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A Major Climate Force Has Been Ignored for Decades

Small mammals play an outsize role in shaping the world around them.By Bathsheba Demuth for The Atlantic, published January 4, 2024

Finding a vole on Alaska’s North Slope takes practice. The open plain pulls the eye upward, toward grand things: the horizon line, the distant shimmer of snow in the mountains. The nearest tree is more than 50 miles away. The low shrubs and sedges toss and wave in the wind. It’s a place where a 600-pound musk ox can look dog-size.

In this landscape, even a very large vole—weighing less than three ounces and no more than nine inches long—is easy to miss. But Nick Patel knows what to look for. Last August, Patel pointed my attention toward a depression worn into the moss, a path that disappeared into a yellowed tuft of sedge. Voles are creatures of habit, scurrying so often over the same route that they wear trails—runways—into the soil. Once you know to look for them, the tundra is laced through with vole runways.

Patel is a field tech with Team Vole, a group of some 20 researchers studying Alaska’s voles and lemmings. Despite their size, these creatures are a force on the tundra. Caribou migrate. So do the geese, ducks, swans, and sandhill cranes that come north by the hundreds of thousands each summer. But voles and lemmings stay put. Unlike many Arctic animals, they don’t hibernate. And as Team Vole is finding, this means that these small mammals—which live throughout the circumpolar north—fundamentally shape the ecosystem around them. In their tiny paws rests a crucial part of the climate’s future: whether the world’s tundra will help pull carbon from the atmosphere, or instead emit more.

Once Patel showed me how to spot voles, I couldn’t stop seeing signs of their work. One day at Toolik Field Station, a science hub 370 miles north of Fairbanks, I watched a pair of them emerge from a knot of grassy fronds. They paused and stood, delicately fingered paws hanging over buff bellies. “Those two are always together,” Audrey Fatone, a field tech with Team Vole, told me. “Although we can’t tell them apart exactly.”

Fatone and Patel were checking on an experiment. On a gentle hillside near Toolik sit three unobtrusive waist-high wire-mesh pens. One pen excludes all voles. The second previously held a large vole population, but now has only a few. The third—in which the duo was now speeding through grasses, mosses, stunted blueberry bushes, and the dozens of other plants that make up the tundra—was stocked with an exorbitant number of voles, caught with live traps on the surrounding hillsides.

The pens attempt to mimic, in 20-meter squares, a curious fact about small Arctic mammals: Their populations change dramatically over time. Lemmings and voles both pulse and crash in three- to five-year cycles. In Utqiaġvik, a community 250 miles northwest of Toolik, Iñupiat Elders remember years so thick with lemmings that people had to actively avoid stepping on them. In other years, Team Vole barely sees a single animal.

The pen with the multitude of voles simulates a boom year. Even at a glance, the tundra inside the pen was transformed: the sedges pruned, the moss trampled, the blueberries nibbled. Here and there along their runways, the voles have piled sedge clippings six or eight inches high; the conical heaps provide food and shelter through the winter. One runway dead-ends in a trampled oval, vole droppings mounded in the middle. The overall effect is a kind of ramshackle coherence. Look close enough, and the tundra suddenly appears built. And not just on a small scale: Scandinavian researchers have tracked Arctic mammals’ transformation of the landscape in satellite images.

All of that construction alters the way that nutrients cycle through the ecosystem, which changes the tundra’s relationship to carbon. Voles cut plants when they’re green and nutrient-rich, so their hay piles are full of nitrogen and phosphorus that the plants would otherwise pull into their roots at the end of the growing season. Hay piles and latrines are basically tiny fertilizer depots. In boom years, they lace the soil with nutrients, allowing microbes to flourish. As the microbes digest, they respire the carbon stored in dead leaves and stems into the atmosphere. A reduced canopy of plants means there are fewer leaves to convert atmospheric carbon into tissue through photosynthesis. It might further boost decomposition by giving soils a hit of sun. In aggregate, Team Vole believes, a high vole year could make the tundra breathe out carbon.

Then the population crashes. No longer pruned by rodent teeth, plants regrow on soils still enriched with nitrogen and phosphorus. Each blade of grass and leaf fixes carbon from the atmosphere into tissue. Decomposition slows. Now, Team Vole researchers told me, the tundra might begin to breathe in carbon.

Before our current era of rapid warming, vole booms and busts helped make the tundra an overall carbon sink, Austin Roy, who worked with Team Vole while a graduate student at the University of Texas at El Paso, told me. That’s in part because boom years set plants up to flourish in bust years, and Arctic cold prevents new leaves and grasses from rotting when they die back in the autumn. Instead, they become part of the permafrost: a layer of ice, mud, and plant matter that, on the North Slope, can be more than 1,000 feet deep.

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