HOMEWARD BOUND: In the last two years, researchers have transplanted four dozen Retuerta horses from the Doñana National Park in southern Spain to the Campanarios de Azaba Reserve near the Portuguese border, with the hope of not only saving the ancient breed from extinction but also of restoring a long-lost ecosystem on the Iberian Peninsula, where genetically similar horses once roamed.© KRISTEL RICHARD/NATUREPL.COM

Last fall, in the far west of Spain, a small crowd applauded as two dozen dark brown horses trotted gingerly out of a temporary holding pen in the Campanarios de Azaba Reserve—5 square kilometers (about 2 square miles) of rolling grassland sprinkled with mature oak trees. The new arrivals are not just any old nags. They are Retuerta horses, an ancient breed with a close genetic resemblance to the wild horses that roamed Iberia millennia ago.

Transported from Doñana National Park in southern Spain, the...

The Retuertas, like the Sayaguesa cattle that managers released into the reserve in 2012, are, in effect, ecological replacements for their extinct cousins—wild horses and large, wild cattle known as aurochs, both depicted in nearby Paleolithic rock carvings. The animals are among the first additions in a small-scale, open-air experiment designed to test the long-theorized and oft-debated idea of rewilding—that selective reintroductions can revive lost ecological processes and restore damaged ecosystems to a more “natural” state.

The Campanarios de Azaba is a pilot project run by Fundación Naturaleza y Hombre (which roughly translates to the Nature and Man Foundation), a Spanish conservation group, in collaboration with Rewilding Europe, a Netherlands-based initiative that aims to rewild one million hectares (nearly 4,000 square miles or 10,000 square kilometers) over the next 10 years. Plans include the reintroduction of large herbivores, such as red deer, ibex, and bison, into 10 designated areas across the continent, from western Spain to eastern Romania, as part of the group’s attempts to allow natural processes, rather than continued human management, to shape the landscape. “It’s about making Europe a wilder place,” says Frans Schepers, managing director of Rewilding Europe, “a place with more scope for natural ecological processes to restore self-sustaining ecosystems.”

Schepers is not alone in his belief that reintroducing megafauna will reinvigorate collapsing ecosystems. Rewilding advocates argue that while such introductions can never recreate the Pleistocene environment, the only way to restore ecosystems to anything like their former glory is to bring in animals similar to those that shaped them.

As even ardent supporters acknowledge, however, it’s not yet clear how best to approach that challenge, and existing projects have produced precious little data. “Scientists have argued back and forth about this, and to be honest, there wasn’t much science involved; it was more based on intuition and personal opinion,” says Josh Donlan, who runs Advanced Conservation Strategies, a nonprofit consultancy based in Park City, Utah, and Paris, France. “In the meantime, practitioners have embraced this proactive approach, so I think science needs to get on board. That’s the only way we can get the data we need.”

With early rewilding projects now maturing and Rewilding Europe launching new ones across the continent, conservationists and ecologists alike now have an unprecedented opportunity to see whether the introduction of large mammals and a hands-off approach really can forge self-sustaining ecosystems in which biodiversity can flourish. In the process, they hope to generate insights into the dynamics of past ecosystems and to study how researchers and conservationists might recreate something of their splendor now in the Anthropocene, the current geological epoch in which human activities have had an indelible impact on nature.

“Rewilding is happening whether we like it or not, and all of these new projects can be experiments,” says Jens-Christian Svenning, an ecologist at Aarhus University in Denmark. “If we do things properly, with proper experimental design and careful monitoring, we can learn a lot. This is a chance to gain a better understanding of how natural ecosystems actually work.”

