REWILDING
CREATED IN 1872 by the United States government, Yellowstone was the first official national park in the world. It was protected because of its natural beauty and outstanding ecological value, yet no one questioned the killing of wolves that lived there. In Europe and North America, deep-seated fears have for centuries portrayed the wolf as the beast to exterminate. Wolves were seen as enemies—and still are by many—that killed livestock, game species like deer, and even people. Government policies cemented that belief, and wolves were systematically exterminated from many countries. We now know that wolves do not kill people—but, ironically, dogs kill 25,000 people globally every year because of rabies, yet nobody calls for the extermination of dogs. It seems as though we are happy with the domesticated version of the wolf, which we manipulated genetically over millennia, but we do not dare to look into the eyes of the wild wolf and try to understand who it is and what it does. But scientists now know that wolves hold the ecosystem together.
No one understood that in the decades after Yellowstone was established. Wolf extermination was the rule, and the last wolf was seen there in the 1920s. Fast-forward 70 years. In 1994, Yellowstone was a beautiful world of geysers and winter snowy landscapes, abundant with elk (Cervus canadensis). But those animals overgrazed the riverbanks. They were eating not only the grasses and underbrush but also the seedlings of cottonwood, aspen, and willow, which used to grow right down to the river’s edge. Park managers tried to manage the elk population, as they put it, by culling them, but reducing their numbers did not reduce their impact.
Ecologists realized there might be a way for nature herself to restore the balance in Yellowstone, since human management was not working. The natural food web needed to be restored by reintroducing the top predator—the wolf. So in 1995, 31 wolves were released in Yellowstone, with the expectation that they would regulate the elk numbers better than humans had.
The plan worked. These 31 wolves and their descendants acted like landscape engineers and transformed the entire park. The wolves did reduce—and regulate—the number of elk, but they also changed the elk’s behavior. By their presence alone, the wolves created a landscape of fear whereby elk spent less time in the open, to avoid being killed—just as the tiger sharks did in Australia’s Shark Bay. Within a few years, the trees on the riverbanks were coming back. With more arboreal habitats, songbird populations increased. Forests of cottonwood, aspen, and willow now shaded and cooled the streams, reducing erosion and providing cover for fish and other aquatic life.
Thanks to the new riverside forests, the beaver population grew 12-fold in only 13 years. More beaver dams and richer habitats allowed otters, frogs, and reptiles to increase. Since the wolves hunted coyotes, coyote prey increased. With more rabbits, mice, and other small mammals throughout the park, the numbers of their predators—eagles and foxes, for example—multiplied.
But there is more. Before the reintroduction of the wolves, many elk died during severe winters and at the end of moderate winters, their winter mortality caused when food resources got buried in deep snow. Scavengers—ravens, eagles, foxes, bears, and coyotes—fed heavily on elk carcasses during these winter seasons. Global heating since 1948 has made the winters get shorter, though. Earlier snow thaws reduced late winter deer mortality, which meant less food for the scavengers. All that changed once wolves were reintroduced. Wolves preyed on elk, which made carrion available throughout even the warmer winters. More carrion meant more scavengers, helping to recycle the recent necrosphere, which nourished living matter once again.
In short, reintroducing wolves enriched Yellowstone’s ecosystem and brought it closer to its mature state. The wolves—Yellowstone’s top predator—are the hero of this story. The return of the wolves to Yellowstone caused the return of all creatures, plants and animals, from small to large, and it is also helping to buffer the impacts of global heating.
Wolves are the epitome of a keystone species. Their role in the terrestrial ecosystem is as provider of life for all types of creatures, in stark contrast to the demonic image that humans tend to project on them. Rewilding Yellowstone with the top predator helped rewind the successional clock and get the ecosystem moving forward toward more mature stages of the ecological succession. Without the wolves, the ecosystem was moving backward.
Top predators are the first to go when humans arrive to a place. We don’t tolerate competition. We crave being the only ones at the top of the food web, sovereigns who prefer shorter and simpler food webs and small species with high turnover rates as our subjects. Our inclination has been either to keep ecosystems stuck or to force them to regress to earlier successional stages. Can we change that habit? Can we help ecosystems move beyond the roadblocks we have put in the way that keep them from maturing?
