer of 1967. Fuller proposed that the difference in the reaction of
the three species to the “hard” winter of 1966-1967 was due to greater
cold-tolerance of C. rutilus.
Another study of Clethrionomys gapperi was undertaken by Elliott
(1969) in the vicinity of Edmonton, Alberta. This study covered the
years 1965-1968. The winter of 1967-1968 was judged most severe by
Elliott because of its thin and unstable snow cover and the greatest
amounts of rain during weeks with freezing temperatures. C. gapperi
densities were the lowest observed in the spring of 1968 for any of the
four years of the study and there was almost no recovery of the population during the summer of 1968. Thus a severe winter was clearly
associated with a population decline.
Fuller andElliott couldonly conjecture what was happening to thevoles
during the winter because their data consisted only of density estimates
in fall and in spring, and survival estimates from animals marked in the