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CHARLES J. KREBS AND JUDITH H. MYERS
however. Litter size does not change over the cycle, except in some
species in which it is lower in the peak year. Females in declining
populations have normal litter sizes. The percentage of females pregnant
during the breeding season also seems to be independent of the population cycle. Length of breeding season is highly variable. Winter breeding
and extended summer breeding seasons occur during the increase
phase of many species. The peak year often has a shortened summer
breeding season, and the decline phase may also have a restricted
breeding season. Age at sexual maturity is the second component of
reproduction to change during a population cycle. Animals in peak
populations reach maturity at older ages and heavier weights, and
young voles and lemmings may not mature at all in their fist summer
if born into a peak population. Age a t maturity may also be delayed in
declining populations. Finally, sex ratios do not seem to vary in any
systematic way over the cycle in numbers.
B. MORTALITY
The reproductive changes discussed in the previous section are
sufficient to generate a population cycle even if the mortality schedule
were constant and independent of density. We here investigate the
mortality schedules of voles and lemmings and attempt to see if there
are patterns of change in mortality which reinforce or cancel the changes
in reproduction.
Mark-and-recapture work is necessary for the estimation of mortality
rates, and relatively little of this has been done on microtine populations
throughout a cycle in numbers. Sampling problems, discussed above
for the estimation of population size, plague estimation of mortality
rates. Marked animals may not respond to traps in the same way.
Juvenile animals are hard to catch in live traps. Animals may move
off the trapping area and since disappearance is equated to death,
measured mortality rates are really “loss rates”.
Mortality rates are very labile in lemming and vole populations.
Mortality varies with age but, given the accuracy of present methods,
we recognize only three age categories: adult, juvenile and nestlings.
We will now discuss each of these three age groups and then discuss
prenatal mortality.
1. Adult mortality
Most of the available data on mortality rates comes from the live
trapping of adult animals. Chitty (1952) estimated mortality rates in
peak and declining populations of Microtus agrestis. Figure 18 shows
some of the earliest data on survival rates in a declining population.
CHARLES J. KREBS AND JUDITH H. MYERS
however. Litter size does not change over the cycle, except in some
species in which it is lower in the peak year. Females in declining
populations have normal litter sizes. The percentage of females pregnant
during the breeding season also seems to be independent of the population cycle. Length of breeding season is highly variable. Winter breeding
and extended summer breeding seasons occur during the increase
phase of many species. The peak year often has a shortened summer
breeding season, and the decline phase may also have a restricted
breeding season. Age at sexual maturity is the second component of
reproduction to change during a population cycle. Animals in peak
populations reach maturity at older ages and heavier weights, and
young voles and lemmings may not mature at all in their fist summer
if born into a peak population. Age a t maturity may also be delayed in
declining populations. Finally, sex ratios do not seem to vary in any
systematic way over the cycle in numbers.
B. MORTALITY
The reproductive changes discussed in the previous section are
sufficient to generate a population cycle even if the mortality schedule
were constant and independent of density. We here investigate the
mortality schedules of voles and lemmings and attempt to see if there
are patterns of change in mortality which reinforce or cancel the changes
in reproduction.
Mark-and-recapture work is necessary for the estimation of mortality
rates, and relatively little of this has been done on microtine populations
throughout a cycle in numbers. Sampling problems, discussed above
for the estimation of population size, plague estimation of mortality
rates. Marked animals may not respond to traps in the same way.
Juvenile animals are hard to catch in live traps. Animals may move
off the trapping area and since disappearance is equated to death,
measured mortality rates are really “loss rates”.
Mortality rates are very labile in lemming and vole populations.
Mortality varies with age but, given the accuracy of present methods,
we recognize only three age categories: adult, juvenile and nestlings.
We will now discuss each of these three age groups and then discuss
prenatal mortality.
1. Adult mortality
Most of the available data on mortality rates comes from the live
trapping of adult animals. Chitty (1952) estimated mortality rates in
peak and declining populations of Microtus agrestis. Figure 18 shows
some of the earliest data on survival rates in a declining population.