Ecologists find plenty of surprises in urban landscapes
Stanley Gehrt took a late-night drive to the cemetery on Chicago’s South Side. Its gate was locked, so he jumped the fence. In the trap he had set earlier, Gehrt found a young male coyote. He drugged it and carried it away.
As he approached the fence, Gehrt spied two men trying to break into his truck. When they saw him leap over the fence and rush toward them, coyote lolling in his arms, the men yelled and ran.
Most people in his field don’t need to worry about urban crime, says Gehrt, an ecologist at Ohio State University. But the hassles are worth it. Since 2000, Gehrt has equipped more than 850 Chicago-based coyotes with ear tags or microchips for identification. He follows the movements of more than 400 of these coyotes using radio and GPS collars.
“When we started, we didn’t think there was going to be much of a study there,” he says. Coyotes usually need large, natural areas to survive, so he expected they would be scarce on the streets of the Windy City. “We were wrong.” He puts Chicago’s coyote population at about 2,000, although he suspects that there are probably many more.
Gehrt is not the first scientist to be surprised by how wildlife can flourish in urban habitats. Most people associate living things with pristine lands far from subways and parking lots, and consider urban territory to be a degraded, beat-up version of nature. But cities are fully functioning ecosystems, and humans are not their only citizens.
While other, more remote environments are sculpted mainly by elements like heat and water, a city’s identity depends largely on humans. People put streets, buildings and tunnels where they want them, and nature fills in the gaps. The resulting mix of habitats — a stand of trees in a park, a green roof on a high-rise, the subway, a sidewalk — is new territory for ecologists. First conceived in the 1970s but slow to gain traction, urban ecology seeks to understand the diversity of life within cities. “Maybe 10 years ago it was considered kind of rogue science,” says Krista McGuire, an urban ecologist at Barnard College, Columbia University. The field is finally making some inroads.
As McGuire, Gehrt and others explore the world underfoot, dealing with would-be thieves and inquisitive onlookers, their research has revealed some unexpected creatures thriving on land that people consider their domain. A few even serve as model citizens, quietly contributing their cleaning services to the benefit of city dwellers.
On human turf
In the 14 years since he started tracking coyotes, Gehrt has followed them from the suburbs into the heart of downtown Chicago. “You have skyscrapers and tremendous traffic volume everywhere,” he says. “There’s just no escape from people or cars anywhere in that landscape.” Yet the coyote population is thriving, and it has grown.
“They show us parts of the landscape that we never notice and never associate with wildlife habitat,” says Gehrt. Those nooks and crannies, he says, “may be critical to their survival.”
He has found coyotes navigating abandoned railway lines, waiting for a stoplight to change and traffic to pause at congested intersections before crossing, and raising kits on top of a parking garage in the shadow of Soldier Field, home of the Chicago Bears. Leery of humans, city coyotes must wait until night falls to venture out to forage. During those limited dark hours, they patrol home ranges more than four times larger than they would need in the suburbs. Clogged with human settlements, city land offers fewer spots to find food.
As he tracks coyotes’ movements, Gehrt also studies their diets. Initially, his analysis of coyote scat indicated that the animals rarely gobble human food. Now, using a technique called stable isotope analysis, he is finding that individual coyotes’ meals can vary. The Chicago coyotes still rely mostly on natural prey, such as rodents, other small mammals and wild fruits. However, discarded food is a larger part of some of their diets than earlier studies suggested, Gehrt’s team reports in an upcoming issue of the journal Oecologia.
“In urban areas, the critical question is how much are they relying on human foods?” says Gehrt. What the coyotes are eating may help explain why they are drawn to certain habitats.
To do the isotope analysis, Gehrt snips a whisker off a captured coyote. That whisker contains a four- to six-month record of the coyote’s food intake. Gehrt analyzes the strand for carbon signatures deposited in the growing whisker after the coyote digested and metabolized its meal.
Human foods are high in certain carbon compounds that are less common in coyotes’ natural food. “The isotope stuff won’t separate French fries from doughnuts or hamburgers, but it will separate all those items from prey or other natural coyote food,” he says.
To do his measurements, Gehrt must capture a live coyote, a particularly difficult task in cities. And he can’t lay the traps in areas where people or pets might trigger them.
