312
CHARLES J. KREBS AND JUDITH H. MYERS
1 9 6 5
1916
1967
FIG. 25. “Fence-effect’’ in Microtus ochrogaster. Grid A is an unfenced control
population, grid D is a two-acre fenced enclosure, which increased to a maximum
of 411 voles, 5.5 times the highest control density. The habitat on grid D was
destroyed by overgrazing during the winter of 1966-1967. (After &ebs et al.,
1969.)
attributed this “fence-effect” to a restriction of dispersal and the
addition of food as trap bait.
Studies of enclosed populations suggest that dispersal may be a key
factor in determining population trends in voles and lemmings. We can
envisage two ways in which dispersal might be important to a population. First, dispersal may act as a safety valve for the population, and
dispersing voles may normally be killed by one hazard or another.
When population density becomes high more and more animals might
emigrate and die, so that the decline phase might be associated with
much emigration. Second, dispersal might act selectively in such a way
that the quality of dispersing voles differs from the quality of residents.
A number of relevant qualities might be involved: ability to avoid
predators, ability to utilize certain food plants, or aggressiveness,
Selective dispersal would be more important early in the population cycle
and, in contrast to the first mechanism, dispersal in the increase phase
might be most important for its qualitative effects. Instead of dispersers
representing a random sample of the population which wilI be eliminated
selective dispersal during the population increase could change the
quality of the population resident at peak densities. In order to obtain
CHARLES J. KREBS AND JUDITH H. MYERS
1 9 6 5
1916
1967
FIG. 25. “Fence-effect’’ in Microtus ochrogaster. Grid A is an unfenced control
population, grid D is a two-acre fenced enclosure, which increased to a maximum
of 411 voles, 5.5 times the highest control density. The habitat on grid D was
destroyed by overgrazing during the winter of 1966-1967. (After &ebs et al.,
1969.)
attributed this “fence-effect” to a restriction of dispersal and the
addition of food as trap bait.
Studies of enclosed populations suggest that dispersal may be a key
factor in determining population trends in voles and lemmings. We can
envisage two ways in which dispersal might be important to a population. First, dispersal may act as a safety valve for the population, and
dispersing voles may normally be killed by one hazard or another.
When population density becomes high more and more animals might
emigrate and die, so that the decline phase might be associated with
much emigration. Second, dispersal might act selectively in such a way
that the quality of dispersing voles differs from the quality of residents.
A number of relevant qualities might be involved: ability to avoid
predators, ability to utilize certain food plants, or aggressiveness,
Selective dispersal would be more important early in the population cycle
and, in contrast to the first mechanism, dispersal in the increase phase
might be most important for its qualitative effects. Instead of dispersers
representing a random sample of the population which wilI be eliminated
selective dispersal during the population increase could change the
quality of the population resident at peak densities. In order to obtain