Baltimore Ecosystem Study
Listen nowAs of this year, more than half of the Earth's six billion people live in cities. In the United States, which in this respect is typical of the developed world, 80 percent of the population lives in urban areas, and metropolitan areas collectively occupy 20 percent of the country's land area.
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| Baltimore’s Inner Harbor fronts the Chesapeake Bay. Water quality in the bay is negatively affected by paved surfaces, and by fertilizer and pesticide runoff from nearby farms and suburban lawns. |
Despite our overwhelming numbers, the proportion of the Earth's surface that humankind has covered with built structures is miniscule, only 0.2 percent of the planet's total land area. Yet the impact of cities on the environment is disproportionately large. One way scientists describe cities is to compare them to coral reefs. Both support high amounts of diversity and cycle through large amounts of material, and both depend on huge inputs of energy and matter from outside their boundaries, such as the open ocean or grain belts and manufacturing centers.
Cities would be worth studying for their impacts on the global environment, but there is more to them than that. As the backdrop of nearly all Americans' waking hours, cities are the only nature readily available to most of us. Yet the usual feeling, even among some ecologists, is that given the pollution, crowding, and ecological disturbance in most urban areas, cities must be environmentally broken, hostile to life, and devoid of biodiversity. That this is not so is perhaps the most incredible finding of a Long Term Ecological Research (LTER) study in Baltimore: In some ways, the creatures in cities are thriving. It's just that until now, we hadn't known what to look for.
Science and the City"Do you ever watch the TV show The Wire?" asks social scientist Morgan Grove, referring to the bleak HBO series that depicts the struggles between police and drug-dealing gangs in Baltimore. "That's our neighborhood."
Hard as it is to believe, Baltimore's infamous west side, which is known to the scientists who study it as watershed 263, is an important part of an effort to foment a revolution in the study of living systems.
"This is one of the most exciting things in environmental science in the last 20 years or so," says Peter Groffman, a soil scientist and Grove's colleague. Both work on the Baltimore Ecosystem Study (BES), which, along with a parallel study of Phoenix, Arizona, is the first large-scale, long-term study of an urban ecosystem ever conducted in the United States.
The BES began in 1997 with a grant from the National Science Foundation. It is one of 26 LTER studies, located in such varied ecosystems as the coast of Santa Barbara and the Arctic, that measure how ecosystems change over time. By studying sites that fall along a gradient from relatively pristine to completely urbanized, scientists can understand how urban environments differ from their less-developed counterparts.
The BES takes place in Baltimore, its surrounding suburbs, and nearby agricultural areas. Like the ecosystem it covers, it is complex and multifaceted, and is unusual in that it employs nearly as many social scientists as ecologists. Both groups collaborate on interdisciplinary research that incorporates traditional sciences such as geography, meteorology, demographics, and modeling, and, because so much depends on humans in urban areas, social sciences, public health, and education.
Twenty years ago it would have been heresy for a team of ecologists to be given tens of millions of dollars in federal grant money to study Baltimore, which was settled by Europeans nearly 300 years ago, and Native Americans centuries before then. Except for Phoenix, all of the other LTER sites are located in habitats where humans rarely venture—the sorts of places in which young graduate students imagine themselves living in a tent and waking to the sounds of songbirds. Or most young graduate students, at any rate.
"I've always worked in urban areas," confesses Charles Nilon, a professor at University of Missouri-Columbia who has studied urban birds for the past ten years as part of the BES. Nilon's unusual pedigree is typically atypical for members of the BES, many of whom enjoy studying environments their colleagues went into ecology to avoid.
Nilon and his fellow BES scientists try to synthesize their research, whether they are ecologists or the many social scientists whose work is integral to the project. However, on their own, they, like the majority of the world's scientists, are subspecialists whose work is a narrowly focused attempt to understand phenomena. In the BES, those phenomena are the features of a city that determine the nature of its most visible nonhuman inhabitants.
