On the train headed north from Amsterdam’s Central Station, be sure to sit to the left. Just past the town of Almere, as you round a right-hand bend, you will find a sight unseen in Europe for centuries, if not millennia: hundreds of red deer, plodding groups of long-horned wild cattle, and skittish herds of low-slung brown horses, all moving through the open landscape like something out of a cave painting. This place goes by the name of Oostvaardersplassen. It is a nature reserve, yes, but it is also a far-reaching experiment. Biologists worldwide are increasingly talking about using large herbivores like the ones sharp-eyed passengers can spot from the train to re-create prehistoric, and sometimes even prehuman, ecosystems.

When keystone species—from ancient mammoths, woolly rhinos, and giant bears to more prosaic grazers like bison, horses, and deer—are wiped out, ecosystems that had sustained themselves in perpetuity collapse. The result is a severe loss of biodiversity. By reintroducing approximations of extinct animals to modern habitats, rewilding advocates want to reestablish dynamic systems that have not existed since the rise of human settlement in Europe. This reserve is the first place where they have done more than talk. Just a short train ride from downtown Amsterdam, nearly 3,000 wild horses, deer, and descendants of prehistoric cattle roam a landscape that is being dramatically shaped by their presence.

The brainchild of a pugnacious Dutch ecologist named Frans Vera, Oostvaardersplassen is challenging some of our most basic assumptions about wildness. Today thick, dense forests are considered synonymous with unspoiled nature. “The current idea is that when you have an area and you do nothing with it, it turns into a forest,” Vera says. Ecologists call this one-way process “succession” and say it rules the unfolding of ecosystems much as natural selection rules evolution. The theory has dominated conservation for centuries, virtually unchallenged.

Until now. Vera says his experiment in rewilding has revealed succession as a human artifact: an unnatural, unbalanced outcome created when people killed off the woolly mammoth and corralled wild horses and cattle. Without free-roaming herds of grazing animals to hold them back, closed-canopy forests took over the land wherever humans did not intervene. The result is a crippled collection of ecosystems that need constant human help to limp along. But Oost­vaardersplassen, some 25 years in the making, stands as a test case of what the wild animals that once roamed Europe might create when left to their own devices.

The existence of a prehistoric wilderness in the middle of one of the most densely populated countries in Europe is remarkable in its own right, but Oostvaardersplassen is much more. By forcing ecologists to rethink traditional ideas of hands-on conservation, which focus on micromanaging and preserving species, it heralds the birth of a new model, one in which natural systems work best when they are left alone.

Calling Oostvaardersplassen a “restored” landscape would be totally wrong. Half a century ago, the fields we are chugging across were underneath a vast inland sea. As part of an engineering project to reduce the risk of floods and reclaim land, Dutch authorities essentially created a new province from nothing. Though the engineering challenges are substantial, the principle is simple: Build a dike to wall off the sea, pump out the water to drain the land behind it, let the soil settle, and build. The reclaimed land, called a polder, was once a shipping route in and out of Amsterdam; Oostvaarders­plassen means “lakes of the ones who sail east.” When it was drained in 1968, this area was slated to be an industrial park.

By a stroke of luck, the Dutch economy in the early 1970s was in the doldrums. The chemical plants planned for the new land never materialized. Instead the drained area sprouted reeds and willows—and attracted birds by the tens of thousands, including endangered species rarely seen in the Netherlands. A coalition of Dutch bird-watchers and nature groups pushed, successfully, to set the area aside as a bird refuge.

Wildlife experts worried that without regular mowing and management, the reed beds, meadows, and marshes supporting such a rich collection of migrating birds would soon give way to bushes and willows. Wait long enough, they predicted, and that growth would in turn give way to dense stands of ash and birch, with the occasional oak managing to push its way through the canopy.

Nature had a surprise in store. In 1978 a few thousand greylag geese landed at Oostvaardersplassen for molting season, the vulnerable spring month when they grow new feathers. The grassy, flat polder was perfect for geese. It had marshy areas for feeding located near open meadows that let geese look out for predators. Within a few years, government experts determined there were an astonishing 60,000 geese molting and breeding at Oostvaardersplassen. They devoured a pound of vegetation a day and stayed for four to six weeks at a time. Everything, from the grass to willow seedlings and reeds, was shorn nearly to the dirt by the ravenous birds.

