POPULATION CYCLES IN SMALL MAMMALS
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3. The periodicity is variable, three to four years is typical, but
4. Some populations may not fluctuate, but none of this type has
some cycles may be two or five years in length.
been stddied carefully.
5. Fluctuations occur in a variety of genera and species from arctic
to temperate areas, from Mediterranean to continental climates,
from snowy areas to snow-free areas.
6. Fluctuations sometimes occur in synchrony ovkr large geographical areas (thousands of square miles). Synchrony is
seldom absolute, however, and local out-of-phase populations
occur. Synchrony is not continental or world-wide.
7. The amplitude of these fluctuations is not necessarily larger
in more northern populations.
8. Populations living in a wide variety of plant communities in a
small geographic area all fluctuate in the same way, often in
phase.
9. Reproductive#rate is highest in the increase phase, owing to (1)
longer breeding season, including winter breeding in some
species, and (2) lower age at sexual maturity. In the peak and
decline phases reproductive rate is reduced.
10. There is no systematic difference between increasing populations
and declining populations in (1) litter size, (2) percentage of
adult females pregnant during the breeding season, or (3) sex
ratios.
11. Mortality rates in all sex and age groups are lowest in the increase
phase.
and high in the decline phase.
13. Juvenile mortality may be high in the peak summer and may
remain high until the end of the peak breeding season, when
it is suddenly reduced. Juvenile losses are very high in the
decline phase.
14. Survival of adult males fluctuates independently of that of
adult females, when viewed on a weekly time scale. Males may
suffer heavy losses in the decline for a few weeks when females
are surviving very well, and vice versa.
12. Adult and subadult mortality rates are low during the peak,
387
3. The periodicity is variable, three to four years is typical, but
4. Some populations may not fluctuate, but none of this type has
some cycles may be two or five years in length.
been stddied carefully.
5. Fluctuations occur in a variety of genera and species from arctic
to temperate areas, from Mediterranean to continental climates,
from snowy areas to snow-free areas.
6. Fluctuations sometimes occur in synchrony ovkr large geographical areas (thousands of square miles). Synchrony is
seldom absolute, however, and local out-of-phase populations
occur. Synchrony is not continental or world-wide.
7. The amplitude of these fluctuations is not necessarily larger
in more northern populations.
8. Populations living in a wide variety of plant communities in a
small geographic area all fluctuate in the same way, often in
phase.
9. Reproductive#rate is highest in the increase phase, owing to (1)
longer breeding season, including winter breeding in some
species, and (2) lower age at sexual maturity. In the peak and
decline phases reproductive rate is reduced.
10. There is no systematic difference between increasing populations
and declining populations in (1) litter size, (2) percentage of
adult females pregnant during the breeding season, or (3) sex
ratios.
11. Mortality rates in all sex and age groups are lowest in the increase
phase.
and high in the decline phase.
13. Juvenile mortality may be high in the peak summer and may
remain high until the end of the peak breeding season, when
it is suddenly reduced. Juvenile losses are very high in the
decline phase.
14. Survival of adult males fluctuates independently of that of
adult females, when viewed on a weekly time scale. Males may
suffer heavy losses in the decline for a few weeks when females
are surviving very well, and vice versa.
12. Adult and subadult mortality rates are low during the peak,