The answer is yes, through the process of rewilding. We were able to rewild Yellowstone, and look what happened. By rewilding, we reintroduce native species to restore the full natural cycle of an ecosystem. By reintroducing apex predators, like the wolves in Yellowstone, we can accelerate the restoration of ecosystems. But apex predators can only effect this trophic cascade if they slot back into an ecosystem with its full complement of prey species—that is, into wilderness areas or protected areas of a certain age. Could we do the same thing by reintroducing herbivores—species that eat plants and are prey to the apex predators? An example from the Serengeti will help answer that question.
A disease in Serengeti National Park provided a natural experiment at the scale of a large ecosystem, something that would be impractical through experimental manipulation. In 1890, within two years, a viral disease called rinderpest killed 95 percent of the wildebeests and buffaloes in East Africa. Passed on from livestock, rinderpest causes fever, diarrhea, inflammation of the mucous membranes, and high mortality. A treatment was established, and by 1963 all the livestock around the park had been vaccinated. When the disease disappeared in livestock, the wild animals bounced back: from 250,000 wildebeest in 1961 to 1.3 million in 1977. More wildebeests helped the Thomson’s gazelles, which prefer to forage in places where wildebeests have already grazed. With more ungulates as prey, the populations of hyenas and lions also increased. In addition, more grazing wildebeests kept the grass short. Fewer plants grew old and withered, which meant the landscape contained less dead plant material and thus less fuel for wildfires. With fewer fires, more acacia tree seedlings survived. Giraffes love to eat acacia seedlings, and the renewed acacia abundance helped boost the giraffe numbers. More browsing giraffes meant fewer acacias grew into tall trees, but more young acacia shrubs sprouted, providing healthy underbrush and more organic debris into the system. Another trophic cascade in reverse, from the bottom up—or, still better, across the food web. In this case, rewilding involved not reintroducing species but ensuring the health and proliferation of species in distress.
Rewilding not only accelerates ecological succession; it also can help mitigate climate change, as mature ecosystems sequester and store more carbon than degraded ecosystems. For example, many trees in tropical rainforests need large mammals to perpetuate their species. Tapirs in South America and elephants in West Africa, for example, eat the fleshy fruit from the trees and disperse the seeds that pass through the animals’ digestive systems. Trees with large fruits tend to be taller and have denser wood, storing more carbon than other trees, which makes the larger mammals who can eat those fruit all the more important. Recent studies suggest that the loss of large fruit-eaters could lead to 12 percent carbon losses in tropical forests—another contributor to global warming. Rewilding our increasingly fragmented tropical forests with these large animals—or simply prohibiting the killing of them—would be beneficial to humanity and the rest of life on the planet.
YELLOWSTONE AND SERENGETI National Parks were protected areas that benefited from rewilding, whether intentional or unintentional. But could rewilding help restore other parts of the planet—even unprotected ecosystems? What about depleted farmland—the closest thing to zero biodiversity our planet has? Would it even be possible to re-create the natural ecosystem that was there before?
In 1987, Charlie Burrell inherited a British farm from his parents. This was not just any farm: It was Knepp, in West Sussex, a 1,400-hectare (3,500-acre) estate that dated back to the 12th century and had seen the visits of kings. By the 20th century, when Burrell got the farm, however, they couldn’t turn a profit because they couldn’t compete with farms on better soils. Both the fields and the dairy business were in ruins. The soil—in cross section, 320 meters (1,050 ft) of clay over a bedrock of limestone—was, according to Isabella Tree, Burrell’s wife, “concrete in the summer and porridge in the winter.” The arable fields had been plowed ad nauseam and subjected to artificial fertilizers, pesticides, and herbicides for a long time before they took over. Trying their best to make a profit, Burrell and Tree did as other farmers these days do: They started using fertilizers, pesticides, and fungicides, to try to make the farm more productive. They tried to diversify, they invested in infrastructure and new machinery, tried new varieties of crops, but after 12 years of hard work, they still failed to make a profit. With few options left, they sold their cows and equipment—whose proceeds cleared their debts—and stopped farming.
