Little loggers make a big difference: the tastes of two small rodents—the meadow vole and the white-footed mouse—can determine what trees grow in a forest

Natural History, May, 2002 by Richard S. Ostfeld

A hundred years ago in the northeastern United States, farms covered one-half to three-quarters of what is today forested land. But working the thin, rocky soils of the Northeast was a marginal existence, and after the opening of the Erie Canal in 1825, farming shifted to the more fertile ground of the Ohio River Valley and the Midwest. Since then, northeastern woods have been regrowing--creating new habitats for forest wildlife, satisfying a human yearning for unmediated natural beauty, and possibly even helping to reduce global warming (young trees have enormous potential to remove carbon dioxide, the primary greenhouse gas, from the atmosphere by capturing the carbon in their burgeoning tissues).

But while some tracts of abandoned farmland became overgrown with woody plants in only a few years, others resisted colonization by trees for decades, and scientists eventually began to wonder what accounted for the difference. The wind would have blown in myriad seeds from the maples, pines, ashes, and other trees bordering all of these "old fields"; were quick-growing grasses outcompeting tree seedlings by creating too much shade and appropriating the available water and nitrogen? Two plant ecologists in upstate New York tried to test this supposition in the 1980s by planting seedlings in areas dense with grasses as well as in areas dominated by taller, slower-growing herbs such as goldenrod--only to have some mysterious nocturnal visitors clip the seedlings, killing them and ruining the experiment. A bit of detective work revealed the unexpected culprits: meadow voles (Microtus pennsylvanicus), small herbivorous rodents that are ubiquitous in North American old fields. The ecologists promptly fenced out the pesky saboteurs and resumed their investigation of plant competition.

Voles are common denizens of virtually every habitat (other than dry desert) in the temperate, boreal, and arctic zones of the Northern Hemisphere. Of the roughly 120 species of voles worldwide, about a dozen are notorious for their boom-and-bust population fluctuations; one of these is the meadow vole. But unlike the populations of some of their more famous relatives (more famous to ecologists, at least), whose population fluctuations follow a regular, three-year cycle, some meadow vole populations irrupt sporadically and others almost always stay high or low. Biologists interested in the radical population swings of voles and their close relatives, the lemmings, have focused almost exclusively on why such fluctuations occur rather than on what the wider impact is. Hearing of the devastation of the researchers' experimental tree seedlings by marauding rodents, my plant ecologist colleague Charles Canham and I decided to investigate whether the fluctuations in vole populations might be important in determining when and whether tree seedlings are able to invade old fields: we wondered whether trees can establish themselves only when the voles go bust. If so, it could be that the old fields that resist tree invasion for decades are those in which the vole populations are chronically high, and those that get overgrown quickly are able to do so because their vole populations crash frequently.

Canham and I established nine enclosures in a grassy field on the grounds of the Institute of Ecosystem Studies in Millbrook, New York. We chose a typical northeastern hayfield--habitat that's wonderful for voles but also likely to get rapidly overgrown if left unmown. The enclosures were each about a third of an acre, and wire-mesh fencing more than three feet high kept all but the most intrepid voles from moving between them. For two years we kept track of the populations every other week by livetrapping and marking each individual with a numbered ear tag. Within each enclosure, we kept the vole population at one of three density levels: about 400, 175, and 80 voles per hectare. We chose such distinct levels to match what happens naturally. To maintain these vole densities over the course of the experiment, we had to keep removing individuals from the lower-density enclosures. (We released these animals in another field about a mile away. Some voles, we learned, can navigate their way home from this distance--even wading through streams, crossing paved roads, and scaling fences to get there.)

Inside the enclosures we planted small seedlings of several tree species that commonly colonize old fields in the eastern United States, Quebec, and Ontario, and we monitored their fates for up to a year. In that time, the densest vole populations eliminated about 95 percent of seedlings, whereas medium- and low-density populations killed about 80 and 65 percent, respectively. The voles, it turned out, preferred red maple, white ash, and tree-of-heaven seedlings and turned up their noses at those of white pine and red oak. Such high levels of destruction, combined with such clear food preferences, suggest that these animals control the species composition of regrowing forests.