338
CHARLES J. KREBS AND JUDITH H. MYERS
of predators to microtine populations. Gross (1947) summarizes the
history of snowy owl migrations from the Arctic into New England.
These migrations followed years of high lemming population densities.
The subsequent movement southward was triggered as food became
scarce when the lemming population declined.
Avian predators can be categorized as restricted feeders which depend
predominantly on one prey species, and general feeders which take a
variety of prey species (Craighead and Craighead, 1969, p. 182).
Restricted feeders travel about looking for high prey densities, and
during a year of high Microtus numbers a great influx of restricted
feeders into the Craigheads’ study area occurred, while populations
of general feeders remained about constant. Other reports of predator
movements in response to fluctuating vole and lemming populations
were made by Honer (1963) for the barn owl, Maher (1970) and Pitelka
et al. (1955) for the jaeger and snowy owl, Mysterud (1970) for the
boreal owl, and Stendell (1972) for the white-tailed kite. Some raptors
are only able to breed when the population of their prey is relatively
high (jaegers and snowy owls, Maher (1970) and Pitelka et al. (1955);
rough-legged hawk, Hagen (1969); and the white-tailed kite, Stendell
(1972)).
But given that predator species can respond to high densities of
voles and lemmings by increasing their own numbers, is mortality
caused by predation sufficient to regulate vole and lemming populations? Opinions are divided on this question. Some studies claim that
predators took a high proportion of the prey population and therefore
me a necessary factor in preventing population increase. Others claim
that predators had very little influence on the prey population (Table
XIV).
A modified version of the predator hypothesis was suggested by
Lack (1954). Lack concluded that predators are too scarce in proportion
to the prey at the time of lemming or vole peak densities, and hence
predators cannot stop the increase of breeding prey populations.
Therefore, it was necessary for another factor, such as food limitation or
intraspecific competition, to stop the microtine increase by shutting
off reproduction before the predators could catch up and exert any
influence on prey population density.
The first serious attempt to quantify the influence of predation on a
population of Microtus was made in 1951 by Brant (1962). This was
done by analyzing raccoon scats which were deposited on his study
area. Brant surprisingly found that twice as many M . californicus were
taken by predators than he had estimated to be present on the study
area. The source of error could be either poor estimation techniques
for Microtus 0; the concentration of predator scats from a larger
CHARLES J. KREBS AND JUDITH H. MYERS
of predators to microtine populations. Gross (1947) summarizes the
history of snowy owl migrations from the Arctic into New England.
These migrations followed years of high lemming population densities.
The subsequent movement southward was triggered as food became
scarce when the lemming population declined.
Avian predators can be categorized as restricted feeders which depend
predominantly on one prey species, and general feeders which take a
variety of prey species (Craighead and Craighead, 1969, p. 182).
Restricted feeders travel about looking for high prey densities, and
during a year of high Microtus numbers a great influx of restricted
feeders into the Craigheads’ study area occurred, while populations
of general feeders remained about constant. Other reports of predator
movements in response to fluctuating vole and lemming populations
were made by Honer (1963) for the barn owl, Maher (1970) and Pitelka
et al. (1955) for the jaeger and snowy owl, Mysterud (1970) for the
boreal owl, and Stendell (1972) for the white-tailed kite. Some raptors
are only able to breed when the population of their prey is relatively
high (jaegers and snowy owls, Maher (1970) and Pitelka et al. (1955);
rough-legged hawk, Hagen (1969); and the white-tailed kite, Stendell
(1972)).
But given that predator species can respond to high densities of
voles and lemmings by increasing their own numbers, is mortality
caused by predation sufficient to regulate vole and lemming populations? Opinions are divided on this question. Some studies claim that
predators took a high proportion of the prey population and therefore
me a necessary factor in preventing population increase. Others claim
that predators had very little influence on the prey population (Table
XIV).
A modified version of the predator hypothesis was suggested by
Lack (1954). Lack concluded that predators are too scarce in proportion
to the prey at the time of lemming or vole peak densities, and hence
predators cannot stop the increase of breeding prey populations.
Therefore, it was necessary for another factor, such as food limitation or
intraspecific competition, to stop the microtine increase by shutting
off reproduction before the predators could catch up and exert any
influence on prey population density.
The first serious attempt to quantify the influence of predation on a
population of Microtus was made in 1951 by Brant (1962). This was
done by analyzing raccoon scats which were deposited on his study
area. Brant surprisingly found that twice as many M . californicus were
taken by predators than he had estimated to be present on the study
area. The source of error could be either poor estimation techniques
for Microtus 0; the concentration of predator scats from a larger