But then I was struck by a thought which now seems obvious. The process is the outcome. The main aim of rewilding is to restore to the greatest extent possible ecology’s dynamic interactions. In other words, the scientific principle behind rewilding is restoring what ecologists call trophic diversity. Trophic means relating to food and feeding. Restoring trophic diversity means enhancing the number of opportunities for animals, plants and other creatures to feed on each other; to rebuild the broken strands in the web of life. It means expanding the web both vertically and horizontally, increasing the number of trophic levels (top predators, middle predators, plant eaters, plants, carrion and detritus feeders) and creating opportunities for the number and complexity of relationships at every level to rise.
One of the most fascinating discoveries in modern ecology is an abundance of trophic cascades. A trophic cascade occurs when the animals at the top of the food chain–the top predators–change the numbers not just of their prey, but also of species with which they have no direct connection. Their impacts cascade down the food chain, in some cases radically changing the ecosystem, the landscape and even the chemical composition of the soil and the atmosphere.
The best-known example is the dramatic change that followed the reintroduction of wolves to the Yellowstone National Park in the United States. Seventy years after they had been exterminated, wolves were released into the park in 1995. When they arrived, many of the streamsides and riversides were almost bare, closely cropped by the high population of red deer (which in North America, confusingly, are called elk*5). But as soon as the wolves arrived, this began to change. It was not just that they sharply reduced the number of deer, but they also altered their prey’s behaviour. The deer avoided the places–particularly the valleys and gorges–where they could be caught most easily.32
In some places, trees on the riverbanks, until then constantly suppressed by browsing, quintupled in height in just six years.33 The trees shaded and cooled the water and provided cover for fish and other animals,34 changing the wildlife community which lived there. More seedlings and saplings survived. The bare valleys began reverting to aspen, willow and cottonwood forest. One apparent result is that the number of songbirds increased: among the resurgent trees a study has found higher populations of species such as the song sparrow, warbling vireo, yellow warbler and willow flycatcher.35
The regrowth of the bankside forests also appears to have allowed the populations of both beavers and bison to expand: beaver colonies rose from one to twelve between 1996 and 2009.36 The beavers then trigger all the effects I have just mentioned, creating niches for otters, muskrats, fish, frogs and reptiles. The returning trees have also stabilized the banks of the streams, reducing the rate of erosion and the movement of channels, narrowing the width of the streams and creating a greater diversity of pools and riffles.37 Similar effects have been recorded in Zion National Park in Utah: where cougars are abundant, the streamsides are stable and fish numbers are high, where they are scarce, the rivers wander and fish numbers are three times lower.38 The soil on the hillsides in Yellowstone, depleted through sheet erosion after the wolves were all killed and deer numbers rose, may now begin to build up again.39 Conversely, on the grasslands where the deer and pronghorn antelope grazed heavily when their predators were absent, five years after the wolves returned, nitrogen in the soil declined by between a quarter and a half. This is because less of it is now recycled through dung.40 This will change the species of plants that grow there and their numbers.
By hunting coyotes, the wolves allow the populations of smaller mammals–such as rabbits and mice–to rise, providing prey for hawks, weasels, foxes and badgers. Scavenging animals such as bald eagles and ravens feed on the remains of the deer the wolves kill. The return of the wolf appears to have increased the number of bears. They eat both the carrion abandoned by the wolves and the berries growing on the shrubs that have sprung back as the deer declined.41 The bears also kill deer calves, reinforcing the impact of the wolves. The reintroduction of wolves to Yellowstone shows that a single species, allowed to pursue its natural behaviour, transforms almost every aspect of the ecosystem, and even alters the physical geography of the site, changing the shape and flow of the rivers and the erosion rates of the land.