A wild idea

Prehistoric cave paintings, such as this one depicting the head and horns of two aurochs found in the Chauvet Cave, France, often depict the ancient species whose ecological roles rewilding advocates now hope to mimic with modern beasts.© JAVIER TRUEBA/MSF/SCIENCE SOURCEThe alpine grassland of the Tarku Mountains in Romania is one site where Rewilding Europe plans to introduce megafauna, such as wild horses, red deer, moose, and European bison.© STAFFAN WIDSTRAND/REWILDING EUROPE WWW.REWILDINGEUROPE.COMAt Yellowstone National Park in the western U.S., wolf reintroductions in the mid-1990s helped restore the area's ecological balance, reducing swelled elk populations and allowing tree populations to recover.NATIONAL PARK SERVICE, YELLOWSTONEWhen searching for an ecological benchmark to which they can aspire, US conservationists have traditionally looked to just prior to the arrival of Christopher Columbus in 1492. But Donlan and colleagues argued in a 2005 Nature commentary that conservationists should instead look to the late Pleistocene, roughly 13,000 years ago, when Eurasians arriving in North America began to wipe out most of the large megafauna that had shaped the continent’s ecosystems for more than 2 million years.1 (Some archaeologists have recently argued that most of the damage was actually done by temperature fluctuations as the Ice Age ended.)

These big animals—mammoths and mastodons, for example—exercised a disproportionate influence on the environment, argued the ecologists, and their disappearance sparked a chain reaction that led to ecosystem collapse and a massive loss of biodiversity. To safeguard the remaining biodiversity and encourage the resurgence of struggling species, Donlan’s team concluded, conservationists should, where and when appropriate, try to restore the ecological and evolutionary potential that was lost in the late Pleistocene.

Although they pointed out that any serious rewilding effort should start small, Donlan and colleagues sketched a daring vision: North American reserves populated with African and Asian elephants, to replace the ancient beasts that once maintained grasslands by suppressing woody plants, and African lions as stand-ins for the extinct American lion that kept the herbivores in check.

Some people were seduced by the idea; others voiced serious concerns. In one of several responses published in ecology journals, Tim Caro, a conservation biologist at the University of California, Davis, argued that the public wouldn’t accept living in close proximity to dangerous animals, and the costs of fencing and management would be astronomical. What’s more, skeptics said, the benefits of such drastic measures are not clear-cut. Researchers have a poor understanding of species composition and abundance in prehistoric ecological communities, meaning there is no clear target for proposed rewilding efforts. Besides, bringing in exotic creatures risks importing diseases or triggering other unforeseen calamities.2

Restoring native species that were recently driven out of a particular area is more palatable, of course, and such efforts are increasingly common. Many of these reintroduction projects are aimed primarily at reestablishing the species itself, but several cases show how they can transform whole ecosystems.

A famous example is the reintroduction of wolves to Yellowstone National Park. After hunters wiped out the predators in the 1920s, elk populations swelled, and the voracious herbivores left fewer trees for beavers to build their dams, which meant fewer desirable habitats for amphibians, birds, and fish. In an attempt to restore ecological balance, researchers transplanted 31 wolves from Canada and Montana between 1995 and 1997. Today, 83 wolves call the park home. As a result, elk populations have declined—3,915 at the last count, down from 16,791 in 1995—and the remaining elk are more timid, munching on young saplings only in areas where they can see wolves coming. As a consequence, populations of aspen, willow, and cottonwood trees have bounced back, providing beavers with an ample supply of wood for dams, which in turn have helped to restore riverine ecosystems in the park.3

“What happened in Yellowstone provides strong support that there are benefits from reintroducing megafauna,” says Donlan. Similar success stories, albeit with somewhat smaller mammals, include the reintroduction of sea otters in southeast Alaska and of Eurasian beavers in western Denmark. But the introduction of exotic substitutes for extinct species has been more controversial and much rarer.

