Suburban areas

Category: Grassland, Tundra, and Human Biomes.

Geographic Location: Global.

Summary: This novel and synthetic biome is defined by the degree to which human activity has dominated and transformed it—but suburban ecosystems still exhibit a high level of ecological activity.

Suburban ecosystems are a unique type of synthetic biome whose defining characteristic is the significant degree to which human activity has dominated and transformed the landscape. The key ecological pressures include replacement of native vegetation with impervious surface such as roads, concrete, and buildings; degradation of native vegetation and/or replacement with nonnative species; fragmentation of natural/green spaces; the corresponding reduction in wildlife habitat; increased pollutant loads; and changes to the hydrologic regime due to land-cover change, for example, increasing flooding during storms.

Suburban areas are not always ecological wastelands, however. Suburban ecosystems can contain novel collections of biodiversity because of introduced species, and increasing trends in ecosystem restoration are placing an emphasis on restoring ecological functions within the built environment. A defining characteristic of a suburban area is that resources, materials, and energy need to be imported from outside the urban area; therefore, suburban areas do not fit the classical definition of an ecosystem. Natural ecosystems are considered to be relatively self-contained systems, where energy and nutrients are cycled within the boundaries of the ecosystem.

To address these unique aspects, suburban ecosystems need to be examined using three different lenses/perspectives: the suburban area as an ecosystem; ecosystems within suburban areas; and the suburban area as an entity within a larger, regional ecological network.

Characteristics of Suburban Ecosystems

The ecological definition of what constitutes a suburban area can vary, depending on the context. A commonly accepted rule-of-thumb definition is that an area is urbanized if it consists of “residential land at densities greater than one dwelling per acre, consisting of housing, commercial and public institutions, railyards, truckyards, and highways.” In this context, both city and suburb are considered to be urban, at the opposite end of the scale from rural areas and/or wilderness.

The ecological differences between urban and suburban areas are relative and are a matter of degree, related primarily to the density and scale of urbanization. There can be outlying areas of a major city that feel more suburban than the inner-core areas of an adjacent suburb; therefore, the delineation from an ecological perspective between city and suburb is not hard and fast. A city and its suburbs can be viewed as lying at different points along a gradient, with the core of the major city at one end, the suburbs in the middle, and farmland and untouched wilderness at the other end of the gradient.

In general, suburban areas are considered to be mainly residential areas, with an abundance of single-family or low-density multifamily housing, where the residential properties often contain yards. This discussion focuses on suburban areas at this medium-density area of the scale.

One of the unambiguous features that definitively distinguishes an urban/suburban ecosystem from its surroundings is the high level of energy use, typically generated from fossil-fuel combustion, that is required to construct and maintain the suburban infrastructure. The energy level in an urban or suburban environment is typically at least an order of magnitude greater than in other ecosystems. While energy use can be relatively complex to measure, it is a helpful metric, as it can help differentiate urban ecosystems from other ecosystems that are also human-dominated but nonurban (for example, intensively managed agricultural land).

Terms and Concepts in Suburban Ecology

Some important terms and concepts used when discussing the ecology of the suburban landscape include:

• • City: A relatively large or important municipality. Typical population ranges for what is considered to be a city can range from 250,000 to 25 million. The defining characteristic of a suburb is that it is in proximity to a large city.
• • Urban region: The area of active interactions between a city and its surroundings. The outer boundary of an urban region is determined by a drop in the rate of flow and movements of people, materials, resources, etc., as one proceeds outward from the city.
• • Metropolitan or metro area: The nearly continuously built, or all-built, area of the city and adjoining suburbs. From the eye of a satellite, a metro area is prominent as a visible object.
• • Built area: Land with continuous closely spaced buildings, as on small properties or plots.
• • Suburbs: Mainly residential municipalities, such as towns, close to a city. A suburb may be entirely within, partially within, or altogether outside a metropolitan area.
• • Peri-urban area: The area on both sides of a metro-area border, where built and unbuilt areas intermix.
• • Urban-region ring: The area outside the metropolitan area and inside the urban-region boundary. This ring is a mosaic of greenspace (unbuilt) land types interwoven with built systems and relatively small built areas. Major highways, railroads, and power-line corridors are the prominent built systems that criss-cross the urban-region ring. Urban regions have a city-center nucleus.
• • Greenspace: Unbuilt area in an urban region—areas without continuous closely spaced buildings—also called open spaces. Greenspace can range from tiny city parks to extensive woodland landscapes, and can range from rounded spots to linear greenways and river corridors.
• • Habitat: A relatively distinct area, and its physical and biological conditions, where an organism, population, or group of species mainly lives.
• • Urbanization: The combination of densification and outward spread of people and built areas.
• • Densification: Increase in density of people and building units, for example, developing/building on greenspace, or changing from low- to high-rise apartment buildings.
• • Sprawl: The process of distributing built structures in an unsatisfactory or awkward, spread-out (rather than compact) manner or pattern.

