One hundred years ago at a meeting of the Ecological Society of America, the botanist Frederic Clements floated a profound and controversial idea. Instead of plants responding individually to the environment, he believed, they formed communities that acted as organic units. Later, in 1939, he took the idea further with fellow ecology pioneer Victor Shelford, making the case that animals as well as vegetation were part of these organic units, which he was now calling biomes. The world’s great landscapes are biomes, he argued. The tundra, the desert, the steppe, and the coniferous forest. The concept stuck. Over the years, other ecologists built on Clements’s definition, adding environmental factors such as climate and soil characteristics to the definition; but for the most part, his original vision of biomes as collections of interrelated plants and animals endured.
Yet fundamental to ecology as biomes are, the traditional definition refers only to natural habitats, as if people and cities did not exist. This oversight is causing some to question whether the traditional definitions still apply.
Leading the push for reframing biomes in light of human development is Erle Ellis, an environmental scientist at the University of Maryland. “The concept behind the classic view of biomes is that these global patterns are shaped by climate, terrain, and soils—that sort of thing, but mostly climate,” he tells me over the phone from his office in Baltimore. “But when you look at the patterns now, they are not shaped just by climate. There’s a huge amount of shaping going on from human activity. The most extensive one is agriculture: crops and pastures and range lands.”
The idea that human influence was being overlooked came to him in the early 1990s, when he went to rural China to study how the move from traditional to industrial farming was altering the environment. “Places like that—they are places that, since the last glacial, have not had a natural history. Humans have been using those landscapes for thousands of years and managing the ecology of the whole landscape. So it became obvious there was a lot of ecology that was essentially a human ecology, and my big question was how much of Earth’s ecology has been transformed by our activity. And I set out to answer that question.”
The answer he arrived at was “almost all of it” and that—on land, at least—natural landscapes have largely ceased to exist. In light of this, he drew a new map of the world’s biomes, one that replaced the traditional grasslands, tundra, and forests with eighteen “anthropogenic” biomes that represented the degree to which humans have altered the landscape. At one end were the few remaining wildernesses, places such as Antarctica. At the other end of the scale, the modern metropolis.
But can cities really be biomes? To fit that definition, cities would need to have similar environmental processes and be home to similar communities of plants and animals regardless of where they are. It would mean that the ecologies of Atlanta, Singapore, and Lagos have more in common with each other than the rural areas surrounding them.
That sounds far-fetched, but the evidence that this could be the case is growing. Take the example of Baltimore and Phoenix. On the face of it, these cities sit in very different environments—Baltimore in the humid East, Phoenix in the arid West. But urbanization has altered their climates. Baltimore has become hotter than the surrounding countryside because of the “urban heat island” effect, while the construction of waterways has made Phoenix cooler. The net result is that air temperatures in Baltimore and Phoenix have become more similar.
Temperature is not the only thing that makes the two cities a closer match than their locations would suggest. The residents of Phoenix and Baltimore have similar gardening tastes, so they opt for the kind of lawns that can be seen in almost every American suburb. The result? Phoenix and Baltimore share similar green spaces with similar plant species.
Water systems also bring cities into line with each other. To build Miami, wetlands were drained, whereas the development of Phoenix saw the construction of lakes and canals. Now, when it comes to water, Miami and Phoenix are more like each other than the Everglades or the Arizona desert.
The similarities between cities extend to fauna, too. The animals that thrive in the urban world share common traits. The ability to keep a low profile is one, but urban animals also tend to be fast breeders with flexible behaviors and diets. Just think of the stone martens of Berlin breeding faster than they can be killed, or the coyotes working out how to cross Chicago freeways, or the red foxes learning to forage on Brighton Pier.
These are the garden weeds of the animal kingdom: adaptable, sneaky generalists that can overcome death by making lots of babies. This isn’t true just of mammals. The most successful city birds are those with the most flexible behavior and an “I’ll eat anything” attitude to life—birds such as pigeons and crows.
Size matters, too. Larger animals such as mountain lions, elk, and bears are more likely to be spotted and removed if they go too far into the city, while opossums and rats are small enough to slip past unnoticed. Bigger animals also have a harder time finding all the food they need in the city, and this principle applies as much at the level of tiny phorid flies as it does to cougars.
“The flies found in the center of the city tend to be small,” says Brian Brown, a phorid fly expert who heads the entomology department at the Natural History Museum of Los Angeles. “You expect that with mammals. You expect that there are going to be deer and bears in the mountains but downtown not so much, because there is a limited resource base available for them. But it’s also the case in the tiny, one-to-three-millimeter-long flies.”