Open-air experiments

European bison returned to the Unteres Odertal National Park in Germany after the resettlement there of wild horses and of cattle back-bred to resemble the ancient auroch.© PATRICK PIEUL/CORBISKonik stallions battle during the breeding season at the Oostvaardersplassen, a 6,000-hectare nature reserve in The Netherlands where this breed of horse was introduced, along with Heck cattle, to replace the extinct species that once roamed the area.© MARK HAMBLIN/WILD WONDERS OF EUROPE/REWILDING EUROPE WWW.REWILDINGEUROPE.COMThe Oostvaardersplassen, a 6,000-hectare (23-square-mile) nature reserve 30 kilometers east of Amsterdam, looks more like an African savannah than like the Dutch countryside: red deer, long-horned cattle, and stocky gray-brown horses plod through the open plains in vast herds as vultures and eagles circle overhead.

In the 1980s, Frans Vera, a government scientist at the Staatsbosbeheer, the government agency responsible for Dutch nature reserves, wanted to revive lost ecological processes on the marshland-like site, which had been reclaimed from a lake in 1968 thanks to a massive drainage project. Vera figured he needed large herbivores that could maintain grassland areas for the vast flocks of greylag geese, which had moved in naturally and were preventing reed beds from becoming swamp forest. With aurochs and wild horses called tarpans extinct, he called in proxies: Konik horses, Polish descendants of wild tarpans, and Heck cattle, bred to resemble the aurochs.

More than 30 years later, most observers agree that from a conservation perspective, the Oostvaardersplassen is a success. The fact that large herbivores roam free in self-regulating populations is itself an achievement, says Vera. Moreover, the presence of grazers, including tens of thousands of greylag geese, continues to prevent reforestation, creating a more diverse habitat that has attracted many bird species, including the rare white-tailed eagle and black vulture. “The proxies have shown they are well capable of creating and preserving grasslands,” says Vera, “and that has proved to be important for other species.”

Vera also thinks that the Dutch experiment can yield insights about the flora and fauna in preagricultural Europe, or at least how one particular ecosystem works when left to its own devices. Contrary to the prevailing idea that the continent was covered in thick forest before humans killed off the megafauna, Vera argues that Europe’s natural state was a mosaic of open grassland, thorny shrubs, and woodland groves. The dense forests that many Europeans associate with untouched wilderness, he says, may have only established themselves once most of the large Pleistocene herbivores exited.4 If that’s true, adds Vera, the continuing veneration of closed-canopy forests is at odds with aims to revive lost biodiversity.

It’s about making Europe a wilder place, a place with more scope for natural ecological processes to restore self-sustaining ecosystems.—­Frans Schepers, Rewilding Europe

Earlier this year, Aarhus University’s Svenning and colleagues published a study that lends support to Vera’s ideas. Analyzing paleoecological beetle data from Great Britain, they found indications of large-herbivore abundance and a wood-pasture mixture in the last interglacial period between 132,000 and 110,000 years ago, long before humans arrived. However, the data for the much more recent Holocene period (12,000 to 5,000 years ago) indicated more closed-canopy forest and fewer herbivores.5

Meanwhile, the Oostvardersplassen experiment has not generated any conclusive evidence. “[It] tells us that wild-living cattle and horses, together with deer, have the capacity to build dense populations with strong ecological impacts,” says Svenning. But the scientific value of the project is limited by the absence of large predators, he adds. “Oostvaardersplassen is one specific setting, and there are no large carnivores, so we cannot tell what kind of vegetation cover [existed in preagricultural Europe], nor can we generalize about the numbers of large herbivores.”

Similar rewilding projects are ongoing in western Siberia—where Yakutian horses, moose, and musk oxen have been brought in to compensate for the absence of mammoths and woolly rhinoceroses—and in Saudi Arabia, where the African red-necked ostrich is used as a proxy for an extinct Arabian subspecies. Again, though, large predators are absent from both field sites. And in all cases there is a common theme: the projects are generating precious few peer-reviewed publications, leaving researchers wondering about the ecological impacts of such initiatives.

“Publishing scientific papers is not a priority for conservationists, which is understandable, but the lack of information makes it hard to learn lessons that could guide future efforts,” says William Sutherland, a conservation biologist at the University of Cambridge who created Conservation Evidence, a search engine designed to support decisions about how to maintain and restore biodiversity. “It means that conservationists are working not completely in the dark, but certainly in the twilight.”