Cities of a Feather
Nilon studies some of the most forgettable species on Earth. They are ubiquitous, and therefore easy to take for granted. You probably saw at least one of them today, squabbling over crumbs or perched on a power line. If you live in a densely urban area, it was likely a pigeon, chimney swift, or house sparrow. If you live in a more suburban area, it might have been a starling, grackle, mockingbird, crow, mourning dove, or robin.
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| Pigeons are commonly found in cities around the world. |
These nine types of birds showed up at more than half of the randomly selected sampling points in Nilon's study of the distribution of birds in Baltimore City. Like many city-dwelling organisms, these birds are truly synanthropic—their relationship with humans surpasses mere cohabitation.
Some synanthropic species have been with humans so long that no one knows exactly where they originated. House sparrows, for example, came from somewhere in Europe or Asia, but exactly where is unknown. And while rock doves—otherwise known as pigeons—still live in some nonurban areas, without humans they would not be such a runaway evolutionary success.
Of the nine types of birds that recurred in his Baltimore study, "you can substitute one or two of those and you'd see those same species in just about any city in the U.S.," says Nilon. "If you go to European cities you might find a slightly different [set] of birds, but it would be a similar group with similar requirements. For example, there are also jay species and crow species that can be found across the world in urban areas—they're slightly different, but they have similar niches."
In a way, it makes sense. Cities everywhere are similar—parks, streets, houses, trash. Nearly all cities are mosaics of built and unbuilt land, natural and artificial surfaces, disturbed and relatively undisturbed areas. Like the Amazon versus the African rainforest, or the Siberian versus the Canadian arctic, all cities are the same, but different.
Cities are, in general, warmer than the surrounding countryside. Scientists at the Phoenix LTER study suggest that cities do not experience hot and cold, or rainy and dry seasons like most ecosystems; that, regardless of their latitude, "cities can be viewed as 'pseudo-tropical bubbles.'"
Due to the patchiness of cities and to people's aversion to having bears, coyotes, and moose around their offspring and pets, urban areas typically lack large predators and herbivores. There are exceptions: Washington, D.C., for example, is inhabited by white-tailed deer and coyotes, because Rock Creek Park runs through the center of town. But in general, without predators and with year-round supplies of water and food, urban squirrels, raccoons, and other animals' biggest challenge is a constantly changing environment that demands flexibility.
From Earthworms to the Ocean
"On campus, near where I am, there are spots where the earth is moving—it's just unbelievable," says Katalin Szlavecz, an ecologist at Johns Hopkins University and one of a handful of BES soil scientists. The roiling earth she describes isn't a geological effect—it's due to the presence of huge numbers of drought-tolerant, six-inch-long earthworms (Amynthas corticis). Invaders from Asia, the worms' explosive seasonal breeding and enormous size are in some respects overwhelming Baltimore's ecosystem.
"They're called ‘snake-worms,'" continues Szlavecz, whose tone suggests the most rambunctious creatures she usually deals with are small, shy, and slow-moving. "They're large, they jump, and they are very fast. They are tropical worms and in summertime can reach really huge densities."
Like Japanese kudzu, A. corticis are typical of the exotic species that have invaded the U.S. South: They thrive in the heat-bubble effect of Baltimore and other cities in northern latitudes where they might otherwise fare worse.
No earthworm species, including large, jumping earthworms, are native to the northern reaches of the United States, although some may be native to Baltimore. According to Szlavecz, earthworms did not exist in areas covered by glaciers during the last ice age, which at one point extended as far south as New Jersey. They arrived in the ship ballasts of the first European settlers, which was shoveled directly onto American shores. A. corticis is a more recent arrival, and its exact means of entering the U.S. is not known.
Earthworms are good for gardens because they eat detritus such as leaves, mulch, and dead plant matter on the ground. Their feces provide a friendly environment for microbes that convert, or fix, nitrogen—a relatively inert gas we breathe every day that constitutes 78 percent of Earth's atmosphere—into ammonium and nitrate. Unlike nitrogen gas, these two fixed chemicals are usable by plants for growth.
