POPULATION CYCLES IN SMALL MAMMALS
351
be transmitted indirectly via the food supply or cover available, the
role of weather may be most difficult to untangle in natural populations, even with field experimentation.
Regular fluctuations in populations presumably require a regular
stimulus, and few would claim that weather variables change in a
regular three- to four-year pattern. However, the imagination is pressed
to conceive of any factor other than widespread weather conditions
which might act as the cue for synchronizing fluctuations of microtines
over broad geographic areas. There are numerous accounts of populations of microtines which are out of phase, but synchrony seems to be
the usual case. Chitty (1952, 1960) and Chitty and Chitty (1962)
record asynchronous populations of Microtus agrestis. In southern
Indiana, Krebs et al. (1969) found some sympatric populations of M .
ochrogaster and M . pennsylvanicus which fluctuated in synchrony, but
other asynchronous populations were also monitored (Keller and Krebs,
1970). Pitelka (1961) reports asynchronous brown lemming populations
in northern Alaska, while Watson (1956) and Krebs (1964a) found
sympatric populations of Lemmus and Dicrostonyx to fluctuate in
phase. Both Mullen (1968) and Krebs (1964a) report peak brownlemming
populations in the summer of 1960. Mullen’s study was done in Barrow,
Alaska and Krebs’ in Baker Lake, N.W.T. over 2000 miles away. The
microtines of Finnish Lapland appear generally to fluctuate in synchrony, but cases of asynchrony have also been observed here (Tast and
Kalela, 1971).
For weather to act as a synchronizing factor it must be postulated
that similar weather conditions can have contrary effects on populations
in different phases of the population fluctuation (Frank, 1957). Chitty
(1967, 1969) considers this to be quite possible if the quality of microtine
populations varies with the density. Therefore peak populations might
be severely influenced by a bout of poor weather while an expanding
population would be hardly affected (Fig. 38).
Leslie (1959) proposes a model which describes how an external
random factor (such as weather) acting on populations which are
geographically isolated brings the oscillations of the population
densities into phase. So we have some reassurance that, theoretically,
weather could be a synchronizing element, but we still lack the biological understanding to determine if it is acting in this way.
A factor which has some bearing on this topic is the degree of
geographical isolation of populations. I n the tundra there are vast
areas of suitable lemming habitat. However, in the temperate zone,
Microtus habitat is largely a relic of farming practices. Fields in
different stages of succession provide habitats of changing suitability.
For this reason we might expect more out-of-phase populations in arem
351
be transmitted indirectly via the food supply or cover available, the
role of weather may be most difficult to untangle in natural populations, even with field experimentation.
Regular fluctuations in populations presumably require a regular
stimulus, and few would claim that weather variables change in a
regular three- to four-year pattern. However, the imagination is pressed
to conceive of any factor other than widespread weather conditions
which might act as the cue for synchronizing fluctuations of microtines
over broad geographic areas. There are numerous accounts of populations of microtines which are out of phase, but synchrony seems to be
the usual case. Chitty (1952, 1960) and Chitty and Chitty (1962)
record asynchronous populations of Microtus agrestis. In southern
Indiana, Krebs et al. (1969) found some sympatric populations of M .
ochrogaster and M . pennsylvanicus which fluctuated in synchrony, but
other asynchronous populations were also monitored (Keller and Krebs,
1970). Pitelka (1961) reports asynchronous brown lemming populations
in northern Alaska, while Watson (1956) and Krebs (1964a) found
sympatric populations of Lemmus and Dicrostonyx to fluctuate in
phase. Both Mullen (1968) and Krebs (1964a) report peak brownlemming
populations in the summer of 1960. Mullen’s study was done in Barrow,
Alaska and Krebs’ in Baker Lake, N.W.T. over 2000 miles away. The
microtines of Finnish Lapland appear generally to fluctuate in synchrony, but cases of asynchrony have also been observed here (Tast and
Kalela, 1971).
For weather to act as a synchronizing factor it must be postulated
that similar weather conditions can have contrary effects on populations
in different phases of the population fluctuation (Frank, 1957). Chitty
(1967, 1969) considers this to be quite possible if the quality of microtine
populations varies with the density. Therefore peak populations might
be severely influenced by a bout of poor weather while an expanding
population would be hardly affected (Fig. 38).
Leslie (1959) proposes a model which describes how an external
random factor (such as weather) acting on populations which are
geographically isolated brings the oscillations of the population
densities into phase. So we have some reassurance that, theoretically,
weather could be a synchronizing element, but we still lack the biological understanding to determine if it is acting in this way.
A factor which has some bearing on this topic is the degree of
geographical isolation of populations. I n the tundra there are vast
areas of suitable lemming habitat. However, in the temperate zone,
Microtus habitat is largely a relic of farming practices. Fields in
different stages of succession provide habitats of changing suitability.
For this reason we might expect more out-of-phase populations in arem