Urban areas

Category: Grassland, Tundra, and Human Biomes.

Geographic Location: Global.

Summary: The urban ecosystem is a synthetic biome whose defining characteristic is the significant degree to which human activity has dominated and transformed the landscape.

Urban ecosystems are a unique synthetic biome with defining characteristics influenced by large populations and commensurate human activities. The key ecological pressures include replacement of native vegetation with impervious surfaces such as roads, concrete, and buildings; degradation of native vegetation or replacement with nonnative species; fragmentation of natural or green spaces with a corresponding reduction in wildlife habitat; increased pollutant loads; and changes to the hydrologic regime due to land cover change, such as increased flooding during storms.

Urban areas are not always ecological wastelands, however. They can contain novel collections of biodiversity from native as well as introduced species. Today, there is an increasing emphasis on restoring ecological functions within urban areas.

Because populous urban areas require resources, materials, and energy to be imported from outside the locale, these communities do not fit the classic definition of an ecosystem—which is a relatively self-contained system where energy and nutrients are cycled within its boundaries.

Characteristics

The ecological definition of an urban area can vary depending on the context. A commonly-accepted definition is that an area is urbanized if residential dwellings exceed more than one per acre (0.4 hectare), and if the designated area includes such components as housing, commercial and public institutions, railyards, truckyards, and highways. In this context, both city and suburb are considered urban. The ecological differences between urban and suburban areas come down to a matter of degree, related primarily to the density and scale of urbanization.

One of the unambiguous features that definitively distinguishes an urban ecosystem from its surroundings is the high level of energy use. Typically this is generated from fossil fuel combustion, which is required to construct and maintain the urban infrastructure. The energy level in an urban environment is typically much greater than in other ecosystems. While energy use can be relatively complex to measure, it is a metric that can help differentiate urban ecosystems from other ecosystems that are also human-dominated but nonurban.

More broadly in the United States, the Department of Agriculture uses the terms developed areas, urban, and built-up areas. They include cities, ethnic villages, and built-up areas larger than 10 acres (4 hectares), as well as industrial sites, railroad yards, cemeteries, airports, golf courses, shooting ranges, and other large developments. Typically, urban areas contain a city, such as New York City, Washington, DC, or Dallas, Texas, with a population of 250,000 to 10 million people. In the United States, the largest city, in terms of population, is New York City, followed by Los Angeles; Chicago; Washington, DC; Boston; and Philadelphia.

Urban regions consist of active interactions between a city and surroundings, with the outer boundary determined by a drop in the rate of flow and movements—people, materials, and resources, etc.—radiating outward from the city center.

Metropolitan areas are nearly continuously built, or completely constructed, and have adjoining suburbs. These areas also contain continuous closely-spaced buildings, mostly on small lots. In contrast, suburbs consist mainly of residential municipalities, such as towns, and may be located entirely or partially within, or altogether outside, a metropolitan (metro) area. Peri-urban areas are positioned on both sides of a metro border, where built-up and open areas intermix.

Urban regions begin with a city center, and also feature a development ring that includes an area outside the metro space but still inside the urban-region boundary. This ring is a mosaic of green­space, interwoven with developed systems, and relatively small built-up areas. Major highways, railroads, and powerline corridors are the prominent systems that criss-cross this environment.

Urban greenspaces, also known as natural areas or natural habitat, are open, unbuilt areas in an urban region that can range from tiny city parks to extensive woodland landscapes, and they can range from self-contained tracts to linear greenways and public river corridors.

Urban Ecology

Cities can be viewed as ecosystems with a metabolism. Using that analogy, the city can be seen from a holistic perspective as a consumer and digester of resources, and a creator of waste products. Total inflows and outflows can be quantified, which gives urban planners tools for ensuring the future availability of resources needed for the city to sustain itself.

Included in this model are inputs, outputs, stocks, and flows of energy, water, nutrients, materials, and wastes. Factors that influence the metabolism of cities include urban density—sprawled, low-density cities have more intensive transportation energy requirements per person than compact dense cities—climate, technology, local policies, programs such as recycling initiatives, and the use of vegetation. Overall, trends in per capita metabolism of urban areas have generally increased during the past 50 years.

Urban areas can be examined in terms of the ecosystems and natural environments located within cities and suburbs. In cities, there are at least seven common types of urban ecosystems that are considered natural, including:

• • street trees—stand-alone trees, often surrounded by paved ground;
• • lawns and parks—managed green areas with a mixture of grass, larger trees, and other plants, including areas such as playgrounds and golf courses;
• • urban forests—less managed areas with a more dense tree stand than parks;
• • cultivated land and gardens—used for growing various food and floral types;
• • wetlands—various types of marshes and swamps;
• • open water areas—lakes and ponds;
• • flowing waters—streams, rivers, and seas.

Ecosystems in urban areas provide many services that have important social and economic value. These services include air quality regulation, filtering, detoxification of pollutants and generation of oxygen; local climate regulation such as reduction of the urban heat island effect; shade that helps decrease cooling-related energy use; noise reduction; stormwater drainage and aquifer recharge; sewage treatment; and recreational, cultural, and aesthetic values. In the case of heating and cooling functions, several studies have found that an increase in urban tree cover would significantly reduce the annual heating and cooling energy costs of the city’s dwellings.