Starting small with giants

Only a couple of taxon substitution projects have produced convincing peer-reviewed evidence that introducing exotic animals as proxies for extinct species can have a positive impact on ecosystems. In 2000, a local conservation group introduced captive-bred Aldabra giant tortoises, endemic to Aldabra Atoll in the Seychelles, to Île Aux Aigrettes, a 25-hectare (less than 0.1-square-mile) island just off the coast of the island nation of Mauritius. The hypothesis was that the tortoises would disperse the seeds of the endemic ebony tree, which had been devastated by extensive logging in the 1980s, roughly 130 years after European sailors had hunted the native giant tortoises to extinction. Upon their arrival, the Aldabra giant tortoises didn’t skip a beat: despite having never seen them before, the tortoises gobbled up ebony fruits with gusto and dispersed the seeds far and wide across the island.

As it turns out, seeds that pass through the tortoises also germinate better than those that were manually scooped from the fruit. Tortoise-dispersed seeds resulted in dense clumps of ebony seedlings in some of the most heavily logged parts of the island.6 “Ebony trees are very slow-growing so it remains to be seen how well they’ll do, but we’ve shown that it’s possible to revive extinct ecological interactions with substitute species,” says Christine Griffiths, an ecologist affiliated with the University of Bristol, UK, who ran the project with the Mauritian Wildlife Foundation. “That should make this ecosystem more resilient.”

Griffiths achieved a similar feat on Round Island, off the northeast coast of Mauritius, where, in 2007, she and her colleagues deployed Aldabras and another giant tortoise species from Madagascar to reprise the vegetation-controlling role of extinct native tortoises. The team reported last year that the tortoises are devouring exotic plants—fast-growing invaders including Spanish needle, crowsfoot grass, and passion flower—and, for the most part, ignoring the struggling native flora.7 And it’s shaping up to be a similar story on the Hawaiian island of Kauai, where David Burney, who manages the Makauwahi Cave Reserve, has released giant tortoises to stand in for moa-nalo, giant flightless ducks that were exterminated when the Polynesians arrived. As Burney predicted, the tortoises are munching on invasive weeds, leaving the native plants, which evolved resistance to the beaks of moa-nalo, to bounce back.

Examples like this offer hope that introducing exotic substitutes into vacated niches can help to protect native species, says Griffiths, but she adds that these are tiny uninhabited islands with simple ecosystems. Attempting to introduce large herbivores, let alone apex predators, into complex ecological networks will undoubtedly be far more difficult. And like the wolves of Yellowstone, the native tortoises of these Indian Ocean islands were wiped out fairly recently, so the researchers are not aiming to restore ancient ecosystems.

Donlan agrees that these small-scale studies cannot predict the outcomes of more complex reintroduction projects. Nevertheless, he says, they can provide inspiration and a model to be scaled up. “We need to develop a framework that we can apply for different scenarios,” he says. “Say we want to rewild this or that landscape, what steps do we take? The only way we’re going to find out is to try.”

Pragmatism, not Pleistocene

Despite the uncertainties—and, in fact, because of them—Schepers is confident that Rewilding Europe’s plans are worthwhile. With more than €2.5 million (US $3.4 million) in funding generated by the Dutch Postcode Lottery, the group plans to introduce wild horses, red deer, ibex, chamois, European bison, and other species to suitable areas in six of ten designated regions over the next five years. These include the Central Apennines in northern Italy, the Danube delta in eastern Romania and the Ukraine, the Velebit mountain range in Croatia, and parts of the Carpathian Mountains, a range that stretches in a great arc from Poland to Serbia.

Rewilding Europe even wants to use animals that aren’t around today. Not satisfied with Heck cattle as a replacement for the aurochs—the bigger, burlier ancestor of domestic cattle that roamed Europe for thousands of years before the last one died in 1627—Ronald Goderie and colleagues at the Netherlands-based Taurus Foundation are working with Rewilding Europe to back-breed today’s cows in an attempt to create a closer approximation of the original.

