In the Owl forum Renno mentioned a three year vole cycle maybe with 2011(I think) being a maximum. I am not sure why it goes in three years -- I think it crashes from from over abundance and then the next year there are lots of predators and not many voles and so the next year there are not so many predators and the voles recover?
i was trying to remember this, too: is it the abundance of food supply for the voles or the predation pressure from the predators that decides the fortunes of voles?
these abundance cycles are known for other animals than voles, too. northern lemmings are a one example of this, and i think the variations in populations of snowshoe hare is a classic example of cyclic variations. i found this in Wikipedia:
"Northern populations of snowshoe hares undergo cycles that range from 7 to 17 years between population peaks. The average time between peaks is approximately 10 years. The period of abundance usually lasts for 2 to 5 years followed by a population decline to lower numbers or local scarcity. Areas of great abundance tend to be scattered. Populations do not peak simultaneously in all areas, although there is a great deal of synchronicity in northern latitudes. From 1931 to 1948 the cycle was synchronized within 1 or 2 years over most of Canada and Alaska, despite differences in predators and food supplies. .... In the southern parts of its range snowshoe hare populations do not fluctuate radically.
Exclosure experiments in Alberta indicated that browsing by snowshoe hares during population peaks has the greatest impact on palatable species, thus further reducing the amount of available foods. In this study there was insufficient nutritious young browse available to sustain the number of snowshoe hares present in the peak years (1971 and 1972) in winter.
hmm... this seems to say that abundance of food or predators doesn't
effect the numbers of snowshoes on the other hand but there is some unknown reason for the variations, and on the other that lack of food reduces the numbers of hares (well that makes sense: if there's no food starvation will inevitably result!!)
the numbers of predators on the other hand depend strongly on the numbers of voles and other prey. we've seen that in Klaara's and Klaus' nest in a tragic way.
a hard winter with lots of snow is good for voles as they can then live under a thick cover of snow in warmth and comfort.