There is no substitute for these complex relationships. Throughout the period in which wolves were absent from Yellowstone National Park, its managers tried to control the deer and contain their impacts–and failed.42 Despite intense hunting and culling, willow trees disappeared from the meadows and aspens were in danger of vanishing from large areas of the park.43 Even when hunting by humans is intense, its effects are likely to differ sharply from those of hunting by wolves. Wolves hunt at all times of the day and night, throughout the year. They pursue their prey, rather than killing it from a distance.44 Wolves and humans hunt in different places and select different animals from the herd. Fencing might keep out the deer, but unlike wolves it does so entirely, while also excluding other animals and reducing the connectedness of the ecosystem.
Where salmon run, the reintroduction of wolves in North America could trigger even wider effects. The wolves create habitats for both salmon and beavers, and the beavers create further habitats for salmon, potentially boosting their numbers. The salmon are caught by bears, otters, eagles and ospreys. Their carcasses are often dragged or carried onto land. The nutrients they contain are distributed in the animals’ dung. One study suggests that between 15 and 18 per cent of the nitrogen in the leaves of spruce trees within 500 metres of a salmon stream comes from the sea: it was brought upriver in the bodies of the salmon.45 Top predators and keystone species unwittingly re-engineer the environment, even down to the composition of the soil.
A starker example is provided by the Arctic foxes introduced by fur trappers to some of the Aleutian islands–the sickle-shaped chain across the northern Pacific between Alaska and Siberia–where they are not native. Those islands with Arctic foxes are covered in shrubby tundra, those without foxes are covered in grass.46 By hunting seabirds, the foxes have ensured that sixty times less guano is brought to the islands. This means that there is three times less phosphate in the soil than where they are absent. As a result, they have changed the entire natural system.
Human hunters might have imposed a similar change in the great steppes of Beringia, the landmass incorporating eastern Siberia, Alaska and the area in between (now covered by the Bering Straits, but exposed during the last Ice Age). Perhaps 15,000 years ago, hunters using small stone blades moved into the region that had hitherto been occupied by people hunting with sharpened bones or antlers.47 Gradually, they wiped out the mammoths, musk oxen, bison and horses that grazed the steppes*6 (When the glaciers blocking their passage into the rest of the Americas melted, they went on to wreak even greater havoc in the New World.) The result, it appears, was that they helped turn the steppe grasslands into mossy tundra. Much of this land has remained that way ever since.
As the Russian scientist Sergey Zimov has shown, grasslands, especially in the far north, are sustained by the animals that feed on them. By grazing, they make the grass more productive (in the steppes it grows five times faster than it does when it is not mowed). They recycle the soil’s nutrients through their dung. The grass dries out the soil and smothers moss and lichens.49 When the animals disappear, the self-reinforcing process goes into reverse. The dead grass, flopping over the soil, insulates it, ensuring that it stays cold, reducing the further growth of grass and encouraging moss to take over. As the moss begins to dominate, the soil becomes wetter and colder–still more hostile to grass. If the animals return, their trampling quickly breaks up the fragile layer of moss and lichens, allowing the grass to dominate again within one or two years.50 The grazers in this habitat, in other words, are keystone species, flipping the entire ecosystem from one state to another.
This suggests, incidentally, that large-scale rewilding of the tundra, which Zimov and others promote, while a fascinating prospe
ct, could have a damaging consequence. Moss is such a good insulator that it prevents even the top layer of soil from thawing.51 It helps to stabilize the permafrost, locking up the methane it contains. If the moss layer is broken up and grasses return, while this might greatly increase the productivity and trophic diversity of the region, it could accelerate the melting which threatens to release large quantities of a powerful greenhouse gas. This is a reminder that rewilding, like any change we contemplate, has costs. In some cases the costs may outweigh the benefits.