Suburban Area as an Ecosystem

There are multiple perspectives and scales at which the ecology of suburban areas can be examined. One approach is to consider the suburb as its own ecosystem, and study the town’s urban metabolism as though it were a discrete organism with its own metabolic processes. The study of urban metabolism examines a town from a holistic perspective as a consumer and digester of resources, and a creator of waste products. When viewing the suburb in this manner, the total inflows and outflows from the town can be quantified, which gives planners tools for ensuring the future availability of resources needed for the town to sustain itself.

Urban metabolism studies typically quantify the inputs, outputs, stocks, and flows of energy, water, nutrients, materials, and wastes. Factors that influence the metabolism of cities and towns include urban density (sprawled low-density cities have more intensive transportation energy requirements per person than compact dense cities), climate, technology, local policies and programs (for example, recycling programs), and the use of vegetation. Overall, trends in per-capita metabolism of urban areas have increased since the mid-to-late twentieth century.

Suburban areas can be examined in terms of the natural environments located within them. In urban and suburban areas, there are at least seven common types of ecosystems that can be considered natural (green or blue, rather than concrete or steel):

• • Street trees: Stand-alone trees, often surrounded by paved ground.
• • Lawns and parks: Managed greenspace 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 items.
• • Wetlands: Various types of marshes and swamps.
• • Lakes and sea: Open water areas.
• • Streams: Flowing water.

Other areas within the city, such as dumps, abandoned backyards, and alleyways, may also contain significant populations of plants and animals.

Ecosystems in suburban areas provide many ecological services that have important social and economic value. These services can include air-quality regulation (filtering/detoxification of pollutants and generation of oxygen); climate regulation (reduction of urban heat island effect, shade contributing to decrease in heating- and cooling-related energy use); noise reduction; stormwater drainage and aquifer recharge; storm protection; erosion control; pollination; sewage treatment; and recreational, cultural, and aesthetic values.

While the ecosystems in suburban areas can be considered natural (as humans are animals that affect their environment, just like any other creatures), they are also novel and relatively new to the planet, as the archaeological evidence suggests that major cities and their suburbs have been in existence for only the past 5,000 years. Therefore, suburban ecosystems are still in a relatively early stage of development. While each suburban area has its own unique aspects, there are still many attributes that can be found in most suburbs.

Suburbs have variables that 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: buildings, pavement, scattered green areas, street trees, gardens, house plants, and often turf-grass lawns, as well as thousands of people and vehicles moving about daily. A 2018 study by the National Science Foundation found that the many suburban lawns contain such common types of grass, shrubs, and flowers that it is greatly reducing biodiversity in suburban ecosystems.

Some species experience population explosions after their habitat becomes urbanized, such as the bluegill fish species in ponds, or some pioneer tree species that thrive in vacant lots after the surrounding area has been cleared. Also, many types of roadside plants thrive in urban and suburban areas due to the abundance of roads that did not exist before urbanization. Many bird species thrive in suburban environments, their populations increasing upon urbanization at the same time that the prior native species decline. In the United States, for example, starlings and house sparrows are abundant in every city, but these birds have been on the North American continent only since the early 1900s.

Some species accompany people wherever they go and colonize a region due to being imported by humans during the suburbanization process. Species such as dogs and cats are introduced intentionally, while many other species are inadvertently introduced, such as cockroaches; rats; mice; beetles that live in stored grain; and insects that arrive on fruits, vegetables, and imported flower pots. Urbanization has also been known to spread invasive pests and diseases that have a devastating effect in their new host environment because the native organisms do not have immunity; examples include Dutch elm disease, chestnut blight, the Asian longhorned beetle, the emerald ash borer, and the winter moth that have attacked tree populations in suburbs across the United States. Suburbanization, along with climate change, has increased human health risks from tick- and mosquito-borne diseases, such as Lyme disease, when natural habitats like forests are fragmented for suburban and urban development.