But they stayed with the land. They had some notion that perhaps they could restore its natural ecosystem. Repton Park, around the house, had been plowed up during World War II as part of the Dig for Victory campaign. Their first step was restoring it—a project for which they received Countryside Stewardship Scheme funding. With those funds they purchased Low Weald native wildflower seeds and began to sow them in the park. “That first year I remember walking out of the door through knee-high wildflowers, and hearing that incredible sound of insects,” Tree said. “We hadn’t known that that was what we’d been missing. And then of course the insects came back, and then the birds came back.” Centuries of land degradation, plowing, and the intensive use of artificial fertilizer and pesticides after World War II had annihilated most life in the fields of England save the monoculture crops being grown there. People had almost forgotten that insects could be plentiful in the English countryside. “I remember in the 1970s, when I was a child growing up in Dorset, having to put the wipers on to wipe all the splatted insects off the windscreen,” Tree told me. “That doesn’t happen anywhere now—except in a very few areas, including Knepp.”
But this was just the beginning of their rewilding experience. They heard about Frans Vera, a Dutch ecologist who hypothesized what Europe’s lowlands must have been like when all the large herbivores roamed in abundance. Once there were the aurochs (wild cattle), the tarpans (wild horses), and the European bison—all depicted in prehistoric cave drawings. They were all long gone, except for the bison, which miraculously survived in captivity after the last animals in the wild were shot in the 1920s, and which are now being reintroduced in rewilding projects across Europe. There were once other herbivores as well, such as red deer, moose, wild boar, and beavers by the many millions—and brown bears, too. We did not drive those extinct, although we overhunted moose, beavers, and bears. The vast numbers of these animals in the past must have had a significant influence on the structure of lowland European ecosystems.
The idea of rewilding Knepp with such creatures appealed to Burrell and Tree. Unable to bring back the old animals à la Jurassic Park, they found modern species that could mimic the role of the wild herbivores in the ecosystem—grazing animals that are, as Isabella put it, “proxies for the animals that would have been present in huge numbers in our landscape in the distant past.” Thus, they introduced longhorn cattle for aurochs, Exmoor ponies for tarpans, Tamworth pigs for wild boars, and red and fallow deer. (Roe deer were already present in small numbers.) This suite of animals represented a combination of different eating preferences and habits. Diversity in action. We don’t know how many species of plants the longhorn cattle feed on, but cattle in general can carry more than 230 seed species in their gut, their hooves, and their fur. So they eat here and poop there, and the seeds are given a jump start with the compost that surrounds them. The cattle don’t eat thistles, but the ponies like them, and they also can stomach rougher grasses than cattle can. And the pigs rootle in search of roots, tubers, worms, and other invertebrates, aerating the soil.
“What we’ve done here is take our hands off the steering wheel, and just stand back and let nature take over,” said Tree. And nature took over indeed. The new herbivores played such a diversity of ecosystem roles, replacing the uniform, depleted fields of the farm with a vastly superior mosaic of habitats, including water meadows, thorny scrub that protects young trees, sallow groves, and dead trees that provide a habitat for many animals. Nightingale territories on the farm increased threefold in a decade. The turtle dove, a species thought to be on its way to extinction in the British Isles, is increasing in numbers. The number of small skipper butterflies increased by 1,100 percent in just one year. The insects returned, and then the little birds came back to eat the insects, and then the predators returned. Now peregrine falcons nest at Knepp, and all five British owls are found there. The little owls are doing great. They eat the dung beetles found in cattle feces, but now the feces contain none of the pesticides or medicines omnipresent when the dairy farm was in operation. In the past, the pesticides and antibiotics given to the dairy herd ended up in their dung, killing any insect that encountered it.
