346
CHARLES J. KREBS AND JUDITH H. MYERS
Walkowa (1971) removed 32% of the population every month and were
not successful in decreasing the size of the population. They in fact
raised the standing crop. This increase was not due to immigration
but was the result of increased survival of young born into the population and longer residency of mice not removed by “artificial predation”.
House mice have a larger litter size and higher reproductive potential
than microtines, but these experiments show that moderate mortality
does not always cause a decline in a small mammal population.
Krebs (1966) removed all Microtus californicus weighing more than
40 g, in many cases removing more than half the number of animals
trapped on the study area every two weeks, but still was not able to
prevent the population from increasing. However, there was considerable immigration of animals from surrounding areas into this population. Krebs et al. (1969) cropped from fenced populations of M .
ochrogaster and M . pennsylvanicus one-third of the adult population
every two weeks and found that these populations still maintained
higher instantaneous rates of population increase than unfenced
control populations. This suggests that a predation rate of approximately 2% per day will not stop the increase of a breeding Microtus
population. A population of M . californicus in a 120 ft2 outdoor pen
had to be cropped at a rate of over 50% a month to maintain a maximum
population of 40 individuals (Houlihan, 1963).
If predation is an important mortality agent in rodent populations,
we should be able to correlate demographic events with predation
pressure. One important aspect of the mortality which occurs during a
microtine decline is that it can be very selective. While two species of
microtines often cycle in phase (Krebs, 1964a; Tast and Kalela, 1971),
sometimes the population decline of one species will precede that of
the other by several months (Krebs et al., 1969; Tast and Kalela, 1971).
Survival of male microtines frequently decreases before that of the
females during the population decline or the mortality on the two
sexes can vary sporadically (Krebs, 1966; Krebs et al., 1969, 1973).
If predators are causing these changes in mortality, they must be highly
selective in their action. We find no support for such selectivity in the
literature. Stendell (unpublished) found that kites took age categories
and sexes in proportion to what was available in the trappable population of Microtus californicus.
To summarize, the role of predation in microtine cycles is limited
to the mortality component of the demographic machinery, and consequently other factors must be invoked to explain reproductive and
growth changes. No one seems to believe that predation can stop a
breeding population in the increase phase, and the major function of
predation is postulated to be in reducing the peak population to low
CHARLES J. KREBS AND JUDITH H. MYERS
Walkowa (1971) removed 32% of the population every month and were
not successful in decreasing the size of the population. They in fact
raised the standing crop. This increase was not due to immigration
but was the result of increased survival of young born into the population and longer residency of mice not removed by “artificial predation”.
House mice have a larger litter size and higher reproductive potential
than microtines, but these experiments show that moderate mortality
does not always cause a decline in a small mammal population.
Krebs (1966) removed all Microtus californicus weighing more than
40 g, in many cases removing more than half the number of animals
trapped on the study area every two weeks, but still was not able to
prevent the population from increasing. However, there was considerable immigration of animals from surrounding areas into this population. Krebs et al. (1969) cropped from fenced populations of M .
ochrogaster and M . pennsylvanicus one-third of the adult population
every two weeks and found that these populations still maintained
higher instantaneous rates of population increase than unfenced
control populations. This suggests that a predation rate of approximately 2% per day will not stop the increase of a breeding Microtus
population. A population of M . californicus in a 120 ft2 outdoor pen
had to be cropped at a rate of over 50% a month to maintain a maximum
population of 40 individuals (Houlihan, 1963).
If predation is an important mortality agent in rodent populations,
we should be able to correlate demographic events with predation
pressure. One important aspect of the mortality which occurs during a
microtine decline is that it can be very selective. While two species of
microtines often cycle in phase (Krebs, 1964a; Tast and Kalela, 1971),
sometimes the population decline of one species will precede that of
the other by several months (Krebs et al., 1969; Tast and Kalela, 1971).
Survival of male microtines frequently decreases before that of the
females during the population decline or the mortality on the two
sexes can vary sporadically (Krebs, 1966; Krebs et al., 1969, 1973).
If predators are causing these changes in mortality, they must be highly
selective in their action. We find no support for such selectivity in the
literature. Stendell (unpublished) found that kites took age categories
and sexes in proportion to what was available in the trappable population of Microtus californicus.
To summarize, the role of predation in microtine cycles is limited
to the mortality component of the demographic machinery, and consequently other factors must be invoked to explain reproductive and
growth changes. No one seems to believe that predation can stop a
breeding population in the increase phase, and the major function of
predation is postulated to be in reducing the peak population to low