Hunting by humans might also have transformed the environment of Australia. Before people arrived on that continent, it teemed with monsters. Among them was a spiny anteater the size of a pig; a giant herbivore a bit like a wombat, which weighed two tonnes; a marsupial tapir as big as a horse; a ten-foot kangaroo; a marsupial lion with opposable thumbs and a stronger bite than any other known mammal, which could prop itself up on its tail in order to stand on its hindlegs and slash with its tremendous claws; a horned tortoise eight feet long; a monitor lizard much bigger than the Nile crocodile. Most of these species, alongside many other marvellous beasts, disappeared between 40,000 and 50,000 years ago. At roughly the same time, the dense rainforests which covered much of that continent began to be replaced with the grass and scrubby trees which populate much of the outback today.
Two debates have raged among ecologists. Were these shifts caused by natural climate change or by humans? If, as now seems probable, they were caused by humans, were the extinctions of the giant animals the result of hunting or of the destruction of their habitats? Research published in the journal Science strongly suggests that humans hunted the large animals to extinction, and that the disappearance of the large animals then caused the destruction of the rainforests.52
Analysing the pollen and charcoal in cores taken from an ancient lake bed, and using the fungus that grows on the dung of large herbivores to measure their abundance, the researchers showed that the shift from rainforest to dry forest took place some 10,000 years before the climate dried out. Both the mass extinction and the change in habitat happened while the climate was stable. They also showed that fire began raging through the rainforests around a century after the large mammal populations collapsed; and that grass and scrub replaced the forests two or three centuries later. When the giant herbivores disappeared, they suggest, the twigs and leaves that would otherwise have been browsed began to build up on the forest floor, creating a fuel supply that allowed wildfires to destroy the rainforests and catalyse the shift to grass and scrub. The herbivorous monsters of Australia, like the mammoths and musk oxen of Beringia, appear to have sustained the ecosystem they browsed.
One of the interesting implications of the discovery of widespread trophic cascades is that removing an animal from a system–especially a top predator–may have counterintuitive and destructive results. For example, in many parts of Africa, people have killed lions and leopards in the belief that this will enhance their chances of survival and (among early European hunters) boost the herds of game. But one result has been an explosion in the population of olive baboons. They inflict such damage on crops and livestock that children have to be taken out of school to fend them off.53 They also transmit intestinal worms to the people whose land they enter,54 and appear to have reduced populations of wild game by preying on the young animals. Similarly, when conservationists in Florida sought to protect sea turtles by culling the raccoons which eat their eggs, they found that it caused the opposite effect. More turtle eggs were lost, as the raccoons were no longer eating the ghost crabs which also preyed on them.55
Perhaps the strangest example of these unexpected effects is the apparent link between the decline of vultures and the spread of rabies in India. In a remarkably short period, vultures have almost become extinct there as an accidental result of the use of a livestock drug called diclofenac, which turns out to be deadly to them when they eat the carcasses. As the number of vultures has collapsed, the carrion they ate is consumed instead by feral dogs. Their population, despite intense efforts to control it, has risen sharply as the vultures have declined. Dog bites are the cause of 95 per cent of the deaths from rabies in India, and the rising population means that more people are likely to catch the disease.56 The vultures were also likely to have helped control animal diseases such as brucellosis, tuberculosis and anthrax, by clearing up infected meat.
Trophic cascades might once have dominated most ecosystems. The old belief among ecologists that natural systems were controlled only from the bottom up–that the abundance of plants controls the abundance of plant eaters, which controls the abundance of meat eaters–arose from the fact that many of the systems they were studying had already been greatly changed by people, not least through the reduction or extinction of top predators. Much of the richness and complexity–the trophic diversity–of these foodwebs was lost before it was recorded. We live in a shadowland, a dim, flattened relic of what there once was, of what there could be again.
7
Bring Back the Wolf
The fells contract, regroup in starker forms;
Dusk tightens on them, as the wind gets up
And stretches hungrily: tensed at the nape,
The coarse heath bristles like a living pelt.