Burrell and Tree let nature take charge and reestablish its wild ecosystem. The only thing they control is the number of herbivores, because they don’t have any natural predators to prevent that population from increasing to the point of decimating the vegetation and starving themselves. The last wolf in England was apparently killed in 1390, and the last wolf in the British Isles was killed in Scotland in 1680. So instead of relying on a top predator to keep the herbivores in check, which would be challenging to reintroduce in populated southern England, Burrell and Tree control the numbers of cattle, deer, and pigs that roam their farm, and they sell their meat—locally and by mail order. A conservation business.
If all those benefits weren’t enough, Burrell and Tree started a safari business at Knepp, offering visitors what they call “microadventures.” Tourists, especially bird-watchers (an abundant breed in Britain), can come to Knepp to observe what has been lost elsewhere. “This is a sustainable business,” said Burrell. “The cash that comes through the safari business seems to be bigger and better than what it was when it was a commercial farm.”
Not only the business but the entire landscape has been refreshed by rewilding—a transformation that has significant implications for the planet. The new diversity of vegetation and the restored soil retain water after heavy rains. The diverse plant communities of Knepp thus help reduce flooding and loss of infrastructure. More plant material and animal feces, together with abundant earthworms, dung beetles, and all the other invertebrates digesting the dung, added to the herbivore grazing and browsing that stimulates roots underground and vegetation regrowth above-ground. All combine to help build healthy soil, with its glomalin, that powerful carbon kidnapper specifically associated with mycorrhizal fungi. Mycorrhizal fungi has tripled in Knepp’s soil since Burrell and Tree rewilded; soil biota and soil carbon have both doubled. Healthy soil rich with all that organic matter locks more carbon up, making the soil a great carbon sink again.
But Burrell and Tree did not return Knepp to its prehuman condition. Located near Gatwick Airport, Knepp is surrounded by roads. Its soils have been affected by nearly 70 years of chemical additives and hundreds of years of plowing. One generation’s efforts cannot undo these influences completely. What we can do, however, is use our increasing knowledge of how pristine ecosystems perform to inform our decisions on how to restore today’s ecosystems. We can use the tools available—the remaining megafauna, the principles of hydrology, plant and animal reintroductions—to create novel ecosystems: ecosystems with conditions and species that have never come together during our lifetimes, but which approach the successional maturity that makes them resilient in response to human impacts. It’s not always a matter of returning to a prehuman ecosystem. The important thing is helping an ecosystem to function and mature. Therefore, rewilding is about the future—not the past.
Imagine if this sort of change could happen over and over around the planet. And if monoculture agriculture shifted to regenerative practices that help build the soil instead of damaging it, the change would create new potent carbon sinks, helping reduce global heating. A recent study suggested that mitigating agricultural practices could lower global temperatures as much as 25 percent of the 2°C (3.6°F) goal set by the Paris Climate Agreement.
WHILE REWILDING HAS proved effective in restoring lost ecological functions and accelerating ecological succession, we have to be careful about what species to reintroduce. Burrell and Tree were right in introducing species with ecosystem roles similar to those of the animals that roamed their land before humans. But not all species that look the same do the same. Let’s go back to Yellowstone, but this time for an example of an ecosystem wrecked by introducing the wrong species.
President Theodore Roosevelt is an icon of American conservation. He created 150 national forests, 51 federal bird reserves, four national game preserves, five national parks, and 18 national monuments, representing more than 230 million acres of publicly protected land in the United States between 1901 and 1909. He also signed the Antiquities Act in 1906, which has allowed American presidents ever since to designate national monuments in order to protect sites of unique historical or natural importance. Ironically, despite his extraordinary record of protecting nature in the United States, Roosevelt was a big game hunter known to have bagged 296 wild animals, including 17 lions, during a single East Africa safari in 1909. His son Kermit killed 216 animals on the same trek.
As an avid hunter, Roosevelt held that the white man’s recreational enjoyment of nature was more important than ecological health—or maybe he had simply not made the connection yet; few had. For the sake of enriching tourists’ fishing experiences, he promoted stocking lakes in the West with game species. Perhaps well-intentioned, this philosophy came to harm Yellowstone National Park some 90 years later.
