urban ecosystem, any ecological system located within a city or other densely settled area or, in a broader sense, the greater ecological system that makes up an entire metropolitan area. The largest urban ecosystems are currently concentrated in Europe, India, Japan, eastern China, South America, and the United States, primarily on coasts with harbours, along rivers, and at intersections of transportation routes. Large urban areas have been features of the industrialized countries of Europe and North America since the 19th century. Today, however, the greatest urban growth occurs in Africa, South and East Asia, and Latin America, and the majority of megacities (that is, those with more than 10 million inhabitants) will be found there by 2030.

The structure of urban ecosystems

Urban ecosystems, like all ecosystems, are composed of biological components (plants, animals, and other forms of life) and physical components (soil, water, air, climate, and topography). In all ecosystems these components interact with one another within a specified area. In the case of urban ecosystems, however, the biological complex also includes human populations, their demographic characteristics, their institutional structures, and the social and economic tools they employ. The physical complex includes buildings, transportation networks, modified surfaces (e.g., parking lots, roofs, and landscaping), and the environmental alterations resulting from human decision making. The physical components of urban ecosystems also include energy use and the import, transformation, and export of materials. Such energy and material transformations involve not only beneficial products (such as transportation and housing) but also pollution, wastes, and excess heat. Urban ecosystems are often warmer than other ecosystems that surround them, have less infiltration of rainwater into the local soil, and show higher rates and amounts of surface runoff after rain and storms. Heavy metals, calcium dust, particulates, and human-made organic compounds (e.g., fertilizers, pesticides, and contaminants from pharmaceutical and personal care products) are also concentrated in cities.

The expansion of large urban areas results in the conversion of forests, wetlands, deserts, and other adjacent biomes into areas devoted to residential, industrial, commercial, and transportational uses. Such conversion may result in the production of barren land. In addition, the conversion process fragments remaining wild or rural ecosystems into ever-smaller patches, and relatively high amounts of suboptimal habitat are found at the boundaries between the remaining native ecosystems and those that have been modified for human use. Such “edge habitats” inhibit specialist plant and animal species—that is, species that can tolerate a narrow range of environmental conditions. In addition, nonurban ecosystems downwind and downstream of urban ecosystems are subjected to high loads of water pollution, air pollution, and introduced exotic species.

Urban animal communities tend to be dominated by medium-size generalists, such as raccoons, coyotes, opossums, skunks, foxes, and other animals capable of surviving across a wide range of environmental conditions. In contrast, nonurban ecosystems tend to contain specialist species and animals that vary across a broader range of sizes. Urban habitats tend to be dominated by introduced plant and animal species that have a long history of association with humans and that show adaptations to urban conditions. For example, birdsong in urban areas often has a higher pitch and louder volume than is heard in nonurban populations of the same species. Louder, higher-pitched song allows birds to communicate in spite of the greater noise levels found in and around cities and suburban transportation corridors.

Compared with plant and animal communities found in wild and rural ecosystems around the world, biological communities found in different urban areas tend to be similar to one another. This ecological similarity is a by-product of the structural similarities among urban environments (comparable building types, landscape designs, and infrastructure) and of the intentional or accidental introduction of similar species into cities, suburbs, and exurban areas and the water and nutrient subsidies provided by people and their activities. Introduced groups include rodents, earthworms, shade trees, weeds, and insect pests. In addition, animal populations in urban areas sometimes show evidence of genetic differentiation from rural populations of the same species.

Differences in design

Although there are many similarities among the world’s cities, differences arise in culture, history, governance, and the effects of the global economy. Some of the remaining differences arise from physical conditions such as aridity, topography, natural hazards (i.e., the risk of damage by earthquakes, volcanoes, and weather-related phenomena), and the amenities represented by the urban area’s unique location on Earth’s surface. Worldwide, city centres are becoming less dense as people spread into suburban areas, which are in turn becoming denser. The spatial footprints of cities, however, differ from one part of the world to the next. North American cities, such as Los Angeles and Atlanta, are notoriously sprawling, whereas European cities, such as Budapest and Paris, tend to be more concentrated around public transportation. Hence, energy efficiency is greater in concentrated cities than in metropolitan areas showing all the signs of urban sprawl.

Although urbanization has traditionally been associated with a higher quality of life for urban as opposed to rural dwellers, a number of ills such as concentrated urban poverty, environmental inequity, and reduced economic opportunity plague new cities in the developing world as well as old postindustrial city centres. Such problems might be addressed by better planning and by ecologically conscious urban design. One solution could involve an increase in “green spaces” and their more effective distribution through the urban landscape. In fact, the amount of green space actually present in cities is difficult to predict based on the density or age of the settlement. In some dense, older American cities, green space is present—as in New York City’s 6,000 acres (2,400 hectares) of parks, which are part of the city’s nearly 39,000 acres (about 15,800 hectares) of open space. Boston and Cleveland are two other older cities with well-developed open space networks. Some sprawling cities, such as Atlanta, which have grown substantially since the 1970s and ’80s, have very little parkland, whereas others, such as Phoenix, have large desert mountain parks nearby but little open space integrated within the urban fabric. Urban green space, including desert and wetland habitats, provides opportunities to improve the ecological processing of pollution and to moderate the local climate. Open space also imparts green amenities—shade from trees, the aesthetic benefits of natural scenery, and recreational space—for all citizens.

Urban ecology

The emerging science of urban ecology, a subdiscipline of ecology that examines the interactions between organisms and the human-dominated ecosystems in which they reside, may provide additional solutions to urban environmental problems. Ecologists first began to perform comprehensive studies of plants, animals, soils, and environmental conditions in cities shortly after World War II, when there were many vacant sites within European cities. They began a tradition of examining open spaces, which supported volunteer plant communities and the animal populations associated with them. Ecological planning emerged as a professional discipline that applied knowledge of the open spaces in urban areas in an effort to enhance biological diversity and amenities originating from green patches in cities.

.In the 1990s, ecological research in urban areas burgeoned. To deal with the complex mosaic of land uses that now make up cities, suburbs, and exurbs, the traditions were combined, along with knowledge taken from other disciplines, to establish a comprehensive ecological approach to the study of urban ecosystems.

Urban ecology has grown increasingly important as a result of the migration of most of the global human population to cities. One of the by-products of this unprecedented phenomenon is that the world’s urban areas are expanding into environmentally sensitive locations, where they alter ecosystem structure through pollution and land-use conversion of natural habitats. The knowledge gained from studying biological communities in cities may assist in the development of improved urban design and decision making in dealing with such problems.