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Abstract
Microtine rodents are known to show extreme population variations (cycles) but non-cyclic
populations have also been recognized during recent years. The cyclic populations
have been widely thought to be regulated by intrinsic mechanisms. However, such predictions
for cyclic populations are usually not applicable to non-cyclic ones and extrinsic
factors may have to be included in any explanation.A hypothesis that the degree of
fluctuations in small rodent numbers is related to the sustainable number of generalist
predators was tested on mainly literature data by computing "indices of cyclicity"
for local populations. These indices were related to latitude and snow cover (two
measures) as these variables will affect the amount of alternative prey available
for these generalists. Within Fennoscandia such indices for Clethrionomys glareolus
and Microtus agrestis were clearly positively related to latitude and snow cover.
The fraction of populations with summer declines in numbers, characterizing highly
cyclic populations, increased in the same way. Cyclicity indices in Great Britain
were similar to those in southern Fennoscandia, both areas being poor in snow, but
were higher at the same latitudes in eastern Europe with more snow. Indices of density
variations were generally low in North American Clethrionomys species and very variable
in Microtus species.The gradients observed and differences between continents are
interpreted as due to microtine-vegetation interactions in northern European areas
poor in generalist predators but with important small mustelid predation, and to similar
snowshoe hare-vegetation interactions in mainly Canada-Alaska, where small rodents
may serve as alternative prey for numerically fluctuating hare predators, at least
in the forests. Western European microtine populations, and probably many others,
seem to be regulated by generalist predators.