“IT WAS a picture postcard of how the English countryside is meant to look,” Isabella Tree tells me. “It was a working farm. We had green fields, manicured hedgerows and ditches, land that was constantly active with maize, barley, rye and grazing cattle. We didn’t realise it at the time, but it was virtually a biological desert. Now it looks much more like Africa.”
She’s talking about her home, the in West Sussex. Seventeen years ago, she and her husband Charlie Burrell stopped trying to coax a living out of its heavy soil. Today, the 1400-hectare estate is the closest thing in southern England to a primaeval landscape: a mosaic of water meadows, thorny scrub, sallow groves and grazing lawns roamed by cattle, ponies, pigs and deer. “The colliding of different habitats has been rocket fuel for biodiversity,” says Tree.
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Knepp is an experiment in “rewilding”, a movement that has swept the Western world in recent years. It takes different forms in different places, but a simple and compelling concept drives it: let nature run things and it can right the wrongs we have done Earth’s wildlife. Habitats will restore themselves and biodiversity will bounce back, along with the vital services that the ecosystems provide, such as pollination and water purification.
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Yet even as experiments like Knepp take off, researchers are voicing concerns about how effective rewilding truly is. Meanwhile, the world has embarked on a huge but largely undirected rewilding project as vast tracts of once-productive agricultural land are abandoned. This is bringing unexpected answers as to what really happens when you let nature run its course.
The original concept of rewilding is attributed to ecologist Michael Soulé at the University of California, Santa Cruz. In 1998, he published an in North America. He advocated creating wildlife corridors between existing reserves, linking them up to create wildernesses large enough to support native carnivores such as wolves and bears, or even introduced ones such as lions.
The idea was underpinned by cutting-edge ecology, and specifically the concept of trophic cascades. This holds that ecosystems are principally shaped by the feeding behaviour of large herbivores and carnivores, the “apex consumers” largely absent from today’s human-dominated ecosystems. Put them back in, so the logic went, and the ecosystem would automatically be restored to a healthier, more biodiverse state.
When Soulé proposed rewilding, the trophic cascade hypothesis was already being tested on the ground in Yellowstone National Park. The creation of the park in 1865 did not stop rangers from shooting wolves, the natural apex predators. By the 1920s, they had been totally eradicated, with huge knock-on effects. The elk population exploded, causing bison to decline owing to competition for food, and beaver numbers to crash as elk overbrowsed trees next to rivers. Coyotes also boomed and gobbled up the pronghorn antelopes.
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In 1995, after a long campaign, wolves were reintroduced to Yellowstone. Almost immediately, the changes went into reverse. Trophic cascades were elevated to an iron rule of ecology. Ecologists John Terborgh and James Estes described them as a “universal property of ecosystem functioning, a law of nature as essential and fundamental to ecology as natural selection is to evolution”.
Of the many big and charismatic rewilding projects that have now been set up, most are essentially attempts to restore trophic cascades by reintroducing large animals, with the hope they will force the ecosystem back to an earlier state. One of the oldest and most famous is , a 160-square-kilometre nature reserve just north of the Arctic circle in Siberia. Since 1996, scientists there have been attempting to recreate the “mammoth steppe” that circled the northern hemisphere at these latitudes at the end of the Pleistocene epoch 12,000 years ago.
Back then, most terrestrial ecosystems were dominated by huge herbivorous mammals such as mammoths, rhinos, mastodons and ground sloths. They ate, trampled and dug up vast amounts of vegetation, kept grasslands free from trees and recycled nutrients through their copious dung. When they were wiped out by human hunters, the ecosystem was pushed into a radically different state: less open, less fertile and less biodiverse. With the megaherbivores gone, predators such as lions, bears and wolves called the shots, keeping populations of smaller herbivores in check through their own trophic cascade – until the predators were themselves hunted almost to extinction.
These waves of losses happened across the world. Some 100 genera of megafauna died out, precipitating a wave of secondary extinctions. The result was an overall simplification of food webs, a process called trophic downgrading. According to a published in Science by Estes, Soulé, Terborgh and others in 2011, “the loss of apex consumers is arguably humankind’s most pervasive influence on the natural world”.
But how do you make a mammoth steppe when mammoths have long since bitten the dust? This touches on one of the most troublesome questions in rewilding biology. Rewilding implies a return to a previous state, yet restoring things to the way they were before humans is often not an option. The original animals may be extinct, or the reintroduction of predators may provoke opposition from people worried about their own safety or that of their livestock.
In Pleistocene Park, the ecological role of mammoths is taken by herds of animals including musk oxen, elk, yaks, Yakut horses, European bison and reindeer, predated upon by existing populations of bears and wolves. According to park manager Nikita Zimov of the North-East Scientific Station in Yakutia, that is bringing results. “I would not call it a steppe ecosystem at the moment, but we are going there,” he says. “It is not an easy task to shift from one ecosystem to another.” The park is more biodiverse and more productive than the tundra it has replaced, he says, with knock-on benefits for ecosystem services, especially carbon sequestration.
