POPULATION CYCLES IN ‘SMALL MAMMALS
34 1
supplement their diets with other species but remain on the area
exerting predation pressure. Secondly, predators do not stop the
increase of a breeding microtine population. Pearson proposes that
predators can be responsible for the amplitude of microtine cycles by
their ability to decrease the populations to very low levels through
continued predation pressure when the microtine populations are low.
Predators can also influence the periodicity of the cycle by prolonging
the period of low numbers. Note that according to Pearson’s ideas, the
critical period to study predation on microtine rodents is during the
late decline and the phase of low numbers. Unfortunately almost no
one seems to do this.
The influence of predation by ermine and weasel on a Microtus
montanus population was studied by Fitzgerald (1972) a t the University
of California research station on Sagehen Creek in the Sierra Nevada.
Vole populations and winter predation by weasels and ermine were
estimated by counting the number of winter nests made by voles (an
estimate of the vole population). The number of Microtus nests which
had been invaded by ermine and weasels as well as the remains of
Microtus left near these nests were counted as an indication of the
predators’ activities. A crucial assumption of this study is that one
vole nest found by Fitzgerald in the spring, indicates the presence of
one vole in the overwintering populations. However, more direct
estimates made by Fitzgerald in a live-trapping study, indicated a
possible ratio of one to three voles per nest. This ratio varied among
years. The formation in the autumn of “great families”, a living unit
composed of parents and several litters in a single nest, is discussed by
Frank (1957) for M . arwalis. This also suggests that one vole per winter
nest may not always be valid.
However, if we assume that one winter nest indicates at least one
overwintering vole, the maximum percentages of M . montanus taken
for the four years of the study are shown in Table XVI. In addition
to the 40% of the Microtus population which were calculated to have
been taken by predators during the winter of the population decline,
9% of the population were found dead in their nests the next spring.
In many cases more than one dead Microtus was found in a nest, which
suggests that the population estimate based on the number of nests
may be an underestimate. There is no way to determine how many of
the deaths in the decline would have occurred if predators had been
absent.
Another study recently completed (Stendell, unpublished) quantifies
the predation by raptors, particularly the white-tailed kite, Elanus
leucurus, on Microtus californicus populations. I n this case populations
of M . californicus on Grizzly Island, Solano Co., California were
hl
34 1
supplement their diets with other species but remain on the area
exerting predation pressure. Secondly, predators do not stop the
increase of a breeding microtine population. Pearson proposes that
predators can be responsible for the amplitude of microtine cycles by
their ability to decrease the populations to very low levels through
continued predation pressure when the microtine populations are low.
Predators can also influence the periodicity of the cycle by prolonging
the period of low numbers. Note that according to Pearson’s ideas, the
critical period to study predation on microtine rodents is during the
late decline and the phase of low numbers. Unfortunately almost no
one seems to do this.
The influence of predation by ermine and weasel on a Microtus
montanus population was studied by Fitzgerald (1972) a t the University
of California research station on Sagehen Creek in the Sierra Nevada.
Vole populations and winter predation by weasels and ermine were
estimated by counting the number of winter nests made by voles (an
estimate of the vole population). The number of Microtus nests which
had been invaded by ermine and weasels as well as the remains of
Microtus left near these nests were counted as an indication of the
predators’ activities. A crucial assumption of this study is that one
vole nest found by Fitzgerald in the spring, indicates the presence of
one vole in the overwintering populations. However, more direct
estimates made by Fitzgerald in a live-trapping study, indicated a
possible ratio of one to three voles per nest. This ratio varied among
years. The formation in the autumn of “great families”, a living unit
composed of parents and several litters in a single nest, is discussed by
Frank (1957) for M . arwalis. This also suggests that one vole per winter
nest may not always be valid.
However, if we assume that one winter nest indicates at least one
overwintering vole, the maximum percentages of M . montanus taken
for the four years of the study are shown in Table XVI. In addition
to the 40% of the Microtus population which were calculated to have
been taken by predators during the winter of the population decline,
9% of the population were found dead in their nests the next spring.
In many cases more than one dead Microtus was found in a nest, which
suggests that the population estimate based on the number of nests
may be an underestimate. There is no way to determine how many of
the deaths in the decline would have occurred if predators had been
absent.
Another study recently completed (Stendell, unpublished) quantifies
the predation by raptors, particularly the white-tailed kite, Elanus
leucurus, on Microtus californicus populations. I n this case populations
of M . californicus on Grizzly Island, Solano Co., California were
hl