While the Rewilding Europe initiative aims to make the continent a wilder place and restore ancient ecological processes, it also wants to provide a sustainable land-management solution as humans desert marginal agricultural lands for urban centers. In western Spain, for example, Schepers hopes that introducing the Retuerta horses and wild cattle will help safeguard the future of species already living there and encourage others, such as the critically endangered Iberian lynx, to reestablish themselves. The idea is that natural grazing will initiate a chain of ecological events that alter local ecosystem dynamics, encouraging the return of species on which lynx prey. In addition, Schepers and his colleagues expect resurgent wildlife to stimulate an influx of tourists that could boost local economies.

Schepers and his colleagues were encouraged by a 2012 paper from ecologists at the German Center for Integrative Biodiversity Research (iDiv) at the University of Leipzig in Germany that makes the case for rewilding as an attractive land-management policy for Europe. With another 10 million acres of agricultural land set to be abandoned between now and 2030, the authors argue that rewilding—in the broadest sense of promoting natural ecological processes—is an approach that, after initial intervention, requires minimal management and should encourage biodiversity and bolster ecosystem processes, such as soil protection, water-cycle regulation, and carbon sequestration.8 “In terms of providing these sorts of services, rewilding is as good, or better, than current management practices,” says coauthor Henrique Pereira of iDiv, who joined the supervisory board of Rewilding Europe last year.

Rewilding is happening whether we like it or not. If we do things properly, we can learn a lot.—­Jens-Christian Svenning,
Aarhus University

Rewilding Europe’s designated areas also represent open-air experiments in which ecologists can answer questions about the impact of wild grazers, the nature of preagricultural European ecosystems, and the question of how large, wild herbivores affect biodiversity. Teaming up with ecologists at several European universities, Rewilding Europe also hopes to facilitate research on the ecological role of carcasses and dung, both of which fuel food webs in ways that are not well understood, particularly at the level of insects, fungi, lichens, and mosses. Finally, researchers hope to address questions surrounding human–megafauna interactions, and how best to manage inevitable conflicts.

“From our point of view we have to be careful that the scientific community does not become prescriptive,” Schepers says. “But we recognize that scientists should have a role in monitoring the impact of reintroductions and conducting research around what we’re doing.”

Svenning agrees. He is currently working with the Randers Tropical Zoo in Denmark to release Asian elephants, which are closely related to the straight-tusked elephants that once roamed Europe, onto a small patch of land on the outskirts of Randers, and he intends to provide a detailed assessment of the animals’ ecological impact. “Most people will think it’s outrageous, but the scientific reasons for trying are clear,” he says. “As long as we start small and monitor carefully, it makes sense to at least try.” 

Daniel Cossins, a former associate editor of The Scientist, is a freelance writer living in Philadelphia, Pennsylvania.


  1. J. Donlan et al., “Re-wilding North America,” Nature, 436:913-14, 2005.
  2. T. Caro, “The Pleistocene re-wilding gambit,” Trends Ecol Evol, 22:281-83, 2007.
  3. W.J. Ripple, R.L Beschta, “Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction,” Biol Conser, 145:205-13, 2012.
  4. F.W.M. Vera, “Grazing Ecology and Forest History,” CABI Publishing, 2000.
  5. C.J. Sandom et al., “High herbivore density associated with vegetation diversity in interglacial ecosystems,” PNAS, doi:10.1073/pnas.1311014111, 2014.
  6. C.J. Griffiths et al., “Resurrecting extinct interactions with extant substitutes,” Curr Biol, 21:762-65, 2011.
  7. C.J. Griffiths et al., “Assessing the potential to restore historic grazing ecosystems with tortoise ecological replacements,” Conserv Biol, 27:690-700, 2013.
  8. L.M. Navarro, H.M. Pereira, “Rewilding abandoned landscapes in Europe,” Ecosystems, 15:900-12, 2012.

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