Ecologists divide animals into two broad groups: K-selected and r-selected species. K-selected species are those whose numbers are limited by the amount of resources in the environment—animals such as elephants and horses. In contrast, r-selected animals are limited by how much they can breed, and most of the successful urban animals fall into this group.
Add to this our tendency to spread species like the European starling or spitting spider around the world, and the animal life of far-apart cities becomes increasingly similar. More evidence is needed, especially from cities outside the most developed nations, but what already exists suggests that urban areas are a biome and, uniquely, one that is almost entirely man-made.
And that raises an interesting question. Since we control, shape, and design the urban biome, can we mold cities into something that fosters the wildlife we want rather than just a gathering spot for animals that are sneaky, smart, and sex-crazed enough to make it their home? It’s an idea some people are already experimenting with.
The boardwalk’s path snakes around the edge of a b-shaped pond within the grounds of the zoo. On either side of the path are clumps of tall, thin grasses and, among them, a smattering of delicate flowers. The calm water offers a clear reflection of the city skyline, and as we walk, we catch glimpses of the Lake Michigan shoreline that lies just across the road from the zoo.
“This used to be just a concrete-lined pond where they did paddle boats,” says Seth. “But then the zoo, some years ago, decided they wanted to renovate it as a native urban pond prairie ecosystem. They reseeded the whole thing and threw out all the concrete. All of these plants are native Illinois prairie plants that attract certain arthropods and birds.”
As we pass under a small bridge that arches over the narrow part of the pond, Seth points to the small ledges above us. “These are for cliff swallows,” he says.
Elsewhere along the path are strategically placed birdhouses. “Those are for black-capped chickadees because they are cavity-nesting birds and we don’t have a lot of cavities out here. We’ve built them in such a way that the aperture size excludes house sparrows but allows black-capped chickadees. That’s been very successful.”
The pond also teems with life. Fish can be seen moving beneath the surface, and shiny dragonflies flit around near the bulrushes lining the water’s edge.
At the southern end of the pond, there’s a small island peppered with trees. “You’ll probably see some turtles around the island,” says Seth. “We introduced painted turtles, but red-eared sliders and snapping turtles have found their way here on their own. We suspect that the red-eared sliders may have had help, since people buy them as pets and then let them go.”
One unexpected resident of the Nature Boardwalk is the black-crowned night heron, a stocky wetland bird with dark red eyes and black feathers that run from the top of its head and down the back of its otherwise white-and-gray body. These birds, which feed on fish and aquatic invertebrates, are an endangered species in Illinois, yet there is a thriving colony of them in Lincoln Park.
The boardwalk is only the most visible example of Lincoln Park Zoo’s urban wildlife work. As well as creating a slice of Illinois prairie in the heart of Chicago, the zoo’s Urban Wildlife Institute is busy piecing together the ecology of the city.
Seth is the institute’s director. Its flagship project is the biodiversity monitoring study. Its goal is to build a complete-as-possible picture of wild Chicago. “We’ve set up over a hundred field stations. They initiate right over there in downtown Chicago and travel out west, southwest, and northwest,” says Seth, as we plant ourselves on a bench overlooking the boardwalk pond. At each field station are motion cameras that take snaps of passing animals, together with alcohol-filled “pitfall” traps to catch spiders and insects. The team also holds regular bird counts at each station.
“When I look back over my body of work so far, the recurrent thread is that things didn’t turn out the way I thought they would,” says Seth. “One of the things we really expected to find in our data was that we would see fewer deer in sites where we see coyotes. Well, not true at all. The sites that have deer tended to also have coyotes.
“We think it’s just that habitat is so limited and resources so limited that if you’re a deer trying to decide where you are going to browse, you may have coyotes in your patch, but leaving involves going across several roads and highways. It’s a hazardous journey and it’s uncertain whether you will find another patch, and even then that patch may have coyotes, too.”
This, he explains, is not how deer and coyotes behave in more natural habitats. “We have this thing there called the ‘ecology of fear,’ where the deer move around and coyotes sort of track them. But that’s not how it works in urban systems.”
The Nature Boardwalk at Lincoln Park Zoo may be a deliberate attempt at conservation within the city, but even when we don’t act, urban areas are supporting many threatened species. The rebound in the number of peregrine falcons across the world is largely due to their success in cities, and Los Angeles has more Mexican red-headed parrots than the part of Mexico they originated in. The city of Jodhpur helped Rajasthan’s Hanuman langurs endure drought.
Stockholm’s golf courses are also helping at-risk species. Close to two-thirds of the Swedish capital’s golf courses boast bird and insect diversity equal to or better than that within nature reserves, and they can attract declining species such as red-headed woodpeckers.