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| A specimen of Amynthas hilgendorfi earthworm, a more common relative of the earthworm A. corticis, found during the BES. (Katalin Szlavecz) |
In areas with adequate rainfall, the availability of fixed nitrogen determines the productivity of plants. For instance, without the miracle of artificial nitrate fertilizers, agricultural productivity would be a fraction of its current level, and it would be nearly impossible to feed the Earth's population. On the other hand, massive amounts of nitrate, mostly in leftover fertilizer, wash into the sea and cause huge blooms of algae that choke oceans worldwide and make the Chesapeake Bay an "impaired waterway."
The problem with A. corticis is that it is so good at turning leaf litter into feces on which nitrate-producing microbes thrive that it is causing nitrate to be produced faster than ecosystems can use it. Much of the surplus is washed away with the rain, at once stealing precious nutrients from forests and dumping them into nearby streams.
The Ecology of Prestige
"You talk to your ecologist friends and they talk about how they stopped cutting their grass because they want it to go back to ‘nature,' and their neighbors hate them—it's like they thrive on becoming outcasts," says BES social scientist Morgan Grove. "So I thought to myself, I'm going to do the opposite—I'm going to nuke my lawn. I'm going to have the greenest lawn possible and see how it enhances my social acceptability."
Grove wasn't merely engaging in an act of crass social climbing when he decided to turn his lawn into a novel shade of day-glo green through assiduous watering and fertilizing. Rather, he was conducting an experiment based on insights gained from his research. By his own account, the experiment was a smashing success. "I'd be at dinner parties and people would say ‘Where do you live?'" explains Grove. "I'd say, ‘Oh I live at the corner of such and such,' and they would say ‘Oh! You're the one with the lawn!'" Their admiration was just one piece of anecdotal evidence that corroborated Grove's previous research, which indicates that a neatly manicured lawn is a powerful force for social status and neighborhood cohesion.
Lawns are the second-most extensively irrigated crop in the United States, trailing only corn and well ahead of wheat and soybeans. And lawns are paradoxes. Although they are energy-intensive and increase pesticide and herbicide runoff into waterways, their bum rap may be somewhat undeserved.
According to results from the BES, lawns may absorb carbon from the atmosphere and sock it away in the soil—a neat trick in this age of concern about rising carbon levels in the atmosphere. But that doesn't take into account the mowing and fertilizing lawns require.
"Using satellite imagery, we can measure the productivity of lawns," says Grove. "So we said, ‘all right, is this a reflection of population density, socioeconomic status, or lifestyle choices?' It turns out that lifestyle choices are the best predictor of lawn productivity. ...When you rely only on ecological variables to explain the variation you see, [you don't get a very good correlation]. But when you include social data, [the correlation] goes way up."
These social factors are part of what Grove calls an "ecology of prestige." He believes they will be essential in understanding how to make cities, where natural and artificial ecosystems intersect, work better. By revealing the forces that shape a city's ecosystem, this approach tells us something about how to wield them. "From a management perspective," explains Grove, "If you want to increase the amount of vegetation in an area, or conserve vegetation…you'd want to appeal to social status."
The Importance of History
By recognizing that humans are a primary driver of the urban ecosystem, the BES is expanding the scope of ecology well beyond what it has ever previously encompassed. It is also opening a Pandora's box of new questions that demand new kinds of data—economic, sociological, and historical.
Why, for example, is a given neighborhood inhabited by people who can afford to lavish attention on their lawns? To answer this question and others like it, scientists from diverse disciplines have traced the roots of Baltimore to its earliest days of European settlement.
In the late 19th century, waterborne diseases killed one in four infants in Baltimore's low-lying areas, so higher parts of the city were considered more desirable for human habitation. In 1931, "a zoning ordinance drew a line and said industrial activity could only happen in [low-lying areas]," says Chris Boone, who specializes in environmental justice.
