348
CHARLES J. KREBS AND JUDITH H. MYERS
fall and recaptured in the spring. However, Whitney (unpublished)
collected survival data for Clethrionomys rutilus and Microtus oeconomus
during three years, a t the same time that he was monitoring winter
climatological features, Two winters were classified by Whitney as
poor for voles even though the conditions were quite dissimilar. The
winter of 1969-1970 began with a rapid freeze but temperatures were
higher than average and ten times during the winter there were thaws.
Snow conditions were poor which resulted in colder than normal ground
temperatures. This would probably be judged by Fuller to be a harsh
winter for voles. There were two periods of loss in numbers for the
C. rutilus population (Fig. 31). The first of these was from September
to November. Survival of females was particularly low in September;
this was approximately six weeks before the winter freeze and the
first snows. For the remainder of the winter the percentage of voles
surviving every two weeks ranged from 70 to 90%. The second period
of poor survival was between February and March, over a month before
the little snow present melted. In general, winter survival by C.
rutilus was superior to summer survival. The period of poor survival
at the start of the breeding season in the spring is typical of microtine
populations (see p. 283) and could not be tied to specific weather
factors.
Whitney reported a similar picture in a sympatric population of
Microtus oeconomus. The survival of males was better during the winter
than in the summer. The deterioration of survival in the spring occurred
a month earlier in this species than in Clethrionomys rutilus on the
same study area.
Summer survival of C. rutilus was low in 1970 (Fig. 31); however, it
improved in August and except for a short period in September
continued to be extremely high during the winter. This winter was not
judged by Whitney to be good for voles either. The fall freeze was
gradual, the snow fall heavy and the subnivean space was not developed
until late in the winter,
Whitney’s study is extremely important because it shows the
necessity of obtaining detailed demographic data in judging the influence
of extrinsic factors on the population. We do not yet know what
characteristics of winters are most stressful for voles. But Whitney’s
data, in agreement with that reviewed earlier, indicates that the
crucial period is really in the fall, with a period of poor survival often
preceding freezing weather and snow, and in the spring, in association
with the beginning of the breeding season. No simple association
could be made between winter conditions and survival of Clethrionomys
rzctilus and Microtus oeconomus.
The spring melt-off might be a time period which is critical to
CHARLES J. KREBS AND JUDITH H. MYERS
fall and recaptured in the spring. However, Whitney (unpublished)
collected survival data for Clethrionomys rutilus and Microtus oeconomus
during three years, a t the same time that he was monitoring winter
climatological features, Two winters were classified by Whitney as
poor for voles even though the conditions were quite dissimilar. The
winter of 1969-1970 began with a rapid freeze but temperatures were
higher than average and ten times during the winter there were thaws.
Snow conditions were poor which resulted in colder than normal ground
temperatures. This would probably be judged by Fuller to be a harsh
winter for voles. There were two periods of loss in numbers for the
C. rutilus population (Fig. 31). The first of these was from September
to November. Survival of females was particularly low in September;
this was approximately six weeks before the winter freeze and the
first snows. For the remainder of the winter the percentage of voles
surviving every two weeks ranged from 70 to 90%. The second period
of poor survival was between February and March, over a month before
the little snow present melted. In general, winter survival by C.
rutilus was superior to summer survival. The period of poor survival
at the start of the breeding season in the spring is typical of microtine
populations (see p. 283) and could not be tied to specific weather
factors.
Whitney reported a similar picture in a sympatric population of
Microtus oeconomus. The survival of males was better during the winter
than in the summer. The deterioration of survival in the spring occurred
a month earlier in this species than in Clethrionomys rutilus on the
same study area.
Summer survival of C. rutilus was low in 1970 (Fig. 31); however, it
improved in August and except for a short period in September
continued to be extremely high during the winter. This winter was not
judged by Whitney to be good for voles either. The fall freeze was
gradual, the snow fall heavy and the subnivean space was not developed
until late in the winter,
Whitney’s study is extremely important because it shows the
necessity of obtaining detailed demographic data in judging the influence
of extrinsic factors on the population. We do not yet know what
characteristics of winters are most stressful for voles. But Whitney’s
data, in agreement with that reviewed earlier, indicates that the
crucial period is really in the fall, with a period of poor survival often
preceding freezing weather and snow, and in the spring, in association
with the beginning of the breeding season. No simple association
could be made between winter conditions and survival of Clethrionomys
rzctilus and Microtus oeconomus.
The spring melt-off might be a time period which is critical to