Rebuilding the broken strands in the web of life

To mark World Nature Conservation Day 2021, author George Monbiot has given us kind permission to share an excerpt from his best-selling book: Feral.

George Monbiot is a journalist, author and activist. His book Feral: Rewilding the Land, Sea and Human Life, was first published in 2013 and has become one of the most influential works about rewilding.

Image credit: John Russell

At first, I struggled to identify the scientific principles that might inform rewilding. To formulate principles, you must know what outcome you are trying to achieve. But rewilding, unlike conservation, has no fixed objective: it is driven not by human management but by natural processes. There is no point at which it can be said to have arrived. Rewilding of the kind that interests me does not seek to control the natural world, to re-create a particular ecosystem or landscape, but – having brought back some of the missing species – to allow it to find its own way.

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 population of red deer (which in the US are confusingly called elk). But as soon as 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.

In some places, trees on the riverbanks, until then constantly suppressed by browsing, quintupled in height in just six years. The trees shaded and cooled the water and provided cover for fish and other animals, 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.

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. The beavers then triggered [many] effects, creating niches for otters, muskrats, fish, frogs and reptiles. The returning trees have also stabilised 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. 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 river wanders and fish numbers are three times lower. 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. 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. 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. The bears also kill deer calves, reinforcing the impact of the wolves. The reintroduction of the 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 rivers and 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. Despite intense hunting and culling, willow trees disappeared from the meadows and aspens were in danger of vanishing from large areas of the park. 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. 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 meters of a salmon stream comes from the sea: it was brought upriver in the bodies of the salmon. Top predators and keystone species unwittingly re-engineer the environment, even down to the composition of the soil.