POPULATION CYCLES IN SMALL MAMMALS
345
Whether or not aerial and terrestrial predators compete for the same prey
has not been investigated. In the M . californicw declines studied by
Stendell (1972) and Myers (unpublished) and the M . montanus decline
studied by Fitzgerald (1972), predation, while possibly accentuating
the decline, was not sufficient to cause it. A prerequisite is the cessation
of reproduction and the presence of other mortality factors which as yet
are not identified.
Two important questions remain: (1) what is the predation pressure
during the period of low numbers, and (2) how much loss can a breeding
microtine population sustain? Unfortunately data regarding the first
question are limited. Maher (1970) claims that in Point Barrow,
Alaska the lemming population is free of avian predation for the first
1.75 to 2.75 years after the decline. While no M . californicus were
trapped on the Grizzly Island study area for over 11 months, cat scats
found on the study area during this time still contained vole remains.
Most of the aerial predators left Grizzly Island following the vole
decline, but kites and marsh hawks still hunted in some parts of the
island (Stendell, 1972; Myers, personal observations).
What experiments can we devise to test the suggestion of Pearson
that predation in the phase of low numbers delays the start of the next
cycle? One experiment which should be done is to add predators to a
population to test whether the periodicity of the cycle is lengthened, or
to remove predators to determine if the cycle is shortened. Since the
reduction of cats on the Pearson study area, the M . californicus have
been exhibiting a two-year cycle (Pearson, 1971) and the populations
have shown type H declines with a slight build-up of the population
after the initial decline. Several replicates of this experiment will be
necessary before conclusive results can be obtained. It has previously
been stated that a population of M . californicus on Brooks Island in the
San Francisco Bay, where there are no terrestrial predators, did not
cycle (Pearson, 1966) but Lidicker (1973) has found evidence of a twoyear cycle. However, densities of voles on this island are almost almost
higher than on the mainland. The addition of a terrestrial predator to
this island might be used to test the influence of predation on the
amplitude and timing of a microtine cycle.
How much loss can small mammal populations sustain? No one has
applied the techniques of optimum yield analysis originally developed
for fisheries (Krebs, 1972, Ch. 16) to microtine rodent populations.
We know that at some loss rate, a population must be driven to extinction, and that populations of different species vary enormously in
their ability to withstand sustained cropping. A few experiments have
been done on rodents.
I n a recent study with house mice, M u s musculus, Adamczyk and
345
Whether or not aerial and terrestrial predators compete for the same prey
has not been investigated. In the M . californicw declines studied by
Stendell (1972) and Myers (unpublished) and the M . montanus decline
studied by Fitzgerald (1972), predation, while possibly accentuating
the decline, was not sufficient to cause it. A prerequisite is the cessation
of reproduction and the presence of other mortality factors which as yet
are not identified.
Two important questions remain: (1) what is the predation pressure
during the period of low numbers, and (2) how much loss can a breeding
microtine population sustain? Unfortunately data regarding the first
question are limited. Maher (1970) claims that in Point Barrow,
Alaska the lemming population is free of avian predation for the first
1.75 to 2.75 years after the decline. While no M . californicus were
trapped on the Grizzly Island study area for over 11 months, cat scats
found on the study area during this time still contained vole remains.
Most of the aerial predators left Grizzly Island following the vole
decline, but kites and marsh hawks still hunted in some parts of the
island (Stendell, 1972; Myers, personal observations).
What experiments can we devise to test the suggestion of Pearson
that predation in the phase of low numbers delays the start of the next
cycle? One experiment which should be done is to add predators to a
population to test whether the periodicity of the cycle is lengthened, or
to remove predators to determine if the cycle is shortened. Since the
reduction of cats on the Pearson study area, the M . californicus have
been exhibiting a two-year cycle (Pearson, 1971) and the populations
have shown type H declines with a slight build-up of the population
after the initial decline. Several replicates of this experiment will be
necessary before conclusive results can be obtained. It has previously
been stated that a population of M . californicus on Brooks Island in the
San Francisco Bay, where there are no terrestrial predators, did not
cycle (Pearson, 1966) but Lidicker (1973) has found evidence of a twoyear cycle. However, densities of voles on this island are almost almost
higher than on the mainland. The addition of a terrestrial predator to
this island might be used to test the influence of predation on the
amplitude and timing of a microtine cycle.
How much loss can small mammal populations sustain? No one has
applied the techniques of optimum yield analysis originally developed
for fisheries (Krebs, 1972, Ch. 16) to microtine rodent populations.
We know that at some loss rate, a population must be driven to extinction, and that populations of different species vary enormously in
their ability to withstand sustained cropping. A few experiments have
been done on rodents.
I n a recent study with house mice, M u s musculus, Adamczyk and