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CHARLES J. KREBS AND JUDJTH H. MYERS
rates. I n order to identify this potential type of disperser it would be
necessary to catch every animal leaving a population. This could be
done by monitoring egress from a semi-enclosed population. No one
has done this yet.
Immigration is more difficult to measure than emigration because of
the difficulties of live-trapping voles and lemmings. It is impossible to
know that you have trapped and removed every single individual from
an area. Thus, new individuals which appear on a live-trapping area
may have been the offspring of females which had avoided being
trapped. On the other hand, they may have moved in from adjacent
areas. Some method of radioactive marking of pregnant females might
be used to get at this problem. If it were possible to label radioactively
all young being produced in surrounding areas, dispersers from these
areas could be positively identified. Genetic markers might be used in a,
similar way. What we would like to determine is the exact source
area of each immigrant. So far nothing has been done along these lines.
We expect that the results of measuring immigration would not be the
converse of those for measuring emigration. Vole populations should be
closed to most immigrants, at least after the increase phase is over, so
that emigrants from one area will not usually be able to colonize an
adjacent area, except if population densities are low or if it is a marginal
habitat or is otherwise vacant for historical reasons. This discussion
leads to another suggestion for studying dispersal: artificial immigration
could be used to measure the ability of a population to absorb immigrants at different phases of the population cycle.
In summary, dispersal is the least studied process in the population
equation for voles and lemmings. Studies on enclosed populations
indicate that numbers reach abnormally high levels when dispersal is
stopped. Almost no one has attempted to measure dispersal rates over a
population cycle. A single study showed highest dispersal during the
increase phase and almost no dispersal during the population decline.
Dispersal may change a population qualitatively as well m quantitatively. This idea will be elaborated in a later section discussing the
possible role of genetic changes in causing microtine cycles.
D. G R O W T H
The growth of individual animals in cyclic populations is important
because it is tied to the primary processes of birth, death and dispersal.
The size at sexual maturity is the most direct linkage between individual
growth and the reproductive rate of a population.
Growth can be measured in many ways but the simplest measurements are changes in weight or length. Length is a good measure of
CHARLES J. KREBS AND JUDJTH H. MYERS
rates. I n order to identify this potential type of disperser it would be
necessary to catch every animal leaving a population. This could be
done by monitoring egress from a semi-enclosed population. No one
has done this yet.
Immigration is more difficult to measure than emigration because of
the difficulties of live-trapping voles and lemmings. It is impossible to
know that you have trapped and removed every single individual from
an area. Thus, new individuals which appear on a live-trapping area
may have been the offspring of females which had avoided being
trapped. On the other hand, they may have moved in from adjacent
areas. Some method of radioactive marking of pregnant females might
be used to get at this problem. If it were possible to label radioactively
all young being produced in surrounding areas, dispersers from these
areas could be positively identified. Genetic markers might be used in a,
similar way. What we would like to determine is the exact source
area of each immigrant. So far nothing has been done along these lines.
We expect that the results of measuring immigration would not be the
converse of those for measuring emigration. Vole populations should be
closed to most immigrants, at least after the increase phase is over, so
that emigrants from one area will not usually be able to colonize an
adjacent area, except if population densities are low or if it is a marginal
habitat or is otherwise vacant for historical reasons. This discussion
leads to another suggestion for studying dispersal: artificial immigration
could be used to measure the ability of a population to absorb immigrants at different phases of the population cycle.
In summary, dispersal is the least studied process in the population
equation for voles and lemmings. Studies on enclosed populations
indicate that numbers reach abnormally high levels when dispersal is
stopped. Almost no one has attempted to measure dispersal rates over a
population cycle. A single study showed highest dispersal during the
increase phase and almost no dispersal during the population decline.
Dispersal may change a population qualitatively as well m quantitatively. This idea will be elaborated in a later section discussing the
possible role of genetic changes in causing microtine cycles.
D. G R O W T H
The growth of individual animals in cyclic populations is important
because it is tied to the primary processes of birth, death and dispersal.
The size at sexual maturity is the most direct linkage between individual
growth and the reproductive rate of a population.
Growth can be measured in many ways but the simplest measurements are changes in weight or length. Length is a good measure of