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Globally, as economic and social change push people from the countryside to cities, swathes of land are being turned over to nature – and it’s thriving. Cal Flynn, bestselling author of Islands of Abandonment, explains: “We’re seeing this all across the developed world, especially in southern Europe. We’re seeing it in East Asia, we’re seeing it in Japan. We’re seeing the return of wolves, bears, lynx in numbers that we really could not have expected a century ago, or even a few decades ago. And these animals do really well in areas where there are not very many people."

When we leave the natural world alone, remarkable things can happen…

While researching her book, Cal visited Verdun in France, where chemical weapons were dumped after the First World War. Even in the most inhospitable conditions, nature had found a way to survive.“There was an area in the woods where the soil was extremely polluted. It was about 17% arsenic, about 13% zinc, 2.6% lead. So, this is a soil that really most plants are unable to grow in, but not all. I did find some sort of scrappy plants making a life there. ”What’s even more fascinating is that some of these plants were detoxifying the land, meaning other species may be able to grow there in future. Cal explains: “We call these ‘metallophyte’ plants – these are ones that love metal… And some of those types of plants will also suck the metal from the soil… and heal over the soil for other plants.”

…But full recovery from pollution and damage can take centuries

We can draw real optimism from nature’s ability to persist in even the most degraded landscapes.

But, Cal says, species are often surviving not thriving, and it can take decades or centuries for ecosystems to fully recover. In the US, New England's celebrated forests have flourished in areas that used to be farmland. From the 1860s onwards, as agriculture moved west, trees took their place. Remarkably, forests now cover 80% of the region, an increase from around 30% in 1850. But where damage is especially heavy, recovery can take far longer. Cal thinks it leaves us with two options for dealing with environmental damage: “we can go in and we can try to fix it… [or] we can leave it to its own devices and be patient.” That brings us on to rewilding…

Rebecca Wrigley, Chief Executive of Rewilding Britain, describes rewilding as, “the mass restoration of naturally functioning ecosystems, until nature can take care of itself, but also take care of us.” She says it’s about supporting and repairing the complex web of life, and finding a balance between people and nature. It’s easy to assume that rewilding means just giving land back to the wild. But it’s not that simple – in fact, Rebecca says, people have a key role in managing ecosystem restoration, particularly at the beginning. Rewilding includes things like allowing rivers to spread and change instead of restricting them, or restoring the natural balance between predators and prey.

The story of the sea otter, a ‘keystone species’ on the US west coast, shows why intervention can be needed. When otter numbers fell, the sea urchins they fed on were able to proliferate. They devoured sea kelp beds that were home to many other species, and the entire ecosystem collapsed. “Until you put back in those keystone species, you can’t see it revive again”, says Rebecca. “So rewilding can be a very active process.”

Rewilding throws up some tricky questions

Rewilding Britain’s polling suggests that 81% of people support or strongly support rewilding. But it’s not popular with everyone. Tom thinks that the word ‘rewilding’ can be problematic for farmers and land owners, as they may feel it undermines the skilled work they do to manage the land. Rewilding projects have hit community opposition too, including a fear of living alongside wild animals.

There’s also a question about whether humans will always have a duty of care to animals we introduce into the wild. Tom says the bison he visited in Kent were managed in a similar way to zoo animals. But a hands-off approach isn’t always realistic: when horses were left to fend for themselves as part of a Dutch rewilding project, they exploded in number, ran out of food and began to starve. The authorities tried not to intervene, but people threw food to the horses anyway.

Tom believes that, to enable nature to thrive while meeting humans’ needs, “we need to look at multi-functional, multipurpose landscapes.” For example, having solar panels in fields of sheep or crops. Jake Fiennes, conservation director of the Holkham Estate in Norfolk, has returned a river to the middle of an ancient flood plain. As well as attracting sea trout, lapwing, curlew, marsh harriers and crayfish, the land will be used for grazing cows and sheep. Jake says that “having farming integrated into this project is key to the success of the project.”

Humans are part of a vast and intricate system

There's one big thing Helen thinks we must remember when making these kinds of decisions. It's that humans are part of Earth's complex web of life, not separate from it, so we need to work with natural systems. “One of the perspectives you get as a research scientist in this area is that you are continually seeing how intricately linked everything is,” she says. “We are all part of a giant linked system and we don’t always see these links… But actually, the oxygen we breathe and the environment required to grow our food and how things are moving around us – you know, the weather – everything is all linked together.”

Rebecca agrees: “People are part of nature. We are just one species within an ecosystem, we just happen to be taking up possibly slightly more than our fair share at the moment.”

Listen to Rare Earth: What Happens If We Leave Nature Alone?