Wild Cities: It’s a Jungle Out There
For most of her career, Christine Padoch did her environmental research in distant, exotic locations like the rainforests of Amazonia and Borneo, while Steven Handel studied evolution in the Galapagos Islands. Now Ms. Padoch, an ecological anthropologist, takes the subway from her job at the the New York Botanical Garden in the Bronx to count exotic vegetables at the green markets of Queens, while Mr. Handel, a professor of evolutionary biology at Rutgers University, is studying the vegetation that grows along the tracks of the New Jersey transit railway — a true test, if ever there was one, of the survival of the fittest.
Their projects are examples of the new frontier of environmental studies: urban ecology.
Until recently, the only real environments thought worth studying were in ”pristine” nature, remote areas as far as possible from the footprint of human beings. Cities, by contrast, were seen as unnatural, nonenvironments, whose parks and gardens, ornamental plants and scraggly sidewalk trees and weeds were of as little interest to ecologists as house cats and lap dogs are to big game hunters.
Now, though, ecologists are finding that cities are interesting, legitimate environments, with surprisingly high levels of biodiversity, and what’s more, that understanding and protecting them may be crucial to our environmental future.
From Paris, Rome and Cairo to New York, Baltimore and Phoenix, cities are all subjects of intense ecological study. Unesco is even thinking of making several major cities, including New York, biospheres, important natural areas to be protected, joining ranks with such traditional natural wonders as Yellowstone and Yosemite national parks.
”If Darwin were alive today, he might be studying Staten Island instead of the Galapagos,” said Mr. Handel, who is, in fact, working to re-introduce native North American species to a portion of what was, until recently, North America’s largest garbage dump, the Fresh Kills waste site.
That ecologists have gone from studying Darwin’s finches to counting papayas and mangos at the Hunt’s Point market in the Bronx, as well as the nearly indestructible flora and fauna that survive inside the vacant lots and abandoned industrial sites of the world’s cities, is part of a much broader maturation of the field, several environmental scientists say.
”The original idea that ecology involved trips to faraway places that people would consider to be pristine reflected a very deep-seated belief that people and nature are separate, which has been dominant in ecology,” said Steward Pickett, a senior scientist at the Institute of Ecosystems Studies in Millbrook, N.Y., who is conducting a major long-term study of the environment of Baltimore. The foundations of ecological thinking, he said, were shaken by studies in the last 25 years that showed that virtually all ”pristine” environments bore clear signs of human intervention: fires, the hunting of animals, the harvesting of plants, herbs, nuts or fruits.
”There is no area left in the world that has not undergone serious human impact, and this makes the whole planet a man-made planet, and cities are only the extreme example of that,” said Christine Alfsen-Norodom, the coordinator of Columbia University and Unesco’s joint program on the biosphere and society.
The shift to urban ecology is also linked to a series of changes in the environment itself: increased urbanization, metastasizing sprawl and global warming. ”The choice is no longer between cities and wildness,” Ms. Alfsen-Norodom said. ”It is, in the face of increasing population, between density and sprawl.”
Density and verticality, the hallmarks of big cities, were once bad words in an ecologist’s vocabulary but are now seen as invaluable allies. By concentrating large numbers of people in limited areas, they leave substantial areas for forests, meadows, wetlands and the wide open spaces needed for many species to survive. ”If large numbers of people didn’t live concentrated in cities, the world would be a nightmare,” Ms. Alfsen-Norodom added.
As ecologists have begun to study city environments, they have been surprised at the level of biodiversity they contain. ”In the New York metropolitan area, within a radius of 50 miles, we have recorded over 3,000 plant species: 2,000 native species and 1,000 introduced to the area,” said Steven E. Clemants, vice president for science at the Brooklyn Botanic Garden.
As the Hudson River has made a comeback, sea horses have been spotted in the Gawanas Canal, and rare centipedes have recently been found in Central Park. New York City is, in fact, a major bird sanctuary. The marshy wetlands of Jamaica Bay near Kennedy airport are among the biggest nesting areas on the East Coast, and big patches of green like Central and Prospect parks look like landing pads to birds migrating north and south with the seasons. Thus, some rather unexpected species — the American White Pelican, the Pied-Billed Grebe and the Red-Throated Loon — can be seen in New York City, while the Downy Woodpecker and the Rock Dove live here year round.
This is not as unlikely as it may seem, Mr. Clemants says, when you consider that New York has a higher percentage of open space — some 25 percent of its land surface — than any major city in the United States.