Colonizing Species

While the ecosystems in urban areas can be considered natural, they also are novel and relatively new to the planet. Each urban area has unique aspects, but there are still many attributes that can be found in most urban locations. These include: opportunities for some species that do not exist in rural areas; new food sources; places to live; species to parasitize; buildings to roost upon; warm, polluted, nutrient-rich waters; underground sewers for breeding; and train and/or auto tunnels and pipes to inhabit.

Many variables within cities influence the size of natural habitat patches and the dynamics of plant and animal interactions. These variables are consistently present, but differ according to local factors such as buildings, pavement, scattered green areas, street trees, gardens, and houseplants, as well as thousands of people and vehicles moving about daily.

Some species experience population explosions after their habitat becomes urbanized. This has been the case with the bluegill fish in urban ponds, and some pioneer tree species that thrive in vacant lots after the surrounding area has been cleared. In addition, many types of roadside plants thrive in urban areas near thoroughfares that did not exist before urbanization. Many bird species thrive in urban environments. In some cases, their populations increase upon urbanization at the same time as the area’s previous native species decline in the same habitat. In the United States, starlings and house sparrows are abundant in every city. However, both of these birds have only been on the North American continent since the early 1900s.

A number of species accompany people wherever they go and colonize a region, imported by humans during the urbanization process; chief among these creatures are dogs and cats, which are intentionally introduced. Many other species are inadvertently introduced into urban environments, among them roaches, rats, and mice; beetles that live in stored grain; and insects—which can arrive in fruits, vegetables, imported flowerpots, and other conveyances.

Urbanization has been known to spread invasive pests and diseases to devastating effect. This has occurred when a pest encounters a new host environment where the native organisms do not have adequate immunity. Among the examples of this scenario are Dutch elm disease and chestnut blight, which destroyed many trees in the United States.

In addition to invaders and pests, urbanization can also have positive, if novel, ecological effects. Nearly 60 percent of many urban areas could be classified as forest; there is also a surprisingly high level of biodiversity in urban areas. One example of this is the heavily-built inner city region of Cleveland, Ohio, which has more than 400 species of plants identified within its city limits.

Environmental Issues

While in many ways there is a relatively high degree of ecosystem functioning in heavily-built urban landscapes, urbanization does pressure everyday functions, as measured by net primary productivity (NPP). This is a key measure of ecological functioning, as it determines the amount of sunlight energy that is captured and fixed by photosynthesis. This process makes the energy available to drive biological functioning throughout the ecosystem.

Urban areas typically have a significantly lower NPP than their natural counterparts, but these areas do contribute to biodiversity in novel ways, as previously discussed. However, urbanization typically fragments native habitats to an extensive degree, which impacts human activities and natural biodiversity.

In general, native biodiversity is increasingly lost the further populations move from a rural fringe to the urban core. Native vegetation generally has very little chance of surviving once the building density exceeds one unit per acre (0.4 hectare). Urban exploiter species typically dominate the urban core, while urban adopters dominate suburban areas, and urban avoiders dominate the peri-urban fringe.

Natural materials and nutrient cycles are affected by urbanization, including nutrient cycling, soil erosion, hydrological flow, and the runoff of pollutants from urban areas. Urbanization also greatly changes the natural ecological disturbance regime, which alters natural succession and introduces biogeographic barriers (e.g., roads, canals) that decrease the patch size of natural vegetation and also introduce chronic stresses such as noise and light.

Urban ecosystems can be viewed as nodes in a network of regional and global environments. The growing discipline of ecological footprint analysis determines the extent that natural ecosystems located outside the actual urban area are exploited to support the urban area. Urban dwellers are often the sole macro-consumers for vast areas of cropland, pasture, and forest outside the urban area. The ecosystems providing a majority of the biophysical life-support for urbanites are rural and other nonurban ecosystems. For example, the ecological footprint—the total land area needed to provide the goods and services consumed—of metro Vancouver, Canada, is estimated to be about 11,495 square miles (29,722 square kilometers), about 300 times the size of the actual surface area of the city.

While the issues that may contribute to the unsustainability of many city environments are becoming increasingly evident, there also are encouraging and accelerating trends toward sustainability and ecological restoration in urban and suburban areas. The trend toward integrated urban planning is accelerating, with the approach explicitly addressing the contributions that healthy ecosystems make to the urban economy and quality of life. The Chicago Plan, initiated at the turn of the 20th century, was an early documented example of accounting for open space in urban planning, and Chicago's Go to 2040 project, implemented in 2010, has intensified this focus by comprehensively addressing the natural environment and its contribution to civic well-being.

In many urban areas, there is a growing focus on green infrastructure, which seeks, for example, to restore natural vegetation to improve stormwater handling and reduce flooding. There is a growing community-gardening movement, which encourages food cultivation in urban areas, providing environmental, economic, and social benefits. Many cities are encouraging the installation of green roofs (vegetated rather than hard-surface rooftops), that help mitigate stormwater flows, reduce the urban heat island effect, sequester some carbon, and also encourage biodiversity. A growing local food, or locavore, movement encourages local farmers markets and community-supported agriculture plans, which help restore the connections between urban areas and the rural ecosystems they depend on. In 2022, a comprehensive climate bill passed by Congress and signed into law, allocated $1.5 billion to the US Forest Service’s Urban and Community Forestry Program, which plants trees in urban areas. The funding is slated to last until 2031.

Bibliography

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