“We’re basically working in a fishbowl with 9 million people able to see what we’re doing out there,” he says of his studies in the Chicago metropolitan area. On farms or federal wildlife reserves, researchers can set snares and padded leghold traps without fearing that people will harass or try to free captured coyotes, he says.
Getting funding for research on urban coyotes can also be difficult. State agencies are more eager to fund management of game species in forested areas, Gehrt says. “People still have a hard time seeing cities as burgeoning, thriving populations of wildlife that also need to be understood,” he says. “They still, I think, view urban areas as wastelands for wildlife as opposed to opportunities for wildlife.”
Like coyotes, bats have managed to thrive in cities, at least in Central Texas. Han Li, an ecologist at Baylor University in Waco, is investigating how bats use the varied urban landscape across the city. He records the bats’ echolocation calls, films them, and occasionally catches them in skeins of fine mesh called mist nets. Within the one small metropolis of Waco, he has identified eight of the nine bat species that inhabit Central Texas.
Inside the city, bats find homes similar to their natural roosts, Li and a colleague reported in the December Urban Ecosystems. Cave-dwelling bats such as Mexican free-tailed bats (Tadarida brasiliensis) are drawn to the tall, old buildings in downtown, which are less carefully sealed than modern skyscrapers. The bats use the height of the buildings to drop into flight.
Bat species that roost in trees, such as evening bats (Nycticeius humeralis), seek out greener neighborhoods. Strangely, these forest-loving bats also gravitate to wealthy neighborhoods regardless of how green they are. Li speculates that landscaping and a lack of hungry stray cats and dogs may be factors.
Like Gehrt, Li has found that people are present at every step of his fieldwork. “You cannot just go in and say, ‘I want to catch bats,’ ” he says. He must clear his work with building owners and the police department, plus he’s got to explain what he’s doing to curious bystanders.
People see him working and pepper him with questions. “Usually by the end people are like, ‘bats are pretty cute if you look at them closely,’ ” Li says. He even called on Waco’s human denizens via local media to help him scout bat territory so that they could tell him where the bats are. “That helped a lot for me, to locate buildings and get permission to sample,” he says.
Amy Savage, an ecologist at North Carolina State University in Raleigh, also sees the benefits of sharing fieldwork with city dwellers. “There are two schools of thought right now in urban ecology,” she says. One says keep the experiment hidden or camouflaged. The other says, “make it as obvious as possible — put signs up, tell people that there’s real science happening.”
The openness approach leaves experiments vulnerable to vandalism. “But it meets another need of science, which is public communication,” Savage says. Her team has already scouted several high school principals who are interested in hosting her experiments about how insects devour discarded food on school grounds. She’s on the fence about whether to conceal or share her experiments with passersby. “I’m still trying to decide which approach is best,” she says.
When most New Yorkers sit on a bench to enjoy a steaming hot dog or a bag of chestnuts, they don’t think about what happens to the crumbs they inadvertently scatter on the ground. But ants and rodents devour much of the food dropped across the city; they may be earning their keep as a mini garbage service.
Savage found an interesting way to measure the impact of cleanup ants. She set up tiny cages in Manhattan covered with mesh to keep out rodents but let in ants and other arthropods. She and her colleagues littered hot dogs, cookies and potato chips both in the cages and on open ground. In parks and street medians, where food was fair game for all species, including rats, squirrels, raccoons and pigeons, more food disappeared from the open sites.
In some of the street-median cages, however, arthropods gobbled up almost as much food on their own as both rats and other critters and arthropods on the outside. The street-median arthropods also scarfed down two to three times as much food per day as those in parks.
These findings indicate that ants (especially the pavement ant, Tetramorium species E) and other arthropods are voracious and are key players in garbage disposal on ultraurban street medians. Rodents, on the other hand, eat more in parks, Savage reported in December in Global Change Biology. The ants may even be competing with rats and other pests for food. She and her team estimate that arthropods alone could devour up to 2,100 pounds of food waste per year just from Broadway and West Street’s medians — the equivalent of about 19,000 hot dogs.
“Urban food waste is one of the biggest problems in cities, and the fact that animals living in cities help us with that has really been underappreciated,” Savage says. Uneaten food ends up in landfills where it decomposes to produce the greenhouse gas methane, she says.
