Intro | Profile | Ecosystem Assessment
It might seem funny to apply the term "ecosystem" to cities, but urban ecosystems are perhaps the most familiar of all ecosystems to us. An urban ecosystem is simply the community of plants, animals, and humans that inhabit the urban environment. It is an area physically dominated by built structures like buildings, roads, sewers, and power lines. But it also contains a rich patchwork of green spaces — parks, yards, street plantings, greenways, urban streams, commercial landscaping, and unbuilt lots — that provide the living heart of the urban ecosystem.
As separate and fragmented as these elements sometimes appear, they work together as a single organism. The urban forest is a good example. It's not hard to envision all of a city's trees — whether in a park, on a street, in an undeveloped parcel, or in a backyard — linked together in a citywide system, just as they seem to be when viewed from an airplane. This urban forest may not be identical to an undisturbed rural forest, but many of its functions are similar.
Of course, there are many differences between urban ecosystems and other ecosystems less dominated by humans. Urban ecosystems are generally highly disturbed systems, subject to rapid changes in soil and plant cover, as well as temperature and water availability. Buildings, roads, parking lots, and other constructions form a largely impenetrable covering of the soil that affects how water flows through the urban landscape and what can survive there.
The plant life in urban ecosystems is different too, characterized by many nonnative yard plantings and weeds. Even in the city's natural or seminatural areas, like parks, the vegetation is often highly altered, with many nonnative and invasive species. For example, in wooded areas, the ground leaf layer may be removed and replaced with shade-tolerant grass, disrupting the natural processes that create healthy soils.
Environmental stresses also modify the natural elements of urban ecosystems. Trees are subject to high levels of air pollutants, road salts and runoff, physical barriers to root growth, disease, poor soil quality, frequent drought, and reduced sunlight. Animal and bird populations are inhibited by the loss of habitat and food sources, toxic substances, and vehicles.
In spite of these harsh conditions, city green spaces are filled with life — some of it quite diverse and biologically significant. In Washington, D.C., a 1993 bird survey identified 115 different bird species — a number almost as high as that found in nonurban sites nearby. The Chicago suburban area, meanwhile, contains some of the best remaining fragments of tallgrass prairie and oak savanna east of the Mississippi River.
What's the Extent of Urban Ecosystems?
Urban ecosystems currently cover about 4% of the world's surface. But with the rapid growth of cities worldwide, urban ecosystems are perhaps the only major ecosystem type that is expanding. The building of roads, housing, power lines, and industrial and commercial sites generates new urban landscapes as it fragments former forests, grasslands, or farmlands in the relentless march of development.
What Goods and Services Do Urban Ecosystems Provide?
The human elements of the city — its man-made infrastructure and economy — provide goods and services of enormous value. Cities are the heart of human commerce and industry, and therefore the primary centers of employment, housing, transportation, and the range of social services from health care to education. But urban green spaces contribute their own unique and essential services to the urban mix.
- Shade and Temperature Control: Street trees and other green spaces help to battle the urban heat island effect in the summer. Temperatures in the heart of a city may be 0.6-1.3ºC (1.1-2.3ºF) warmer than in rural areas due to the large heat-absorbing surfaces of buildings and asphalt, as well as high energy use. Trees provide shade and also transpire large amounts of water that, when evaporated, provide a cooling effect. When combined, these effects can lower local temperatures — sometimes by many degrees in the vegetated area. The shading effect of trees translates to energy savings and reduced regional pollution. One study in Chicago estimated that planting about three more trees per building lot could save $50-$90 in air conditioning costs per house.
- Air Filtering: City plants are efficient air pollution removers. In a park, the leaf surfaces of trees can filter out as much as 85% of the ambient air pollution — mostly particulates. Street trees can also be effective air purifiers, removing up to 70% of particulates on a tree-lined street. Urban forests in the Baltimore-Washington region remove some 17,000 tons of air pollutants per year, worth at least $88 million annually.
- Noise reduction: Trees and shrubs can help filter out noise pollution too. A 30-meter belt of tall dense trees combined with soft surfaces can reduce local noise levels by 50%.
- Stormwater Control: City watersheds function differently than more rural watersheds because they are covered with buildings and paved areas that water can't penetrate. Urban forests, wetlands, and streamside vegetation help to restore some of the natural balance by buffering stormwater runoff, absorbing pollutants, and recharging groundwater reservoirs. Forests in the Washington-Baltimore corridor are estimated to have saved the region about $1 billion in costs for constructing stormwater control facilities like retention ponds and reinforced stream banks.
- Biodiversity and Wildlife Habitat: Cities support a variety of plants and animals. In fact, the extent of "urban wildlife" surprises many people. Deer and small herbivores such as squirrels are prevalent. Muskrats and beavers may be widespread in urban water areas, and some smaller predators like bats, opossum, raccoon, coyote, fox, mink, and weasels adapt well to the habitat changes wrought by development. Urban ecosystems also provide critical habitat to many migratory species. For example, urban green spaces and parks are critical to many migratory birds that otherwise would face long stretches with no resting and feeding places. In many instances, these wildlife provide us with recreational opportunities. More than 40 million people — almost a third of urban residents in the United States — participate in wildlife watching activities within 1 mile of their homes, according to the US Park Service.
- Recreation, Aesthetic, and Spiritual Values: : Parks and green spaces provide city dwellers with invaluable recreational opportunities. Just ask the bikers, joggers, skaters, and dog-walkers in any urban park, any day of the week. Also, we tend to dote on the patches of nature we surround ourselves with — our lawns and yards. Urban real estate costs continually bear out the value we place upon landscaping, yard size, and the proximity to parks.