William Dunlop
Landscape as Werewolf
We associate elephants, rhinos, lions and hyenas with the tropics. But until very recently (in geological terms) they lived in climates much colder than north-western Europe is today. Until around 40,000 years ago, the straight-tusked elephant (Elephas antiquus), closely related to the Asian elephant, roamed across much of Europe.1 The woolly mammoth, which had an entirely different ecology, grazing on cold steppes (rather than browsing, like the straight-tusked elephant, in temperate forests) lasted longer: one relict population, isolated from human hunters in the fastness of Wrangel Island off the north coast of Siberia, survived until the Bronze Age.2
Three species of rhinoceros–the woolly, the Merck’s and the narrow-nosed–lived in Europe at the same time as humans. Until roughly 40,000 years ago, Russia was haunted by two monstrous beasts, Elasmotherium sibiricum and Elasmotherium caucasicum. They were humpbacked rhinos the size of elephants, eight feet to the crest, weighing perhaps five tonnes. Elephants roamed across Europe, Asia, Africa and the Americas; rhinos never populated the Americas, but they lived throughout the Old World. Across the past 50,000 years the range and variety of these species have shrunk as humans have hunted them. They were exterminated first in Europe; then (in the case of elephants) in the Americas; then in the Middle East and North Africa; then in most of Asia; eventually in most of Africa. The animals conservationists are now desperately trying–and often failing–to save are the last, tiny populations of creatures which once dominated most of the earth’s surface, so recently that we can almost stretch out our fingers and touch them.
When Trafalgar Square was excavated in the nineteenth century, presumably to build Nelson’s column, the river gravels the builders exposed were found to be crammed with hippopotamus bones; these beasts wallowed, a little over 100,000 years ago, where tourists and pigeons cluster today. The same excavations–and those conducted in the square in the twentieth century–also revealed the bones of straight-tusked elephants, giant deer, giant aurochs and lions.3 Lions raised their heads where the monument now stands long before Sir Edwin Landseer got to work.
They were larger than those now living in Africa but probably members of the same species. They hunted reindeer across the frozen wastes of Europe,4 and survived in Britain until 11,000 years ago:5 the beginning of the Mesolithic, when humans returned to the land after their long absence. Spotted hyenas (also still living in Africa) survived in Europe until roughly the same time6 (their fossilized faeces have been found in Trafalgar Square7). Scimitar cats (Homotherium species), lion-sized with great curved fangs, preyed perhaps exclusively on young elephants and rhinos. These species–elephants and rhinos and the cats which ate them–are likely
to have dominated the ecosystem during the previous inter-glacial period, which ended around 115,000 years ago (a blink of an eye in geological terms). The curious features that some of our plants possess may be ghostly adaptations to the way they fed.*1
Elephants’ habit of snapping or uprooting trees could explain why species such as oak, ash, beech, lime, sycamore, field maple, sweet chestnut, hazel, alder and willow can regrow from the point at which the stem is broken.†2 In eastern and southern Africa there are dozens of tree species which resprout–or coppice–from the snapped trunk, and ecologists recognize this as an evolutionary response to attacks by elephants.9 By breaking African trees such as mopane or knobthorn acacia, elephants improve their food supply, as the shoots the damaged trees produce are easier to reach and more nutritious than older branches.10 Trees that can survive the attention of elephants often come to dominate the places in which the animals live: the ability to coppice confers powerful selective advantages.
But somehow the obvious link–between coppicing and elephants–appears to have been missed by people studying European ecosystems. It is another example of Shifting Baseline Syndrome. Ecologists are not always aware of the extent to which the systems they study have been altered by humans: that the life they describe has been greatly simplified and diminished.
Elephants could also explain why understorey trees in Europe, such as holly, yew and box, are so resistant to breakage and have such strong roots, though they carry less weight than canopy trees and are subject to lower shear forces from the wind. They have to be tough, as they take much longer to become massive enough to withstand toppling or for their branches to grow out of the reach of trunks and tusks. The ability of some trees to survive the removal of much of their bark could be another adaptation: elephants often strip bark with their tusks. Elephant-proofing could account for the birch tree’s pied coat: the black fissures make the white skin harder to strip cleanly.
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