Wild Side: Low on the food chain

But meadow voles are mighty in ecological terms.

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Somewhere between 20 and 100 voles may inhabit an acre of good habitat. — Wikimedia Commons

A high percentage of the meadow voles that I see are in a difficult situation indeed: dangling from the talons of a red-tailed hawk or a northern harrier. Small rodents about the size and shape of an Italian sausage, only sporting a short tail, voles are abundant, and they’re important prey for a wide range of predators. But like so many other common, important animals, voles typically go undetected unless you’re actively looking for them.
If you do go vole-hunting, take your cue from the experts. Hawks and harriers spend a good portion of their time working open grasslands — anything from native bluestem meadows to hay fields. Harriers typically cruise low over these habitats, ready to drop down onto anything small that moves. The red-tails are more likely to perch-hunt, parked nonchalantly on a post or snag, but also ready to swoop. In either case, these birds have zeroed in on the preferred habitat for meadow voles. There rodents prefer settings dominated by dense grasses and forbs, which may be either wet or dry, but have as little woody vegetation as possible.
Lacking the phenomenal eyesight of birds of prey, humans have much more trouble finding voles. But it’s often easy to spot signs of their activity. As they scuttle about on their personal business of feeding and mating, voles tend to use the same paths over and over. After a while, these paths are readily visible, tunnels about an inch in diameter running under pressed-down grass or through leaf litter. Vole trails are also easily visible after a light snowfall, or after a heavier snowfall has nearly melted. These rodents remain at least somewhat active year-round, trundling through or under the snow, and once again leaving their characteristic tubes.
These tunnels can be plentiful, reflecting the abundance of these restless little animals. Typically, researchers have found, somewhere between 20 and 100 voles may inhabit an acre of good habitat. But this species undergoes periodic explosions in population, which are not fully understood by biologists, and prompt behavioral changes — increased aggression and an understandable tendency to wander in search of less crowded lodging — in the voles themselves. At the peak of such bursts of abundance, scientists have documented vole densities greater than 1,400 per acre. That is a lot of rodent.
That voles can achieve such abundance, or even that they can maintain their much lower normal abundance, in the face of ongoing predation by hawks, cats, crows, snakes, and other predators, speaks volumes about the irrepressible randiness of these little mammals. They basically live to reproduce, reaching sexual maturity just a few weeks after birth; females then crank out successive broods averaging six, and if you grasp the notion of exponential growth, you can see how quickly that can add up. If it weren’t for predators, we’d be buried in voles.
But it’s a hard life low on the food chain, and voles, which eat mainly vegetable food, are on a low rung indeed. (You might say that in ecological terms, one of their main functions is to convert vegetable matter into a form that predators can catch and eat.) Mortality of newborns due to starvation or predation is high (88 percent, according to one study), and few adults survive more than a year or so. In addition to seeds, leaves, and stems, voles feed occasionally on small invertebrates, and this species is not above a little recreational cannibalism when population densities get high. Given their abundance and broad diet, it’s not surprising to find that voles can sometimes be agricultural pests, eating enough to do significant damage to crops or to the bark of young fruit trees.
In purely ecological terms, vole feeding exerts strong pressure on the plant mix in vole habitat, significantly lowering the abundance of their preferred food plants. Meanwhile, vole droppings are an important means of seed dispersal for some plants. Between these roles and their function as food for predators, voles turn out to be extremely important animals, shaping the ecology of their preferred habitat. Voles are also hosts (although not very efficient ones, it appears) for some of the well-known tick-borne diseases, including Lyme and babesiosis. These animals may weigh only an ounce and a half, but their numbers multiply the impact of each individual vole.
The meadow vole is a reminder of how much important stuff goes on in the natural world that humans don’t notice. Small animals, largely nocturnal, and usually concealed under matted-down vegetation, these mammals have every incentive to lead lives of secrecy: Discretion is their only defense against ending up in the talons of a hawk or owl. But they’re out there, sometimes in huge numbers, and the natural world we’re familiar with is largely the result of the ceaseless workings of thousands of these humble animals.