“Anything that species living in cities can do to remove that food … is really going to help people,” she says. “It is exciting to consider that ants, which seem small, may have very strong influences on ecosystem process and that these effects may be particularly important inside cities.”
Like Stanley Gehrt, Savage is using stable isotope analysis to find out how much the ants rely on human food. “Most of their carbon is coming from human food. That is not the case for the ones living in forests,” she says.
New York also has tenants that are even easier to miss than ants, but might offer a different type of utility. McGuire, of Barnard College, has found a surprisingly wide diversity of microscopic fungi and bacteria on New York City’s green roofs, where plants and soil insulate the buildings beneath them and absorb storm water before it becomes runoff.
Many of these microbes are beneficial to plants growing in stressful environments — such as the shallow, nutrient poor, sun-drenched soil on Manhattan rooftops. The beneficial fungi snuggle in and on plant roots, growing filaments that search the soil for nutrients the plants require. In turn, the plants provide their resident fungi with sugars from photosynthesis.
Some of the fungi that McGuire identified in the soil are also thought to protect plants from pathogens and drought. Other fungi she found, among them species of Penicillium and Aspergillus, have the capacity to degrade certain pollutants, such as the hydrocarbons in car and factory exhaust.
But whether the microbes are meeting their genetic potential remains to be seen. “Even though microbes may have DNA sequences for all these functional genes, it doesn’t mean they’re using that program,” McGuire says. “We’re trying to figure out if they’re actually performing that function.”
She is also cataloging the diversity of microbes in soil from additional green places across the city, including the popular High Line elevated park, and investigating whether microbes in ground-level green spaces are cleaning incoming storm water.
Slowly, ecologists like McGuire are beginning to fill in the gaps about how animals, plants and microbes share space with city-dwelling humans and provide services in return.
“The fact that we don’t understand even the basics of patterns of diversity across cities is somewhat mind-boggling,” Savage says. Urban ecology will become increasingly vital as our world becomes more and more developed. “More than half the world’s population lives in cities and it’s only projected to increase,” McGuire says. “So it’s pretty important to get a sense for what kind of diversity an urban environment can harbor.”
In the 19th century, Charles Darwin and Alfred Russel Wallace embarked on trips across the seas to survey life in unfamiliar habitats. “We’re kind of like the Darwins and Wallaces of the urban microbial environment,” McGuire says.
Voyagers of a sort, urban ecologists are called back from the “pristine” areas traditionally prized by ecologists to chart habitats much closer to home.
A different kind of laboratory
Skyscrapers and sidewalks don’t provide the same breathtaking scenery as rainforests, mountains or deserts. But then again, big cities like New York are the only field sites where ecologists can gather data in the morning and catch a Broadway show in the evening.
“The historical perspective has been that you have to get away in a remote area to really get a good picture of ecology and appreciate the world around you,” says Amy Savage of North Carolina State University in Raleigh. “But I like that I study a system that I can learn about when I walk to work in the morning.”
Her city work resonates with people she talks with as well. “When I talk to people about tropical rainforests, they are interested. They know deforestation is a problem — but it seems so far removed from their daily lives. This project is much more personal and close to home for people.”
Plus fieldwork in cities lacks the dangers that come with more remote habitats. “In the tropics you have to worry about malaria and dengue and leeches and snakes,” says McGuire, who also works in Malaysia and other tropical locales. “I’ve had amoebic dysentery and many giardia infections.”
While some ecologists happily roam urban and remote terrain, others prefer to stay within city limits. Working in a city, surrounded by people, suits Han Li, an ecologist at Baylor University in Waco, Texas. “I don’t get any pleasure from camping,” he admits.
Jonathan Hogg agrees. “I grew up in cities — I’m not quite as hard-core as some of the other people I’ve known in wildlife biology who are more attuned to that Grizzly Adams sort of lifestyle,” says Hogg, who has surveyed birds of prey in St. Louis’ business parks for the University of Missouri.
“I think as people start to realize that there’s more going on in our cities and urban areas, there will be a growing interest” in urban ecology, he says. “Especially for people like me who don’t necessarily want to camp out for weeks at a time but are still interested in wildlife and conservation.” — Kate Baggaley
This article appeared in the January 10, 2015 issue of Science News with the headline "The Concrete Jungle."
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