Vera, then a young biologist working for the forest service, read about the winged invasion and began to wonder if the succession model might have a key weakness. In all the traditional models of unmanaged wilderness, the variable was humans; animals were an afterthought. Take people out, the thinking went, and forests will follow. And since dense forests cannot support many large herbivores, large herbivores could never have been very numerous.

The more Vera considered that model, the less sense it made. If prehistoric Europe was densely forested, how had meadow-loving geese evolved in the first place, without people mowing to keep their habitat open? How had grazing animals thrived in shadowy, thick woods, let alone evolved to prefer grass? “People argue that animals follow succession; they don’t influence it,” Vera says. “But Oostvaardersplassen shows animals steering the succession.”

Vera saw the reserve as an opportunity to test his theory. If geese alone could shape the landscape, what would happen if the animals that inhabited Europe before humans arrived were introduced to the reserve and allowed to graze freely? From within the forest service, he began a campaign to expand the reserve and reroute a planned train track, which would have cut the reserve in half. He won the battle. (“I was committing the two biggest sins in the civil service,” he says now. “I didn’t obey my superiors, and I turned out to be right.”)

The railroad was diverted in 1982, effectively carving out a 15,000-acre wildlife reserve less than 20 miles from Amsterdam. Vera set out to find stand-ins for extinct European grazers like aurochs (ancestral to today’s cows) and wild horses. A year later he introduced 32 Heck cattle, bred by Germans in the 1930s, to approximate the aurochs; a year after that, 20 konik ponies, a Polish-bred version of the wild horses painted on Paleolithic caves, were set free. Forty-four red deer followed in 1992.

Since then the animal populations have exploded. There are now close to 3,000 deer, cattle, and horses living wild in the reserve, which is one of Europe’s largest. The free-roaming herds are not given extra food or shelter during the Dutch winters, which can be cold and long. There are no big predators at the reserve, so more than 20 percent of the large herbivores starve during the winter, numbers that mirror annual deaths at African game reserves.

The decision to let nature take its course initially drew fire from Dutch animal rights activists, who complained that letting horses and cows starve to death was cruel. In a concession to those concerns, rangers now stalk the reserve with high-powered rifles, finishing off animals clearly too weak to survive another week. The carcasses are quickly stripped to the bone by foxes and carrion birds, including the first breeding pair of white-tailed eagles seen in the Netherlands since the Middle Ages.

For Vera it is evidence of a system in balance. The herds have been about the same size for five years, swelling with new calves, foals, and fawns in the spring and shrinking again by winter’s end. When I visit in early May, Hans Breeveld, a wry park ranger with a ruddy beard, takes me for a ride across the polder. The open fields, which are closed to the public, are so closely grazed they remind me of a putting green. “They haven’t been mowed in 12 years,” Breeveld tells me.

As we bounce across the polder, there is constant motion. Flotillas of geese shepherding unruly goslings launch themselves into ponds as we approach. Dozens of cattle stare, then turn and hurry away from the car. The deer are the strangest sight. I’ve seen large groups of cattle before (though usually in stockyards) and small herds of horses at pasture. But I am used to deer as nearly solitary creatures, flitting through the woods in groups of two or three at most. Conventional wisdom holds that three deer per a couple of hundred acres is pushing a forest’s capacity. Oostvaardersplassen’s fields support more than 16 times that many, creating what could be a scene from an old Wild Kingdom special on Africa’s Serengeti: hundreds of red deer bounding in tight herds across the open landscape, turning and running away from Breeveld’s battered green Suzuki 4×4 in unison.

As we drive I borrow Breeveld’s binoculars and stare. Three hours ago I was in central Amsterdam, and now I’m in what looks like a chilly, gray savanna. I ask Breeveld if such huge herds of deer are normal. He looks at me with a slightly mocking smile, as if he is wondering whether I’ve been paying attention for the last few hours. “What is ‘normal’? What’s your reference point? We’ve never let them be in an area this open and large before,” he says.

Oostvaardersplassen is the world’s largest and most advanced exercise in rewilding, but others could soon follow. North America offers some prime settings for another test. Today it is very different from what it was like when humans first arrived some 14,000 years ago. Within a few millennia, the continent lost 59 species weighing more than 100 pounds—from mammoths and horses to lions, saber-toothed tigers, and giant bears.