As ecologists have come to accept the ”impure” nature of urban environments, they have been re-evaluating the role of exotic plants, imported species that have traditionally been regarded as noxious, aggressive pests crowding out native North American species. As a result, there is an intense debate between ecological purists who are trying to conserve and bring back native species and those who feel that exotic plants are the victims of biological xenophobia.
”If we value the composition and the diversity of native species, then we do have to be concerned about exotic species,” Mr. Pickett said. ”The other side of the story is that in many urban environments, the exotics are particularly good at dealing with harsh urban conditions.”
Because the sidewalks, paved streets and tall buildings make the city hotter and its soil drier, plants originally from tropical environments do extremely well here. If you look through a subway grate and see a tree growing at the slimy bottom, ecologists say, it will almost certainly be an Ailanthus, known as the Tree of Heaven, from China and the Korean peninsula. They grow at a rate of two meters a year and survive in soils that would kill most plants. Indeed, of the 3,000 plant species found in New York, fully a third were introduced from somewhere else.
”We owe a lot to invasive species,” said Charles Peters of the botanical garden in the Bronx. ”They are able to grow and reproduce in the extremely screwed-up environment. Over the decades, there is an increase of nonnative species. The natives are dying out or having trouble reproducing. The exotics, like the Tree of Heaven, are continuing to prevent soil erosion, maintaining the cycling of nutrients, pumping out oxygen and providing shade where native species don’t go.”
Mr. Handel, who is co-director of the Center for Urban Restoration Ecology at Rutgers, insists that while obviously an exotic species in a place where nothing else will grow is fine, people should not be too cavalier in accepting the demise of native species. Because they are often less aggressive, they allow for a higher number of different plants in their midst, while the most successful exotic plants are so aggressive that they leave no room for others.
”The final result is a poorer, more monotonous environment,” Mr. Handel said. But with care, as in Fresh Kills and in Flushing Meadows on grounds once occupied by the World’s Fair, native species have made a solid comeback, he added.
While Mr. Peters supports some of these efforts, he thinks some of restoration ecology’s hopes are unrealistic. ”Already, 20 percent of the trees in the Brooklyn Botanic Garden are exotic, and that’s a protected island in an urban sea,” he said. ”We’ve examined the soil, and it’s filled with dormant seeds of the Tree of Heaven just waiting for an opportunity to grow. The average oak may grow 1,000 acorns a year and they all fall near the tree, while the average Tree of Heaven will send off gazillions of winged spores that go everywhere. The acorns all get eaten by squirrels because there are no predators around. You can subsidize the plants that are not very good in this environment or just let the best man win.”
But exotic species can also create problems. In Colorado, Patrick Bergeron, a French ecologist at the University of Colorado in Boulder, said that the sprawl around cities like Denver and Boulder has radically changed the landscape: ”Diversity has increased. The trees mitigate the effect of pollution and attract birds and animals, but they are pumping water like crazy and the whole area is thirsty for water.”
Moreover, he explained, because much of Colorado is a semiarid landscape that will not naturally support trees, the added ”biodiversity” has made the area extremely vulnerable to fire. In a more sparse, unpopulated landscape, fires can serve to regenerate the soil, but in a populated area with lots of trees, fires can spread more easily and become much more dangerous, as they have become in recent summers in the Western United States.
Nonetheless, efforts to support native species do not always work, Mr. Bergeron points out. Ecologists in Phoenix, for example, discovered that people there who were trying to cultivate native species ended up watering them as much as cultivated, exotic plants rather than allowing them to rely on their own resources.
One of the reasons for the shift to urban ecology, says Cynthia Rosenzweig, a senior research scientist at NASA and the Earth Institute, is an increased awareness of global warming. ”Climate change is a global phenomenon,” she said, ”and so it becomes easier to see the links between cities and other environments.”
Moreover, says Ms. Rosenzweig, who helped direct the first extensive study of climate change in the New York metropolitan area, global warming has already begun to affect the city environment. Tropical plants like kudzu, the highly invasive Japanese vine that covers almost every tree in Southern states like Georgia would never have survived an old-fashioned New York winter, but in recent years it has been found growing in downtown Manhattan.
”As we are thinking about the effort to restore the harbor estuary, the Fresh Kills dump site and the wetland areas,” Ms. Rosenzweig said, ”we need to take projection on climate change into account, or we may be attempting to save or restore something that may not be there.”
– November 23, 2002
Published at The New York Times