The only member of the trout family—the salmonids—native to Yellowstone Lake is the cutthroat trout. There were an estimated 3.5 million of them there in the 1970s, making them the dominant predatory fish in the lake. These trout live in shallow waters, eating mostly cladocerans—water fleas just a few millimeters long, but larger than most lake plankton. Cladocerans eat smaller crustaceans (copepods), and copepods eat algae, thus keeping the lake population of microscopic algae—phytoplankton—balanced and making the lake productive. The cutthroat trout is in turn eaten by otters, bears, bald eagles, and ospreys. During their spawning season, cutthroat trout migrate from the lake to its tributary streams, transporting nutrients as they do.
In 1994, the lake trout—not native to Yellowstone—was first observed in the lake. Sportfishermen had introduced the species illegally into a nearby lake in the 1980s, intending to improve game fishing in the area. It proved to be an example of the wrong type of rewilding, caused by the myopic belief in man’s right to dominate nature.
As the lake trout population boomed, cutthroat numbers plummeted, declining by more than 90 percent by 2012. The game fish had become an invader—and a top predator, gobbling up the native trout. In 1998 alone, lake trout ate between three and four million cutthroat trout. The National Park Service quickly started a gillnetting program to rid Yellowstone Lake of the introduced species, and between 1998 and 2012, more than a million lake trout were removed from the lake. Yet that did not prevent the introduced species from increasing exponentially: from 125,000 adult fish in 1998 to 953,000 fish in 2012.
The introduction of lake trout and the decline of the cutthroat, the native top predator, caused trophic cascades within and outside of the lake. In the lake, without the cutthroat to eat them, the cladocerans increased in abundance and ate small copepods. Without small copepods, large copepods—too large to be eaten by cladocerans—boomed, and in turn ate more phytoplankton. With less phytoplankton, the primary productivity of the lake—the base of the food web—declined, affecting everything above. Since the lake trout lives in deeper water than the cutthroat, it is not accessible prey for bears, otters, and waterbirds including ospreys and eagles. All indicators showed these species declined. In 1997, 38 nesting ospreys lived at Yellowstone Lake; only three remained in 2017. In the late 1980s, grizzly bears caught more than 20,000 cutthroat trout in the spawning streams per year; by the late 2000s, that number had dropped to only 300. The abundance of river otters was the lowest ever estimated in 2008.
One new species added to the mix, strictly for human entertainment, caused a trophic cascade that reverberated through the entire Yellowstone Lake ecosystem, from microscopic phytoplankton to large mammals like otters and bears. Monumental efforts have been applied to reduce the abundance of lake trout in Yellowstone. In 2017, more than 9,200 kilometers (5,700 mi) of gillnets were set—the distance from New York to Cairo as the bird flies—during a grueling season lasting from May to September. Lake trout catches started to decline, and some experts believe we have reached a turning point. While full eradication of the lake trout is deemed impossible, continued efforts will likely reduce its population over time, and the local cutthroat trout will hopefully rebound. But we cannot drop our guard. The moral of this story: Rewild with care.
Rewilding with the right species—native herbivores or carnivores—at the right time can restore ecological functions that were lost—natural grazing or predation—and, under the right conditions, help an entire ecosystem self-restore. In a protected area where everything is missing except the keystone predator, its reintroduction can accelerate ecological succession and restore the complexity and maturity of the ecosystem. Even in a degraded area, like a moribund industrial farm, the right species can restart progress toward a healthy, productive, and ultimately mature ecosystem. The introduction of non-native species can on the other hand wreak havoc within an ecosystem. Inside a protected area, introducing the wrong species can actually reverse the gains accumulated by protection.
Our world has become less wild because we have transformed intact ecosystems into agricultural fields, grazing ranges, hunting and fishing destinations, and cities. Losing these wild places, we have lost most benefits they provide for us as well, such as flood protection, water security, water filtration, clean air, and naturally fertile soil. Rewilding the world could help us regain some of these benefits. But is utility for humans the most important way to measure the value of recovering the wild? Are there more powerful reasons why we should stop our assault on the natural world? Are there deeper reasons why we should do all we can to rewild wisely and protect and preserve the wild? In short, why and how should we value the natural world?