When conservationists worry about the prospect of a world without wildlife, they often focus on two related developments: the sprawling growth of crowded cities and suburbs and the push to farm more land, and farm it more intensively, to feed those cities. Together, these two forces have worn the natural world down to tattered remnants.

So it may seem contradictory to suggest that cities can also be part of the solution. But conservationists, who used to focus on protecting landscapes that were pristine and full of wildlife, now often work instead to improve the margins—to make roadsides, backyards, idle fields, and working waterfronts wildlife-friendly. They argue that with a little effort, cities can provide habitat for birds, butterflies, pollinators, and other creatures great and small. According to this line of thinking, re-wilding the cities will be better not just for wildlife but for the cities. The idea is that the metropolis is a far richer place to live—more magical even—to the extent that it is also a zoopolis.

It’s a grassroots movement—or maybe, an anti-grassroots movement, with urban landowners planting habitat in place of lawns. But it’s official too. At its 2010 meeting in Nagoya, Japan, the United Nations Convention on Biological Diversity set targets to reduce the loss of habitat and to restore degraded natural areas. The plan includes a City Biodiversity Index, now being tested by 50 cities around the world as a tool for measuring and improving urban ecosystems.

So which cities are leading the movement to a zoopolitan future? What lessons are they learning that other cities can apply?

1. Over the past 30 years, Singapore has increased its natural cover—trees, parks, and green roofs—to almost half its land area, even while doubling its population to 5 million people. The original intent was commercial, because carefully maintained street trees and parks are signs to outside investors of a stable, prosperous community. That way of thinking is still evident, for instance, in the weirdly futuristic World's Fair appeal of the 250-acre Gardens by the Bay project, a hugely popular park recently developed on reclaimed land.

The effort has since extended beyond self-interest: Singapore has also successfully reintroduced Oriental pied hornbills, and officials there have plans to boost populations of other species, including its critically endangered banded leaf monkey. Authorities have declared their intentions to make Singapore “a city in the garden,” and they seem to recognize that a garden is an empty stage if there are no birds or other creatures chattering in the trees.

2. In 2003, officials in Kampala, the capital of Uganda, wanted to drain the 1,300-acre Nakivubo Swamp and convert the land to industrial development and housing. Then a study pointed out that the wetlands provide essential treatment of sewage and other wastewater cast off from the urban area. Over and above the cost of construction, operating a treatment plant to replace what the swamp was doing for free would have cost the city $2 million a year. The wetlands are now a protected area.

3. The 1.5 million Mexican free-tailed bats inhabiting the nooks and crannies of the Congress Avenue Bridge once seemed like a nuisance to planners in Austin, Texas. But a campaign by Bat Conservation International has turned the bats’ nightly emergence into a major attraction. Evenings in August, picnicking families assemble on grassy areas nearby, and tour boats jostle on the river below like Land Cruisers on the Maasai Mara, waiting for the moment the bats take wing. The result is an ecotourism business worth an estimated $10 million a year. The city has taken the bats to its heart, and it now has a hockey team named the Austin Ice Bats. The Texas Highway Department, not otherwise known for progressive environmental thinking, has also recognized a good thing and is working to make other bridges in the state bat-friendly.

4. Sao Paulo is the third-largest city in the world, with 11 million people. Yet 21 percent of its land area is still covered with dense Atlantic rainforest including the Green Belt Biosphere Reserve. As a result, the city is home to cougars, capybaras, howler monkeys, and more than 430 other animal species.

5. Cape Town comprises just one-fifth of a percent of South Africa’s total land area while supporting half of South Africa’s critically endangered vegetation types and 3,000 plant species, many of them found nowhere else in the world. The city’s Strandfontein Sewage Works may not sound like such an appealing destination, but birders can spot a hundred species in a good morning there. The city is home to blue cranes, penguins, malachite kingfishers, and many other species. Nearby waters harbor a colony of Cape fur seals, with their attendant great white sharks.

6. My favorite case study is New York City’s Bronx River, for very personal reasons. My father grew up on the banks of the river in the 1920s, and the stories he told were all about going out on the water with his Italian-immigrant grandfather to gather botanicals and to hunt. It was still a wild river then. (In the magical stories my father passed down to his children and grandchildren, it was inhabited not just by a variety of wildlife but also by a menagerie of imaginary creatures, led by a walking, talking green melon ball named the Growly.)

For much of the rest of the 20th century, the Bronx River became a ruin of rusting bedsprings and junked cars, along with sewage and industrial pollution. But an extensive cleanup effort by the Bronx River Alliance and other groups has restored the eight-mile-long lower river, with turtles, alewives, glass eels, great blue herons, and other species back at home there. Beavers returned in 2007—after an absence of several hundred years. City programs now focus on making the river a source of green pleasure for neighboring residents, many of them, like my great-grandfather, immigrants.

The restored habitat is providing homes for wildlife—but it’s no doubt also producing new stories to entertain children, and to be passed down for generations. That makes the city a much richer and more magical place for everyone.