364
CHARLES J . KREBS AND JUDITH H. MYERS
How can one demonstrate that a surplus population of microtines
exists? The best way to do this is to crop a resident population of
breeding animals, and to see if new animals take up the positions
vacated by removals. This experiment was first done by Smyth (1968)
on Clethrionomys glareolus at Oxford, and he found that extensive
immigration offset his removals. Krebs (1966) cropped a two-acre
grassland of 1758 Microtus californicus over one year and yet found
little difference in density between the cropped population and its
control. Myers and Krebs (1971b) described a substantial influx of M .
pennsylvanicus into cropped areas in Indiana. Watts (1970) cropped a
population of Clethrionomys gapperi of adult males during a phase of
increasing density, and found little difference in the rate of growth of
the control and experimental populations. Elliott ( 1969) removed
males from another population of C. gapperi and found that they were
replaced by other adult males. He concluded that spacing behavior
was important in determining breeding densities. Dahl (1967) did the
converse experiment of adding M . pennsylvanicus to a resident population and found that he could not increase density by adding voles.
These cropping experiments can be criticized because the “surplus”
voles moving into the experimental areas might be the resident breeding
animals from surrounding areas. Hence the breeding density of the
whole area might be depressed by filling in the evacuated habitat, and
no truly “surplus” voles might exist. This did not appear to be the case
in the Krebs (1966) experiment, since not a single marked individual
was drawn from the control area to the removal area 300 ft away.
Surplus animals in rodent populations presumably disperse and
are largely lost to various agents of mortality. Consequently the
study of dispersal in vole and lemming populations is also a study of
the movements of “surplus” animals. Because of the general habits of
rodents, we are not able to study their social organization in the same
way we can for territorial birds. Hence the demonstration of surplus
voles or lemmings will probably never be as elegant in experimental
terms as similar experiments on birds.
The surplus population of microtine rodents always seems to be
capable of breeding, thus satisfying condition (2) above. The immigrants
of Microtus californicus which colonized the removal area were active
breeders (Krebs, 1966), and the same was generally true in the study
by Myers and Krebs (1971b), although some young animals not yet
sexually mature also dispersed. Cyclic rodents seem to have adopted
the general strategy of decreasing the population of breeding individuals
as they go from the increase phase to the peak phase. Reproductive
changes, such as increase in the age at sexual maturity, all seem to
force more of the population into a “surplus”, non-breeding category.
CHARLES J . KREBS AND JUDITH H. MYERS
How can one demonstrate that a surplus population of microtines
exists? The best way to do this is to crop a resident population of
breeding animals, and to see if new animals take up the positions
vacated by removals. This experiment was first done by Smyth (1968)
on Clethrionomys glareolus at Oxford, and he found that extensive
immigration offset his removals. Krebs (1966) cropped a two-acre
grassland of 1758 Microtus californicus over one year and yet found
little difference in density between the cropped population and its
control. Myers and Krebs (1971b) described a substantial influx of M .
pennsylvanicus into cropped areas in Indiana. Watts (1970) cropped a
population of Clethrionomys gapperi of adult males during a phase of
increasing density, and found little difference in the rate of growth of
the control and experimental populations. Elliott ( 1969) removed
males from another population of C. gapperi and found that they were
replaced by other adult males. He concluded that spacing behavior
was important in determining breeding densities. Dahl (1967) did the
converse experiment of adding M . pennsylvanicus to a resident population and found that he could not increase density by adding voles.
These cropping experiments can be criticized because the “surplus”
voles moving into the experimental areas might be the resident breeding
animals from surrounding areas. Hence the breeding density of the
whole area might be depressed by filling in the evacuated habitat, and
no truly “surplus” voles might exist. This did not appear to be the case
in the Krebs (1966) experiment, since not a single marked individual
was drawn from the control area to the removal area 300 ft away.
Surplus animals in rodent populations presumably disperse and
are largely lost to various agents of mortality. Consequently the
study of dispersal in vole and lemming populations is also a study of
the movements of “surplus” animals. Because of the general habits of
rodents, we are not able to study their social organization in the same
way we can for territorial birds. Hence the demonstration of surplus
voles or lemmings will probably never be as elegant in experimental
terms as similar experiments on birds.
The surplus population of microtine rodents always seems to be
capable of breeding, thus satisfying condition (2) above. The immigrants
of Microtus californicus which colonized the removal area were active
breeders (Krebs, 1966), and the same was generally true in the study
by Myers and Krebs (1971b), although some young animals not yet
sexually mature also dispersed. Cyclic rodents seem to have adopted
the general strategy of decreasing the population of breeding individuals
as they go from the increase phase to the peak phase. Reproductive
changes, such as increase in the age at sexual maturity, all seem to
force more of the population into a “surplus”, non-breeding category.