In addition to invaders and pests, urbanization can have positive, if novel, ecological effects. Many suburban areas can be classified as forest. In many places, there can be a surprisingly high level of biodiversity in suburban areas.

In North America, common wildlife species found in suburban areas include tree frogs, snails, butterflies, turtles, raccoons, fireflies, moths, newts, muskrats, spiders, rabbits, skunks, honeybees, crickets, wasps, beetles, bats, moles, woodpeckers, dragonflies, foxes, sunfish, toads, squirrels, snakes, woodchucks, praying mantises, crayfish, hawks, mice, opossums, lizards, shrews, flies, otters, worms, freshwater sponges, fairy shrimp, mayflies, ants, and lacewings.

While in many ways there is a relatively high degree of ecosystem functioning occurring in even heavily built suburban landscapes, suburbanization does present pressures that affect urban ecosystem function. Net primary productivity (NPP) is a key measure of ecological functioning, as it determines the amount of sunlight energy captured and fixed by photosynthesis, making the energy available to drive biological processes throughout the ecosystem. Urban and suburban areas typically have a significantly lower NPP than their natural counterparts.

Suburban areas do contribute to biodiversity in novel ways, but urbanization typically fragments native habitats to an extensive degree, and fragmentation of natural habitat patches is one of the best-known effects of human activities on natural biodiversity. In general, native biodiversity is increasingly lost the farther one moves from a rural fringe area to the urban core; native vegetation generally has very little chance of surviving once the building density exceeds 1 unit per 1 acre (0.4 hectare). Urban exploiter species typically dominate the urban core; urban adopters dominate suburban areas; and urban avoiders dominate the peri-urban fringe.

Natural materials and nutrient cycles are also affected by urbanization, including nutrient cycling, soil erosion, hydrological flow, and the runoff of pollutants, such as fertilizers, pesticides, wastewater, and road salt from suburban areas. Urbanization also greatly changes the natural ecological disturbance regime. This alters natural succession and introduces biogeographic barriers (paved roads, canals, dams, ditches, utility infrastructure, and so on) that decrease the patch size of natural vegetation, disrupt natural processes like flooding, and introduce chronic stresses such as noise and nighttime light.

Larger Ecosystems

Suburban ecosystems can also be viewed as nodes in a network of regional and global ecosystems. The growing discipline of ecological footprint analysis determines the extent and area of natural ecosystems, located outside the actual suburban area, that are exploited to support the suburb. Suburb dwellers are often the sole macro-consumers of vast areas of cropland, pasture, and forest outside the immediate area. The ecosystems providing a majority of the biophysical life support for suburb dwellers are rural and other nonurban ecosystems.

In the Canadian metropolis of Vancouver, British Columbia, the ecological footprint—the total land area needed to provide the goods and services consumed by Vancouver city and suburb residents—is estimated to be about 11,476 square miles (29,722 square kilometers), which corresponds to about 300 times the size of the actual surface area of the metro area.

Trends in Urban Ecosystem Restoration

While the issues contributing to the unsustainability of many suburban environments are becoming increasingly evident, there are also encouraging and accelerating trends toward sustainability and ecological restoration in suburban areas. The trend toward integrated urban planning has accelerated, with contemporary planning approaches explicitly addressing the contributions that healthy ecosystems make to the economy and quality of life. In many areas, there is a growing focus on green infrastructure, which seeks to restore natural vegetation to improve stormwater handling and reduce flooding. Conservationists in various parts of the world are encouraging coexistence with wildlife on the fringes of suburban and urban areas and seeking to protect migration corridors.

There is also a growing community gardening movement that encourages food cultivation in suburban areas, providing environmental, economic, and social benefits. Many towns are encouraging the installation of green roofs (vegetated rather than hard-surface), which helps mitigate stormwater flows, reduces the urban heat island effect, sequesters carbon, and encourages biodiversity. And a growing local food, or locavore, movement encourages farmers’ markets and urban farming, which help restore the connections between suburban areas and the ecosystems they depend on.

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