After decades of focusing on climate as the prime mover in shaping the North American landscape, scientists are increasingly recognizing that animals may have played a major role in shaping their own habitats. Jacquelyn Gill, a University of Wisconsin at Madison paleoecologist, recently used pollen records from an Indiana lake to prove that the disappearance of mammoths and other large herbivores had a major impact on the types of trees that flourished in the region more than 15,000 years ago. Another change: Major wildfires began only after the mammoths were gone, suggesting that the herbivores may have eaten up all of the fire-prone biomass. “We lose so many of our large herbivores, it’s intuitive that the landscape would notice, but the ecological consequences have been largely ignored,” Gill says. “It’s a big question mark as to how much animals were creating and maintaining that habitat.”

Cornell biologist Josh Donlan has proposed running experiments on private land or within nature reserves in the United States to answer that question, using “analogue species” for what he calls Pleistocene rewild­ing. Elephants from zoos would stand in for mammoths and mastodons, and herds of buffalo and wild horses are already on hand to step back into their Pleistocene places. Donlan has proposed creating protected enclaves similar to Oostvaardersplassen, areas where the impact of large herbivore analogues could be studied. He notes that private game-hunting reserves stocked with everything from gazelles to cheetahs already exist in the American West. So far, though, no one has been willing to let him try. “We pointed to Oostvaardersplassen as a model,” he says. “If Vera can do it in the Netherlands, we can certainly do it in the United States.”

At a remote Siberian research station 100 miles south of the Arctic Ocean, Russian biologist Sergey Zimov is already running a similar experiment. He has been monitoring small herds of moose, horses, and reindeer at what he calls Pleistocene Park for the past 20 years. In 2005 Zimov argued in Science that establishing herds of large herbivores in Siberia might one day change the region’s scrubby, swampy tundra back to the grasslands that once stretched from one side of Eurasia to the other. So far, Zimov is seeing landscape changes similar to what is going on at Oostvaardersplassen.

It may take quite a few of those demonstrations to establish the idea that closed-canopy forests, which most people regard as the normal state of nature, may actually be man-made. In fact, those forests are forbidding places for migrating birds. The forest floor is too barren to support large numbers of grazers, and the canopy is too dense to let light-hungry trees like oaks sprout and grow. They are leafy deserts. Yet traditional forest management usually winds up culling deer and bison—not to mention beavers and boar—when their behavior starts to affect trees. “The tragedy is that biodiversity is sacrificed on the altar of the closed-canopy forest,” Vera says. “There’s this crazy idea that no animals should damage trees, as if trees are made by God not to be eaten.”

Vera, Donlan, and Zimov all say that large animals are the keystones of entire ecosystems. Take them out and things begin to fall apart. Setting the system in motion again, whether with the original species or with modern equivalents, is a boon for biodiversity. Many species flourish on the edges between forests and fields. Ironically, suburban America—landscaped with small stands of trees and wide-open lawns—creates a rough approximation of Vera’s mosaic of forest and field. No wonder there is a plague of deer in America’s backyards.

The day after Breeveld takes me on a tour of the reserve, Vera drives over from his home near Utrecht to explain the science behind Oostvaardersplassen. In the cluttered break room of the ranger station, he pours a cup of coffee and pulls out a map to illustrate his plan to expand the reserve via a corridor to a forest 10 miles away, roughly doubling the area the animals will have access to and opening up forested space.

In 2000 Vera’s doctoral thesis was translated and published in English as Grazing Ecology and Forest History. The book made an immediate splash, dividing the ordinarily sedate field of forest ecology into Vera supporters and everyone else. “It’s the nearest I’ve come to being involved in one of those great Victorian debates,” says Keith Kirby, a forestry expert at Natural England, England’s conservation authority. “Vera is really the first person to develop a coherent alternative to the closed-forest idea.”

Grazing Ecology is not your typical biology text. Vera draws on everything from pollen analysis and ecology to medieval woodcuts, etymology, and Latin grammar to prove that we have let shifting perceptions blur what “wild” really means and that, as a result, we are working to conserve an artificial, dysfunctional landscape. In Vera’s vision, the Europe of the past looked more like a city park than an impenetrable thicket of trees. Before humans altered the landscape, it was a mosaic of grasslands and marshes dotted with stands of trees and the occasional isolated oak or lime tree, two species that need ample light to grow.

Presumably, herds of everything from mammoths to deer would have roamed this European savanna, keeping it open by grazing and eating all but the luckiest saplings. Once herds of bison, deer, wild horses, and cattle were wiped out or domesticated, land that was not farmed or managed rapidly turned into thick forest. Over time the dark, menacing woods of fairy tales and Renaissance paintings came to define everything we see as uncivilized and wild.