- Food Production: Although it is not so prevalent in the US, urban agriculture is very important on a global basis. Worldwide, some 800 million city residents grow food in backyards, vacant lots, roadsides, and small suburban farms. In Kenya and Tanzania, 2 out of 3 urban families are engaged in farming. In Taiwan, more than half of all urban families are members of farming associations. In Cuba in 1999, urban agriculture produced 800,000 tons of fresh organic produce and employed 165,000 people.
Intensive and rapid urban growth is the greatest pressure on urban ecosystems. The prospects for global urban growth are startling. Urban areas will add some 2 billion new residents worldwide by 2030, growing from the current urban population of 2.9 billion to 4.9 billion. In fact, in today's world, nearly all population growth is urban growth. Rural populations will remain steady over the next 30 years, while growth is channeled into cities and suburbs.
Growth of giant cities — megacities with more than 10 million people — will be one factor in future urban growth. However, most urban growth will not occur in these megacities, but in smaller cities. What that means for the landscape is that urban areas will be spread over a significantly larger area than today, changing natural areas like forests, grasslands, and farms into urban and suburban environments. Within existing cities, more people will use parks and other green areas, and development will gradually fill in vacant parcels, increasing the stress on the remaining green areas.
To put a more local face on urban growth, consider that over the next 3 decades, the population of US cities will increase by more than 2 million people per year. Today, three quarters of the United States' population is urban — about 215 million people. By 2030, that will increase to 280 million — about 85% of the population.
The growth pattern in the US, which is characterized by sprawl, is particularly disruptive of the landscape. As a rule, US metropolitan areas have grown much faster in area than in population. For example, the land area covered by the Detroit metropolitan area grew by 65% between 1960 and 1998 as new rural land was incorporated by the suburban matrix, but the Detroit population grew just 26%. At the same time, the budget for caring for urban forests and green spaces is not increasing at the same rate as urban pressures are growing. As a result, the vital green assets of many cities — their green infrastructure — are declining in health.
Pressures on urban ecosystems in other nations, particularly in Asia, will be even more intense. China will have to cope with an estimated 11 million new urban dwellers each year. Chinese officials estimate that some 40 new cities will spring up each year to help absorb this onslaught of urbanization.
Can Urban Areas Be Managed as Ecosystems?
Managing urban areas as ecosystems begins with a better understanding of what these green oases contribute to urban life, as well as an appreciation for the pressures they face and the rapid changes now occurring. Many communities still see green areas as more a luxury than an essential component of the city's infrastructure like its roads or sewers. The result is a lack of planning and budgeting for the care of green spaces. For example, many cities lack systematic tree-care programs, and little attention is paid to the effects on the urban forest of soil conditions, restrictions to root growth, droughts caused by the channeling off of rain, the heat island effect, and the lack of undergrowth. Even if attitudes toward green areas are supportive, most communities manage their green areas as individual units rather than as an interconnected citywide system.
But practices have started to change. In some urban areas, city managers and civic groups have begun to seriously inventory the biological wealth of their urban green spaces and to monitor how they are changing over time. In Chicago, an umbrella organization called Chicago Wilderness unites more than 130 different groups — from butterfly collectors to planning agencies, scientists to volunteer prairie stewards — in an effort to monitor and restore the metropolitan area's rich legacy of more than 200,000 acres of parks, preserves, and waterways. The goal is to manage all the green spaces as a single living ecosystem that benefits both wildlife and people. In Baltimore, scientists funded by the National Science Foundation try to advance the infant science of urban ecology by studying the city's biological metabolism — how its green elements interact with surrounding rural areas, as well as with the city's human inhabitants. These fledgling efforts point to a new way to assess and manage urban green spaces, but such efforts are still few and far between.
Any attempt to improve the sustainability of urban ecosystems must identify ways for cities to exist in greater equilibrium with surrounding ecosystems. Like all other ecosystems, urban ecosystems are not isolated islands, but part of a larger web of life. What goes on in cities affects all the ecosystems that surround them — the nearby forests, farms, and waterways. More than any other ecosystem, cities are permeable in their environmental effects. They draw resources from other ecosystems all around them — food, water, building materials — and often export wastes and pollution. They can be a tremendous burden on the surrounding natural ecosystems.
The good news is that urban areas present tremendous opportunities for greater efficiencies in energy and water use, housing, and waste management. Strategies that encourage better planning, mixed-use development, urban road pricing, and integrated public transportation, among other efforts, can dramatically lessen the environmental impacts of billions of people.
The fact that land use changes rapidly in urban areas is a management and planning challenge, but also an opportunity. For example, the million or more brownfields that scar cities worldwide offer the chance to create new green spaces or lessen congestion and development pressure on remaining green areas. If well managed, urban green spaces can add to the already proven health and education benefits of urban ecosystems.
| CITY | TREE COVER (%) |
| Baton Rouge, Louisiana | 55 |
| Waterbury, Connecticut | 44 |
| Portland, Oregon | 42 |
| Dallas, Texas | 28 |
| Denver, Colorado | 26 |
| Los Angeles, California | 15 |
| Chicago, Illinois | 11 |
| Lancaster, CA | 0.4 |
Tree cover in cities varies because of differences both in management and in the natural environment, particularly precipitation.
Source: This profile is adapted from the companion book, World Resources 2000-2001.
For comprehensive data about the world's ecosystems, visit EarthTrends at http://www.earthtrends.wri.org/.
The Value of Ecosystems
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