POPULATION CYCLES I N SMALL MAMMALS
333
eaten out the natural food supply and were on the brink of starvation.
Three animals died without losing weight. This points out the fallacy
of observations that symptoms of starvation were not observed in
declining populations (Rauch, 1950, p. 176) and suggests that observations of this sort are not useful in judging the relation between an
animal and its food supply.
An alternative to providing supplemental food to a microtine
population is to determine if a habitat can in fact support higher
microtine densities than it naturally does. Two-acre enclosures of
grassland in southern Indiana supported densities of Microtus ochrogaster
and M . pennsylvanicus several times greater than did a similar habitat
just outside the fence. Although eventual habitat destruction resulted,
immediate recovery occurred at the beginning of the next growing
season, and introduced Microtus always responded with rapid population growth (Krebs et al., 1969, 1973). These experiments showed
that higher densities of voles could be supported by the Indiana
habitat than normally were. Even after extensive habitat utilization
there was no delay in recovery to forage vegetation sufficient for
growth and reproduction of Microtus populations.
A further test of the influence of standing crop, amount of food
available and percentage cover on microtine populations is provided by
the work of Batzli and Pitelka (1970, 1971). I n this study two populations of Microtus californicus were monitored, one in Richmond,
California bordering the San Francisco Bay and the second in the
Briones Hills, ten miles east of Richmond. Peak population densities
of M . californicus in these two areas were almost the same in 1967-1968
when the study was conducted and both populations showed similar
declines in the fall of 1968. However, these workers report that the
standing crop, average height of vegetation and the percentage cover
were all greater in the Briones study area, and the volume of Bromus
rigidus, a major component of the food of M . californicus, was ten
times greater at this area than in the Richmond plot. Unfortunately
the study was terminated just as the Microtus population decline began,
but it is obvious that in this case even ten times the amount of one
food plant did not allow a greater peak density nor prevent the
beginning of the population decline.
Field experiments have so far suggested that food limitation is not
the factor stopping the increase phase of microtine populations. But we
still must analyze the nutrient and lemming cycles described by
Schultz (1969) in terms of what is cause and what is effect. The
nutritional threshold hypothesis of Schultz is as follows: During the
summer of peak lemming population the nutrients in the forage
are at high concentrations, and high production and consumption
333
eaten out the natural food supply and were on the brink of starvation.
Three animals died without losing weight. This points out the fallacy
of observations that symptoms of starvation were not observed in
declining populations (Rauch, 1950, p. 176) and suggests that observations of this sort are not useful in judging the relation between an
animal and its food supply.
An alternative to providing supplemental food to a microtine
population is to determine if a habitat can in fact support higher
microtine densities than it naturally does. Two-acre enclosures of
grassland in southern Indiana supported densities of Microtus ochrogaster
and M . pennsylvanicus several times greater than did a similar habitat
just outside the fence. Although eventual habitat destruction resulted,
immediate recovery occurred at the beginning of the next growing
season, and introduced Microtus always responded with rapid population growth (Krebs et al., 1969, 1973). These experiments showed
that higher densities of voles could be supported by the Indiana
habitat than normally were. Even after extensive habitat utilization
there was no delay in recovery to forage vegetation sufficient for
growth and reproduction of Microtus populations.
A further test of the influence of standing crop, amount of food
available and percentage cover on microtine populations is provided by
the work of Batzli and Pitelka (1970, 1971). I n this study two populations of Microtus californicus were monitored, one in Richmond,
California bordering the San Francisco Bay and the second in the
Briones Hills, ten miles east of Richmond. Peak population densities
of M . californicus in these two areas were almost the same in 1967-1968
when the study was conducted and both populations showed similar
declines in the fall of 1968. However, these workers report that the
standing crop, average height of vegetation and the percentage cover
were all greater in the Briones study area, and the volume of Bromus
rigidus, a major component of the food of M . californicus, was ten
times greater at this area than in the Richmond plot. Unfortunately
the study was terminated just as the Microtus population decline began,
but it is obvious that in this case even ten times the amount of one
food plant did not allow a greater peak density nor prevent the
beginning of the population decline.
Field experiments have so far suggested that food limitation is not
the factor stopping the increase phase of microtine populations. But we
still must analyze the nutrient and lemming cycles described by
Schultz (1969) in terms of what is cause and what is effect. The
nutritional threshold hypothesis of Schultz is as follows: During the
summer of peak lemming population the nutrients in the forage
are at high concentrations, and high production and consumption