Vera’s multidisciplinary approach was not well received. One reviewer sniffed at his attempt “to demonstrate not only his ecological competence but also his linguistic interests.” His sharpest critics say that he cherry-picked and misinterpreted his pollen data. Prehistoric pollen taken from the bottom of lakes and peat bogs is considered the best evidence we have for what primeval Europe really looked like, and Vera points out that hazel and oak predominate in the pollen record. Both trees need ample light to regenerate—a strong indicator that the past landscape looked very different.

In one of the few studies to directly address Vera’s claims, paleoecologist Fraser Mitchell of Trinity College in Dublin compared Irish pollen records to those from mainland Europe from around the same time. Even though there is no evidence that large herbivores ever grazed in isolated Ireland, the Irish pollen profile from oak and hazel is essentially the same as the rest of Europe’s. “Ireland is full of both hazel and oak but no large grazing animals,” Mitchell says. “The implication is that grazing animals are not the cause.” Others have chipped away at the edges of Vera’s argument, quibbling with his take on the data or his interpretation of historical documents. And others have conceded that Vera may have some good points, while noting that allowing herds of wild animals to roam free across Europe is impractical.

But no one has directly confronted the heart of his argument: that we have wildly underestimated the impact that animals, especially large ones, had on the environment. Even Vera’s critics say they appreciate the debate he has stirred up, if only because it has made them reexamine their convictions. “It’s paradigm shifting; it challenges everything we used to think,” says Peter Szabo, a Czech Academy of Sciences ecologist who took on Vera’s analysis of medieval records in a recent article. “Most people kind of welcome the idea.”

The hardest part of Vera’s argument to accept is that individual species may come and go, as long as the system stays stable. Oostvaardersplassen suggests that ecosystems are complete only when they need no human help. “Frans has taken more than his fair share of criticism because it is so at odds with some of the conservation philosophy in Europe,” says Kathy Willis, a professor of long-term ecology at Oxford University. “It’s in people’s psychology that they want to manage, and this is very hands-off.”

Rewilding cuts against just about everything conservationists have been taught. Over and over, people intervene to help species in jeopardy, altering the environment piece by piece without looking at the big picture. Vera is willing to let individual species suffer if it means restoring the balanced dynamics of an entire system. That often means going against a host of special-interest groups. “Here in the Netherlands, we still have biological apartheid: There are bird-watchers, tree-watchers, insect-watchers, butterfly-watchers,” he says. “There is no tradition of looking at a site from the point of view of a system.”

Take spoonbills, which thrived in ponds and ditches at Oostvaardersplassen until a dry season a few years ago eliminated their habitat. Old-school conservation might have called for flooding the area temporarily, something bird-watchers did in fact insist on when the spoonbill population crashed. Vera just shrugs. “Most nature conservationists don’t deal well with dynamics,” he says. “New species come when the old ones go. People say you should keep them both, but that’s impossible.” (Some concessions have been made. Vera would very much like to see wild boar at the reserve, but so far, fears that boar would eat the spoonbills have kept them out.)

The larger issue may be that we like our forests dark and leafy—that our image of wild nature is hard to change, even if it is wrong. “Because we have modified the landscape and been the dominant force for so long, the wildlife patterns we value depend on human practices,” Natural England’s Kirby says. “If we want to maintain those particular ones, then we have to maintain these practices.” In other words, the “wilderness” we imagine and crave requires constant, costly maintenance, from fire suppression and deer hunting to protect trees to annual mowing to keep meadows open for birds. When Vera was first maneuvering to establish Oostvaardersplassen, critics told him the idea of such a large reserve was impractical. “They said you can never have such a large nature reserve in the Netherlands because you can’t manage it,” he recalls. “That’s the absurd consequence of the old system.”

After we talk, Vera offers to drive me back to the train station in Lelystad, a city of 70,000 just north of the reserve. On the way we drive along the dike that separates Oostvaardersplassen from the sea. It is a reminder that this re-creation of primeval Europe is taking place on land that has no past. Oostvaardersplassen is several meters below sea level. A massive pumping station works constantly to keep it from flooding. If it were to stop, the reserve’s 15,000 acres would be underwater again within a year.

It may be impossible to settle the debate over what kind of landscape our ancestors encountered when they first walked across prehistoric Europe. But Oostvaardersplassen is proving that, given a little time and autonomy, nature can take care of itself. “If you want openness, you can cut and mow,” Vera says. “Or you can say, ‘If this is a system that worked for hundreds of thousands of years, why not reinstall it?’”

In other words, reboot. Reintroduce large herbivores. Rewild.