In North America, however, Soulé’s original vision has been slow to materialise. An organisation called the Wildlands Network is gradually piecing together four separate wilderness areas across Canada, the US and Mexico, but progress is glacial. Soulé, now in his 80s, is philosophical. “Conservation is always an uphill battle,” he says. “There are so many elements in society that don’t want any sort of restoration to occur because it interferes with economic activity.”
In parts of Europe, though, Soulé’s ideas are alive and well, largely thanks to the fact that large predators are already making a comeback. This means trophic cascades can be restored simply by reintroducing herbivores, a “cannon fodder” approach that is the key to one of Europe’s grandest rewilding projects, in the straddling the Bulgaria-Greece border. The idea here is to create corridors between the area’s existing national parks to build a wilderness covering 2500 square kilometres, and to restore the deer population to provide food for the wolves and bears already living in the mountains.
Deli Saavedra of the NGO , which oversees the project, calls this trophic change restoration. “It is not about looking back, but looking forward,” he says. “It is impossible to try to restore ecosystems to how they were, so we will end up with something new, but we still want them to be more natural – more wildlife, more natural processes – where nature can manage itself.”
The expectation is that deer and wolves will eventually recover to their natural population densities, and black vultures will follow thanks to the carcasses left after wolf kills. With plenty of deer to eat, the wolves should also be less likely to attack livestock, so human-wildlife conflict will be reduced. “We’re now seeing results, with much higher densities of deer impacting the wolves and vultures,” says Saavedra.
Full trophic restoration of this kind requires vast areas for predators to roam – a wolf pack typically needs around 200 square kilometres. In landscapes fragmented by human activity, such as those of Europe and North America, this is rarely an option.
The answer can be to go halfway, restoring the herbivores, but leaving out the carnivores. This is the approach taken at Knepp, in England’s densely populated south, and at another well-known rewilding project, in the Netherlands. This 56-square-kilometre site east of Amsterdam was reclaimed from the sea in 1967 and earmarked for industrial development. Nothing happened and it gradually ran wild, becoming an important bird habitat.
To keep this habitat from being swallowed by forest, in the early 1980s its managers introduced cattle and horses to intensify the grazing. Red deer were added in 1992. Their activities have helped to create an ecosystem that ecologist Frans Vera, the site’s manager, has claimed is analogous to late Pleistocene Europe. But the lack of carnivores to devour the herbivores, and provide a natural brake on the amount they consume, means they must be culled or given supplementary food from time to time. Similarly at Knepp, some of the grazers are culled periodically and their meat sold, meaning humans are effectively acting as apex predators.
That raises questions as to whether such projects are true to the spirit of rewilding. But the concept of trophic cascades has itself recently come in for criticism. “Why do we assume that all ecosystems in the world are controlled from the top?” asks David Nogués-Bravo at the University of Copenhagen in Denmark. “This is one of the main scientific assumptions in rewilding, but there are many exceptions.”
200 km2
The space a wolf pack typically needs to roam in
He points to digging deep into the causes of ecological change at the end of the Pleistocene at five sites in Britain and Ireland. This concluded that climate was much more influential than the presence or absence of megaherbivores.
Even in Yellowstone, the quintessential example, the strength of the trophic cascade effect has been questioned. “It’s safe to say that, generally, wolves reduce their prey populations and that has varying effects on the vegetation that prey feed on,” says L.David Mech of the Northern Prairie Wildlife Research Center in North Dakota. “But below that trophic level, effects are more diluted and variable.” The increase in beaver numbers following the reintroduction of wolves, for example, probably had less to do with the end of elk overgrazing and more to do with a separate programme to bring back beavers that took place soon after the wolves were returned.
Another sceptic is at the Chinese Academy of Sciences’ Center for Integrative Conservation in Yunnan. He says that rewilding programmes have got ahead of the science and there is an urgent need for long-term experiments. “I’m a general supporter of rewilding,” he says. “The romantic aspect appeals to anyone involved in ecology who has seen everything go downhill in the past 50 years. But the lack of science is a major issue.”
Despite these concerns, rewilding is becoming a fixture of conservation biology. Most of it is happening entirely by accident, however, as huge swathes of farmland are taken out of productive use.
According to some estimates, within the European Union alone about between 2000 and 2030, an area about the size of Italy. Abandonment is also happening in the Americas, Australia and some developing countries, driven by low productivity, agricultural intensification and demographic change. Much abandoned land is in marginal areas such as uplands. Once deserted, most is simply left to its own devices.
