Lessons from Travels: Rodent Cycles

by Carl Strang

One of the phenomena of wildlife in the far North is the dramatic cycling of small rodent populations. I had the opportunity to witness this when I was doing my graduate research in western Alaska. I was studying glaucous gulls rather than small mammals, but there was some relevance because the gulls feed heavily on tundra voles early in the season, when the thaw floods the voles into exposed positions.

Tundra vole, enjoying a snack provided by a colleague at the tent frame.

Lemmings were present in small numbers as well, but the only rodent that we saw undergoing violent population fluctuations was the vole. At the low point in the cycle one was hard pressed to find an active runway, and sightings of the voles themselves were few and far between. At the high point the pingos (ice-elevated rounded hills) were riddled with runs.

The voles dug into the soil, chewed a dense maze of runways, and seemed to be everywhere.

I was there in four consecutive summers, and saw one high-density year.

In the peak year they invaded my home.

Now I want to refer back to my recent literature review on food web stability. Species diversity is relatively low in the North, and in general there is a gradient of diminishing diversity from tropics to tundra. Low species diversity is associated with lower stability, and stability clearly is lacking in the vole population. It is not, however, simply a matter of few predators available to exert top-down control. In addition to glaucous gulls there were mew gulls, parasitic and long-tailed jaegers, less common predators like short-eared owls, and foxes. The last were represented by two species.

Red foxes were larger. This one got muddy.

Arctic foxes were the smaller species. This one, which appears to have a tundra vole in its mouth, hasn’t yet molted to its summer pelage.

Furthermore, all of these predators have broad diets and so can switch to focus on the most abundant prey (birds and their eggs being the chief alternative for most of them). Switching, however, isn’t stabilizing the voles. I haven’t followed the literature on this, so I don’t know where the current consensus is, but I think it’s important to point out that for much of the year the voles are protected by a deep layer of snow, and the avian predators all are gone outside the relatively short breeding season. The long winters are depauperate of species indeed, voles can breed in every month, and that surely plays a role in this food web.

While modest cycling of small rodents occurs in Illinois, it doesn’t come close to matching what we saw in Alaska. We have many more kinds of plants, rodents and predators here, and the rodents are vulnerable year round. This seems to be enough to account for the difference in food web stability of the two places.

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