Bigger animals have a harder time finding all the food they need in the city, and this principle applies as much at the level of tiny phorid flies as it does to cougars. One shining example of urban conservation is the city of Bakersfield, California, which is helping the San Joaquin Valley kit fox survive. This rare subspecies of kit fox has been facing extinction due to habitat loss but, somewhat ironically, has found refuge in Bakersfield, where they live in shipping yards, parks, and golf courses and on undeveloped land.
Life in Bakersfield is good for these small, buff-furred desert foxes, which look as if they evolved to be muses for Japanese anime artists with their short snouts and their extra-big pair of ears that keeps them cool in the heat. There are fewer predators to worry about, plenty of sites for dens, and a steady supply of human food, insects, and ground squirrels to eat. All of which has led the kit foxes of Bakersfield to live longer and breed more than those outside the city.
The kit foxes rarely cause problems for people, either. They are quiet and rarely knock over trashcans. Their most heinous crime is nothing more serious than occasionally stealing golf balls during play—and getting themselves tangled up in soccer nets.
Their fox-cub looks and lack of antisocial behavior have won them plenty of supporters. Some Bakersfield residents have taken it upon themselves to defend the foxes, stopping people from disturbing the animals and even installing artificial dens on their property for them to use.
The idea of using cities for conservation is, however, easier said than done. Mexican red-headed parrots might be abundant in Los Angeles, but what’s the point of shipping them to northeast Mexico to rebuild the original population if the problems that caused their decline there remain? But having cities that act as life rafts for troubled species does at least offer a potential means of doing that.
Of course, urbanization itself has pushed out plenty of species, a good proportion of which now face extinction. But the realpolitik of the situation is that cities are not going to vanish or stop growing unless there’s some cataclysmic nuclear war. We might not be able to reverse the damage already done by urbanization, but that’s no reason not to use cities to supplement our wider efforts to help struggling species, especially when there’s plenty of evidence suggesting that they can do this.
The reasons to use our cities in this way don’t just stop with maintaining biodiversity. Having urban areas that are more wildlife-friendly makes cities and towns better places to live in. Urban wildlife can sometimes be irritating or messy and, in cases such as the leopards of Mumbai, genuinely scary; but for the most part, these unexpected encounters with the animals among us are positive, a cheering reminder that we are not alone and that our cities are far from sterile or divorced from nature. It’s hard not to have your day brightened by a glimpse of a bushy-tailed fox running down the street or a flock of monk parakeets flying across the skyline or a wild boar with piglets in tow holding up traffic.
Working out how we can engineer wilder cities is tricky, though. Urban ecology has gone understudied for many years and, as Seth’s work shows, much of what we know about how ecosystems function in the wild or in rural areas just doesn’t apply in cities. More funding for urban ecology research is going to be needed before we understand city environments well enough to really start designing cities that encourage wildlife effectively.
But that research need not be confined to the halls of academia. The efforts of groups such as Chicago Bird Collision Monitors have, through dedication and studious recording of bird strikes, proven the effectiveness of Lights Out programs and helped both architects and urban planners make cities more bird-friendly. Citizen-science studies, such as the Arthropods of Our Homes project, offer people a way to learn about what animals live around them while also helping to increase our wider understanding of urban wildlife.
We already know that some approaches to fostering urban wildlife work. One approach with plenty of evidence behind it is the creation of green or brown roofs. The idea of creating rooftop wildlife gardens started in Germany back in the 1970s, and the worldwide movement that followed has plenty of success stories to tell. In the Swiss city of Basel, where green roofs are now compulsory on new flat-roofed buildings, these rooftop gardens have become home to significant numbers of rare beetles and spiders. Brown roofs of crushed brick and concrete also take much of the credit for the return of the black redstart, a robin-sized bird with gray-black plumage, to London.
The potential is huge. If a small number of brown roofs in London can bring back the black redstart, just imagine what a city full of green and brown roofs could achieve. And what if we embraced Berlin’s long-held vision for “coherent greenery” and started linking green roofs and green spaces together via green walls? We could also think about the Bakersfield kit foxes and cliff swallows in Lincoln Park Zoo and create artificial dens and nesting sites for the wildlife we want to encourage in our parks and yards.
But before we can do any of this, we’ve first got to stop thinking of cities as barren, anti-nature zones. This environment we’ve built, this urban biome, is teeming with life, but all too often we just blank it out. “I was in a meeting just yesterday, and a woman was there from another zoo, and she made this statement that ‘I love it when kids come to the zoo. For many of them, it’s the first time they’ve ever seen a wild animal,’” Seth tells me as we sit on that bench looking out over Chicago.
“I just had to stand up and say, ‘That’s not true! They have all seen squirrels, they have all seen pigeons … It’s just that you are so attuned to them being around, you no longer think of them as wildlife.’”