322
CHARLES J. KREBS AND JUDITH H. MYERS
pennsylvanicus. Poa pratensis from gravel, clay and silt loam areas
was judged by Thompson to be unfavorable, suboptimal, and optimal
based on the color, vigor and succulence of the plants. Pieces of sod
with grass from these three areas were presented simultaneously
to Microtus pennsylvanicus. The percentages of the stems which were
clipped by the animals were 63, 63 and 58%. This test suggests that
microtines may not be very selective in choosing the quality of their
food.
Microtus ochrogaster in Indiana takes a wider variety of plant
species as food than does M . pennsylvanicus, and this is correlated with
the greater diversity of plant species in habitats where M . ochrogmter
are common (Zimmerman, 1965). Batzli and Pitelka (1971) confirmed
that for M . californicus the species common in the habitat were also
common in the diet.
If food supply is a critical influence on the dynamics of rodent
cycles, we might look for differences between microtine species with
different food habits. An attempt at assessing the relation of voles
and their habitats to population phenomena can be made by comparing
Microtus, which are grassland dwellers, to Clethrionmys, which live
in scrubby or wooded areas. Comparisons of food habits of C. rutilus
and M . oeconmus dwelling in a white spruce forest near College,
Alaska were made by Grodzinski (1971). Berries, fruits, tree seeds,
fungus and lichens were preferred by the red-backed voles (C. rutilus),
while greens were most preferred by the tundra vole ( M . oeconmw),
although berries, fruits and seeds were also taken to a large degree.
The results of laboratory preference tests agreed well with those of
stomach content analyses. But all species of Clethrionomys are not
similar in their feeding habits. Tast and Kalela (1971) state that
C. rufocanus is a greens eater while C. rutilus is a seed eater. All species
of Microtus seem to feed on greens.
That Microtus and Clethrionomys compete where sympatric has been
demonstrated by observations that when only one species is present,
for example on an island, it will invade the habitat usually occupied
by the other species (review in Morris and Grant, 1972). Also experimental manipulations have been carried out to show that M.pennsy1vanicus tends to exclude C. gapperi from grasslands and C. gapperi
tends to exclude M . pennsylvanicus from wooded areas. Therefore,
although their preferred habitats are different, there is some overlap
in the ecological requirements of these two species.
Clethrionomys populations tend to exist at lower densities than
Microtus populations (Table XII). Some but not all species of Clethrionomys seem to fluctuate in regular cycles, but whether fluctuations are
related to feeding habits is not clear. One of the longest series of popda-
CHARLES J. KREBS AND JUDITH H. MYERS
pennsylvanicus. Poa pratensis from gravel, clay and silt loam areas
was judged by Thompson to be unfavorable, suboptimal, and optimal
based on the color, vigor and succulence of the plants. Pieces of sod
with grass from these three areas were presented simultaneously
to Microtus pennsylvanicus. The percentages of the stems which were
clipped by the animals were 63, 63 and 58%. This test suggests that
microtines may not be very selective in choosing the quality of their
food.
Microtus ochrogaster in Indiana takes a wider variety of plant
species as food than does M . pennsylvanicus, and this is correlated with
the greater diversity of plant species in habitats where M . ochrogmter
are common (Zimmerman, 1965). Batzli and Pitelka (1971) confirmed
that for M . californicus the species common in the habitat were also
common in the diet.
If food supply is a critical influence on the dynamics of rodent
cycles, we might look for differences between microtine species with
different food habits. An attempt at assessing the relation of voles
and their habitats to population phenomena can be made by comparing
Microtus, which are grassland dwellers, to Clethrionmys, which live
in scrubby or wooded areas. Comparisons of food habits of C. rutilus
and M . oeconmus dwelling in a white spruce forest near College,
Alaska were made by Grodzinski (1971). Berries, fruits, tree seeds,
fungus and lichens were preferred by the red-backed voles (C. rutilus),
while greens were most preferred by the tundra vole ( M . oeconmw),
although berries, fruits and seeds were also taken to a large degree.
The results of laboratory preference tests agreed well with those of
stomach content analyses. But all species of Clethrionomys are not
similar in their feeding habits. Tast and Kalela (1971) state that
C. rufocanus is a greens eater while C. rutilus is a seed eater. All species
of Microtus seem to feed on greens.
That Microtus and Clethrionomys compete where sympatric has been
demonstrated by observations that when only one species is present,
for example on an island, it will invade the habitat usually occupied
by the other species (review in Morris and Grant, 1972). Also experimental manipulations have been carried out to show that M.pennsy1vanicus tends to exclude C. gapperi from grasslands and C. gapperi
tends to exclude M . pennsylvanicus from wooded areas. Therefore,
although their preferred habitats are different, there is some overlap
in the ecological requirements of these two species.
Clethrionomys populations tend to exist at lower densities than
Microtus populations (Table XII). Some but not all species of Clethrionomys seem to fluctuate in regular cycles, but whether fluctuations are
related to feeding habits is not clear. One of the longest series of popda-