What is a park? For most of us, a park is a place apart—a reserve of nature in a world increasingly dominated by human activities and arranged to fulfill human needs and desires. But a park is also for people— a place of refuge for the human soul, which tends to wither when long separated from green and growing things.

John Muir, the great naturalist, captured this dual purpose at the dawn of the national parks movement. “Thousands of tired, nerve-shaken, over-civilized people are beginning to find out that going to the mountains is going home; that wildness is a necessity,” Muir wrote in 1901. Our concept of parks, especially in North America, Europe, and Australia, has remained largely unchanged since.

Daniel Raven-Ellison, a self-described “guerrilla geographer” and National Geographic explorer, would like to change it.

Raven-Ellison’s home isn’t the mountains—it’s London, a city founded in 43 AD, a metropolis today of almost nine million people, with 14,000 of them, on average, living in each square mile. Raven-Ellison is lobbying for the entire city to be declared a National Park.

In spite of its teeming streets and liberal use of concrete, he points out, London has many features we associate with parks. If you count not only the designated urban parks but also the backyards and the untended bits of land, the city is already 47 percent green space. What’s more, it’s highly biodiverse—and in many spots, quite wild.

A walk through Epping Forest at the edge of the city might turn up a badger, a bat, or a browsing fallow deer. Red foxes stroll the sidewalks and raise cubs in back gardens. Some 8.4 million trees dot the city: birch, lime, apple, sycamore, oak, hawthorn, and many more. The London Underground has even spawned its own biodiversity—Culex molestus, a mosquito that evolved into a new species in subway tunnels.

Meet the People Behind a Movement to Diversify Our National Parks

Unlike most large parks, London is not separate from people and their houses and cars. But that’s not a bug, Raven-Ellison says; it’s a feature.

“London is the most biologically diverse place in the United Kingdom precisely because people are there,” he says. Why shouldn’t that amazing diversity be valued alongside that of more remote and less altered places?

“Rainforest national parks are very different from desert national parks,” he says. “A city is very different from both of those but it is not necessarily less valuable.” By redefining what a park can be, Raven-Ellison hopes to open our eyes to the nature that’s already around us—and expand our ambition for adding more.

View Images Another famous urban park is the Presidio in San Francisco, which offers gorgeous vistas and recreational opportunities like skateboarding. Photograph by Simon Roberts, Nat Geo Image Collection

Biophilia in the City

Raven-Ellison is not the only one calling for an end to the conceptual estrangement between humanity and the five million or so species with which we share the planet. With more than half the human population already living in cities, and that fraction increasing every year, there’s a growing movement to recognize the importance of urban nature.

Timothy Beatley, an urban planner at the University of Virginia, heads up a consortium of “biophillic cities”— including Singapore; Wellington, New Zealand; Vitoria-Gasteiz, Spain; Birmingham, UK; San Francisco, Portland, and Milwaukee—that have committed to weaving ever more greenness, diversity, and wildness into the urban fabric. The group’s moniker derives from biologist E. O. Wilson’s 1986 book Biophilia. In it Wilson argued that humans have an innate love of nature, a connection to other species that derives from our long evolutionary history of living among and relying upon them.

“The basic idea is that nature in cities is not optional but absolutely essential to leading a happy, healthy, meaningful life,” says Beatley. His cities are planting trees, draping the walls and roofs of high-rise buildings in moss and ferns, and installing bird-friendly window panes. They’re landscaping with native plants or with non-natives that support pollinators and other wildlife, and they’re offering incentives to homeowners to plant drought-tolerant and wildlife-friendly gardens.

They’re also boosting childhood natural history education, and encouraging their residents simply to walk out the door in the morning with a different perspective. Do you know the species of trees on your city block? What about the birds in the park? When is the last time you held a worm?

That kind of awakening can happen even in Los Angeles, the West’s biggest, greyest, most-car oriented metropolis. At the city’s Natural History Museum, the citizen science program has collected thousands of observations of slugs, reptiles, squirrels, and other species in Los Angeles, through social media and the app iNaturalist. To give kids a place where they can “put their feet in a pond or their hands in a compost pile,” says Lila Higgins, the manager of the Citizen Science program, the museum also built a busy, buzzy, blooming garden.

Wilson was ahead of his time when he proposed the concept of biophilia. A growing body of research suggests that exposure to nature increases mental and physical energy, makes people more generous and cooperative, boosts the immune system, reduces stress, and lowers blood pressure. The lightbulb that Raven-Ellison and Beatley want to turn on is that you don’t have to leave the city to reap these benefits.

View Images The oldest of the city's eight Royal Parks, St. James's Park offers lovely views. Photograph by Simon Roberts, Nat Geo Image Collection

Half London

Raven-Ellison has collected support from prominent London politicians, including mayor Sadiq Khan, for his idea of a “National Park City.” It would be more like an honorary park, he says—a vision that flows from the government down to the people, through rules and regulations, but also the other way, from a mass movement of city dwellers. “It’s achievable by lots of small and everyday actions,” Raven-Ellison says. “People can do something very simple: Punch a hole through the fence to let the hedgehog go through, pave a driveway with succulents; let a garden go a bit wild.”

Raven-Ellison’s most measurable goal is to make London 51 percent green, up from the current 47 percent. It’s a goal he came up independently from Wilson—who in his most recent book, Half Earth, proposes that half the planet should be set aside as “inviolable natural reserves” in order to preserve species and wild, diverse ecosystems. Humans would visit the reserves only occasionally, although they could observe them more frequently via webcam.

A paper published last week in the journal BioScience by an international team of conservationists argues that the goal of protecting half of Earth is “not only aspirational but also achievable” —thanks in part to the urbanization of the planet, which is depopulating the countryside in many regions and allowing more room for nature.

For the aspiration to become reality, though, city-dwellers need to be inspired to love nature, not just to leave it. Hands-off reserves need to be supplemented, says Raven-Ellison, with green spaces inside cities, spaces where nature speaks to humans daily through their eyes, ears, nose, and skin. Half Earth needs Half London.