350
CHARLES J. KREBS AND JUDITH H. MYERS
in northern Bulgaria, an area subject to periodic summer droughts,
M . arvalis populations sometimes reached high densities. The determining factor seemed to be the amount of rain during the second half of
the summer. Good rains and associated forage growth permitted
breeding by the voles into the autumn, and therefore dense overwintering populations.
The area near Berkeley, California is characterized by summer
droughts of varying lengths. The study of Batzli and Pitelka (1971)
of M . californicus included two years in which rainfall was below
normal. In one of these years the voles were at low densities and in
the other at peak density. Thus summer drought does not appear to
determine population trends in the California vole.
As part of a study of M . penmylvanicw and M . ochrogaster in
southern Indiana solar radiation, rainfall, soil temperature, and
humidity were recorded for the five years of the study (Krebs, unpublished). No correlations could be found between these weather
characteristics and Microtus population fluctuations. Furthermore,
the two species living under the same weather regime did not always
fluctuate in synchrony. Weather appears to exert little limitation on
vole populations in Indiana, where extremes of drought or cold do not
While it is impossible to vary winter weather experimentally and
thus to test its influence on microtine cycles, populations which are
out-of-phase could be used for this test. This experiment requires an
increasing population contiguous with a peak or declining population.
One means for accomplishing this is to introduce microtines to enclosures, where they respond by increasing in numbers. The influence
of a particular winter on an enclosed and an adjacent free-living
population could then be observed. The relation of drought to population phenomena of southern microtines could be tested by observing
populations in irrigated areas. For example, we could ask whether
irrigation abolishes the cycling of M . californicus populations.
One of the conceptual difficulties in recognizing the role of weather in
microtine cycles is the diversity of populations in which cycles occur.
Voles from arctic to temperate areas seem to go through population
cycles which have many demographic attributes in common. If we
explain a fluctuation in M . oeconomus by winter snow conditions, must
we seek another explanation for M . pennsylvanicus in areas where
snow is rare?
We recognize that weather has received too little attention from
students of microtine rodents. We need to look both at the destructive
aspects of weather on survival rates and at the permissive aspects of
weather in allowing reproduction. Because many of these effects can
occur.
CHARLES J. KREBS AND JUDITH H. MYERS
in northern Bulgaria, an area subject to periodic summer droughts,
M . arvalis populations sometimes reached high densities. The determining factor seemed to be the amount of rain during the second half of
the summer. Good rains and associated forage growth permitted
breeding by the voles into the autumn, and therefore dense overwintering populations.
The area near Berkeley, California is characterized by summer
droughts of varying lengths. The study of Batzli and Pitelka (1971)
of M . californicus included two years in which rainfall was below
normal. In one of these years the voles were at low densities and in
the other at peak density. Thus summer drought does not appear to
determine population trends in the California vole.
As part of a study of M . penmylvanicw and M . ochrogaster in
southern Indiana solar radiation, rainfall, soil temperature, and
humidity were recorded for the five years of the study (Krebs, unpublished). No correlations could be found between these weather
characteristics and Microtus population fluctuations. Furthermore,
the two species living under the same weather regime did not always
fluctuate in synchrony. Weather appears to exert little limitation on
vole populations in Indiana, where extremes of drought or cold do not
While it is impossible to vary winter weather experimentally and
thus to test its influence on microtine cycles, populations which are
out-of-phase could be used for this test. This experiment requires an
increasing population contiguous with a peak or declining population.
One means for accomplishing this is to introduce microtines to enclosures, where they respond by increasing in numbers. The influence
of a particular winter on an enclosed and an adjacent free-living
population could then be observed. The relation of drought to population phenomena of southern microtines could be tested by observing
populations in irrigated areas. For example, we could ask whether
irrigation abolishes the cycling of M . californicus populations.
One of the conceptual difficulties in recognizing the role of weather in
microtine cycles is the diversity of populations in which cycles occur.
Voles from arctic to temperate areas seem to go through population
cycles which have many demographic attributes in common. If we
explain a fluctuation in M . oeconomus by winter snow conditions, must
we seek another explanation for M . pennsylvanicus in areas where
snow is rare?
We recognize that weather has received too little attention from
students of microtine rodents. We need to look both at the destructive
aspects of weather on survival rates and at the permissive aspects of
weather in allowing reproduction. Because many of these effects can
occur.