This is known as passive rewilding. It may not get pulses racing in the same way as releasing charismatic mammals into a wilderness landscape, but in terms of land area it is much more significant. “That is the future of rewilding,” says Corlett. “If you look at the estimates of land likely to be abandoned, they are as big as current protected areas and much bigger than classic ecological restoration projects.” That is a huge opportunity to revive biodiversity, but it needs to be supported by good science. “We need to know what happens when we just abandon land,” says Corlett.
There are some indications. In 2014, a team led by of the Stockholm Resilience Centre in Sweden reviewed 276 studies on the effect of farmland abandonment. Surprisingly, the researchers found that while some areas saw an increase in biodiversity, most did not, especially in Europe.
That is because traditionally farmed landscapes often create a wide range of habitats for wildlife. When human intervention stops, these biodiverse “cultural landscapes” disappear.
A classic example is Portugal’s Côa valley, which has one of the highest levels of land abandonment in Europe. The area was once used for grazing cattle and running pigs, and also produced cork, honey, firewood and wild foods such as mushrooms. This low-intensity agriculture created a mosaic of habitats with high levels of biodiversity, supporting endangered Iberian lynx and Spanish imperial eagles. But as abandonment progresses, much of the Côa valley has become choked with dense scrub and forest, and is under constant threat of wildfires.
It is a similar story in Japan. There, traditional terraced rice paddies are rapidly being deserted as the population dwindles and people switch from rice to wheat. Since 1961, half of the land once turned over to rice, amounting to around 2700 square kilometres, has been left to run wild. Another 1600 square kilometres is expected to follow in the next decade. Management of the paddies, through flooding and mowing, maintains a diversity of habitats and wildlife. When it ceases, the fields become choked with vines and invasive bamboos. Over the past 15 years, Japanese ecologists have documented a steady decline of insects, birds, amphibians and plants.
“Passive rewilding is not problem-free,” says Corlett. “You don’t get an original, native ecosystem. You tend to get dominated by invasive species and you lose species that have adapted to human-made habitats.”
For some of these areas, a certain amount of active rewilding may be the answer, as at Knepp, where species surveys show that the activity of herbivores has driven an increase in biodiversity. The Côa valley is one of . It plans to reopen the landscape by introducing horses and cattle. But given the scale of land abandonment, such active, well-planned intervention will remain a rare luxury.
300,000 km2
Estimated farmland area to be abandoned in the EU between 2000 and 2030
To find out more about the effects of passive rewilding, I visited a nature reserve in the uplands of northern England, where experiments have been going on for more than 60 years. Moor House-Upper Teesdale National Nature Reserve was established in 1952. A vast area of bleak and soggy moorland, criss-crossed by streams and scarred by the remains of 19th-century lead mines, it represents exactly the sort of hardscrabble land where passive rewilding will mostly happen. “There’s a large proportion of land that is very marginal for agriculture, and it’s likely to be this marginal land that goes out of production,” says my host at the site, of the Centre for Ecology and Hydrology in Lancaster.
Starting in 1953, scientists began fencing off areas to see what happens when they are left ungrazed by sheep. In total, eight areas were enclosed and have now been undisturbed for 50 years or more. This experiment has been particularly relevant to the rewilding debate in northern Europe, Rose tells me. For example, a common view of the British uplands, promoted by the writer and activist and others, holds that the Lake District National Park, a little to the west of Moor House, is a biodiversity desert destroyed by overgrazing, and that the answer is to take sheep off the land.
We trek over to an enclosed plot called Bog Hill. Sheep have been excluded from this mossy mire for 65 years, but to the naked eye, the land inside and outside the enclosure is identical. Detailed studies of the soil and vegetation confirm that very little has changed. The story is repeated at all the plots. “A lot of the landscapes here are very resilient to change,” says Rose.
This suggests that bringing back nature by doing nothing can take an awfully long time, and taking the sheep away from areas such as the Lake District will do little or nothing for biodiversity. Perhaps the reverse: at Knepp, they kept one plot out of bounds to the big animals and did not see the same explosive rebound of nature, says Tree. “Without the heavy-hitting herbivores, the land takes ages to move,” she says.
In many places, then, passive rewilding looks to be an oxymoron. Just leaving land to go its own way may actually lead to a further reduction in biodiversity, not an increase. For that reason, Corlett advocates active intervention, at least in the beginning. “We have to work out something cheap to do on abandoned land that enhances conservation. I suspect it will turn out to be some selective reintroductions,” he says.
If we do work it out, abandoned farmland represents our best chance yet of substantially rewilding large areas. Despite the challenges ahead, Tree is optimistic. She says that Knepp is the start of something bigger, something not dissimilar to Soulé’s vision of joining up existing reserves to create vast areas.
“Farmers now, at least on marginal land like ours, are considering joining forces and becoming farm clusters, pulling up their borders and doing projects like this,” says Tree. “We need more space for nature. Rewilding can provide the webbing to link the isolated spots and connect up the bigger landscape. We’re very excited.”
This article appeared in print under the headline “The call of rewilding”