If Baltimore had been like other cities Boone has studied, including Los Angeles and Phoenix, the low-lying industrial areas would subsequently have been inhabited by groups socially and economically disadvantagedby their race—in the case of Baltimore, African Americans. But a funny thing happened in Baltimore: Whites who preferentially held jobs at Bethlehem Steel and other waterfront plants ended up living close by so they could walk to work. Later, this preference was reinforced by legal residential segregation and zoning laws that grouped Baltimore's citizens by race.
As a result, Baltimore is the inverse of most cities, where toxic waste dumps and other environmental hazards tend to end up in communities of color; in Baltimore, whites are more likely than African Americans to live next to Toxic Release Inventory sites. "[The 1931 zoning ordinance] put an institutional stamp on where those future Toxic Release Inventory sites would be," says Boone.
This finding, which is impossible to generalize to other cities, has profound implications. It illustrates how cities evolve: Just as historical contingency is the only logical explanation for humans' vestigial organs, which are really just modified versions of old adaptations, history has a powerful effect on the current shape of an urban ecosystem's built areas. In turn, people who live in cities make decisions that shape the lives of the area's flora and fauna.
Although historical contingency makes it difficult to generalize some observations about cities, the class and lifestyle differences it creates often have consistent results. For example, even though higher-income residents in Phoenix do not live in old industrial sections of town as they do in some parts of Baltimore, their socioeconomic status has an effect on their surrounding ecosystem.
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| Rowhouses in Baltimore's Union Square, part of the BES study area. (Baltimore Ecosystem Study) |
"[In Phoenix] one of the things I found was that higher-income neighborhoods had high plant-species diversity, while low-income neighborhoods had low species diversity," says Paige Warren, a research associate professor at the University of Massachusetts Amherst and one of the few scientists to conduct studies in both the Baltimore and Phoenix LTER sites. Warren hypothesizes that this difference in plant diversity drove the variability in bird species she discovered.
So it's not just humans that find higher-income neighborhoods more desirable—birds have the same preference. The effect is more pronounced in Phoenix than in Baltimore, where areas that include row houses with lots of long-standing vegetation exist in both rich and poor areas. Regardless, Warren's finding is an example of how social and economic factors have direct and not always intuitive effects on urban ecosystems.
Applying Lessons Learned
"One can think of plant and animal communities that have coexisted in nature for tens of thousands to millions of years, but in urban areas we're bringing species together that have never coexisted before," says Richard Pouyat of the U.S. Department of Agriculture's Forest Service. These communities of organisms are known as "emerging ecosystems," and they are as young as modern cities.
Considering how little of our time on Earth has been spent in a state of urbanized civilizationand what a tiny fraction of the history of life on Earth that represents, it is a marvel of the adaptability of ecosystems that so much life has already taken root in cities. Countless writers of science fiction have tried to imagine what might happen next—mutant cockroaches overrunning cities, the adaptive radiation of pigeons into dozens of new species specialized by varieties of trash—and here urban ecology, itself in its infancy, offers only a few clues.
It does tell us that in the future, the urban canopy will be subtly different. Gone will be the mighty oak and the stately maple; saplings of these species are already absent from the understory in urban areas. In their place, more adaptable, disturbance-tolerant breeds such as cherry will shade the streets and parks of centuries hence.
Not all the changes humans make to the urban ecosystem have to be negative. By using the knowledge that the members of the Baltimore and Phoenix studies are gathering, urban planners, architects, local politicians, and ordinary citizens will have more power than ever to predict the consequences of their actions on their immediate environment. Perhaps, if they can be convinced that it really is an ecosystem, and not just a heap of buildings and roads imposed on uncooperative nature, they will understand the importance of applying that knowledge.
—Christopher Mims is a freelance science writer based in Brooklyn, New York.
ZooGoer 3(3) 2007. Copyright 2007 Friends of the National Zoo. All rights reserved.
