Urban and suburban systems

Last reviewed: August 2016

An ecosystem is a community of living organisms interacting in complex ways with the physical environment, such as soils, geography, chemistry, and weather, to form a relatively cohesive functional unit. This definition suggests a certain degree of similarity across a given area such that what happens in an ecosystem is fairly predictable. Cities meet that definition and, in fact, there are several urban ecosystems being studied under the international Long Term Ecological Research Network. This article describes urbanization and explores characteristics, issues, and some solutions common to urban ecosystems.

The simple definition of a city is a large and densely populated urban area. But how large or dense, and what do we mean by “urban”? These questions have been surprisingly hard to answer because there are many different land uses in urban areas and there is not a clear or widely accepted division between urban, suburban, and rural.

Smaller cities, for example, those with 5,000 to 50,000 people, typically include residential, business, and industrial areas and have formal governance such as a city council and mayor. In a large city there is a gradient of urbanization, from high-rise downtown areas transitioning slowly—or sometimes more abruptly when an urban growth boundary is in place—to lower-density, more sparsely populated suburban and rural areas, with other towns and cities within the area of influence. Therefore, it is useful to think about urban ecosystems in terms of a metropolitan area, where concentrations of people live in large cities, suburbs, and the satellite cities and towns close enough to provide the jobs, goods, services, and cultural experiences important to people.

Over many centuries, human population has expanded exponentially to the rapidly growing global population of approximately 7.4 billion people in 2016, with a corresponding shift from hunting/agriculture to urban areas. Prior to the eighteenth century, 3 percent of people lived in cities. In 2008, for the first time half of the world's population lived in cities. In 2014, approximately 54 percent of the total global population lived in urban areas. By 2050 about 70 percent of all people are expected to live in cities. That is a great number of people, acres, and major impacts on the environment, so it warrants critical and scholarly attention.

Key Characteristics of Urban Ecosystems

Ecologically, urban ecosystems have both positive and negative aspects. On the good side, concentrating people in one area reduces time and expense in commuting and transportation while improving opportunities for jobs, services, education, housing, and transportation. Concentrating human population can also reduce the impact on the rest of the environment. On the other hand, urbanization takes a heavy toll on air and water quality, fish, wildlife, and habitat. The key is to reduce these impacts without substantially increasing the urban footprint. Success does not mean that a metropolitan area resembles the original natural environment. Rather, it accommodates the needs of people, provides contact with nature, and conserves the biological resources and diversity. These goals can be achieved using a strong foundation of science, social and political compromise, and a variety of tools including urban planning, conservation, and regulation.

Compared to natural ecosystems, characteristics shared by most cities include changes in land cover (less vegetation and more hard, or impervious, surfaces), changes in natural disturbance regimes, air pollution, warmer air and water temperatures, water quality and quantity issues, changes in the amount and type of habitat, invasive species issues, and wildlife communities where generalist species prevail. This article will explore the causes and effects of these changes, offer some solutions, and present a case study from the Portland-Vancouver metropolitan area in the northwestern United States.

Land Cover Changes

Land cover is material at the surface of the earth, such as trees, grass, pavement, or water. Converting natural habitats to urban land cover is the overarching, and ecologically overwhelming, reason cities are similar to one another and different from other ecosystems. Much of the original landscape, whether forest, desert, prairie, or some other type of ecosystem(s), is now characterized by significant impervious land cover such as roads, parking lots, driveways, sidewalks, and rooftops.

In cities the air, habitat, and water quality are products of the cumulative effects of past and present human constructs and activities. Not all urban areas are the same, however. In rural residential areas, approximately 5 to 10 percent of the land cover is impervious, with the majority of land cover being pervious natural or agricultural areas, lawns, or landscaping. Land cover in low-density suburban areas is typically 20 to 35 percent impervious, while high-density suburban areas near urban development is typically 40 to 60 percent impervious. In urban areas, 75 to 90 percent of land cover is impervious, and pervious areas include small landscaped areas, street trees, small yards, and parks and other open spaces. Land cover in city centers may be 85 to nearly 100 impervious.

Altered Disturbance Regimes

Disturbances are events such as fires, floods, landslides, or wind storms that disrupt and can change an ecosystem or community. Ecosystems are adapted to certain types of disturbances that occur relatively predictably over time and space—a disturbance regime. When land cover changes, it changes the timing and spatial characteristics of disturbances and introduces new types of disturbance. Eventually, the ecosystem stabilizes under a new, relatively predictable disturbance regime associated with urbanization. Now it is an entirely different kind of ecosystem.

For example, larger river systems include extensive floodplains to accommodate increased water during the wet season. There are different floodplain levels, or “benches,” adapted to floods occurring annually and less frequently, perhaps inundating an area every 10, 50, 100, or 500 years on average. Frequently flooded areas are characterized by fast-growing plant species such as grasses, sedges, and shrubs, as well as species such as cottonwood and willow that can physically withstand the force of the floodwaters and survive underwater for periods of time. A 50-year floodplain can sustain longer-lived species less adapted to flood disturbance, and so forth. The floodwaters deposit sediments, nutrients, rocks, and woody debris on the floodplains as the water slows and recedes. The substantial sediment deposits can form some of the richest farm soils in the world.

Urbanization has often occurred in floodplains because they are flat and close to water sources and shipping channels, a key means of transporting goods for import or export to support concentrations of people, industries, and jobs. However, the paving, vegetation removal, dikes, dams, levees, and floodwalls that come with urbanization alter the natural disturbance regime of the floodplain. Most times, the water is intentionally confined in the river channel; when it is not, substantial economic and structural damage, and sometimes loss of life, occurs.

Other types of natural disturbance are intentionally disrupted in metropolitan areas. For example, whether the original ecosystem was forest, desert, shrub, or grasslands, fire suppression is nearly universal, particularly around the fringes of a metropolitan area where significant natural habitat remains, along with homes and other rural uses. Fire suppression in these areas reduces danger to humans and economic damage and blocks the fire from spreading closer or into the urban area.

Nonnatural types of disturbance also characterize urban ecosystems. The process of changing land cover is a disturbance, but the new land cover is not; humans disturb the ecosystem in predictable ways. For example, every so often a building might be demolished in a downtown area and rebuilt. While this is not likely to happen again at that particular site for decades, it will happen in various other places in the city. Demolishing a building creates substantial noise for days or months, creates extra construction traffic, reroutes car and transit routes, and creates many tons of waste. This disturbance is regular within the system. A more short-term type of disturbance is freeway or light rail traffic, in which rush hour sets a regular pattern of higher disturbance. Ball games in a lit stadium, people walking in a park, blasting at a rock quarry, plane traffic around an airport—these are all part of the urban ecosystem's disturbance regime. They influence the ecosystem and organisms living there.

Often an ecosystem has been altered from its original state prior to urbanization. The most typical example is conversion from the original mix of habitats to agriculture, and then to an urban area. In such an area, essentially three different ecosystem types have characterized exactly the same area, often over a few decades to a few hundred years: the original ecosystem type, an agricultural ecosystem and, finally, an urban ecosystem.

Urban areas often have high levels of phosphorus, nitrogen, carbon dioxide, and other nutrients. For example, nitrogen and carbon dioxide concentrations are high near busy roadways. These nutrients can allow certain plants to thrive at the expense of others. In a study near San Jose, California, an endangered butterfly species—the Bay checkerspot—declined in numbers near busy roads because invasive plants thrived on higher nitrogen levels, pushing out the butterfly's host plant. In this case, limited cattle grazing has been proposed as a solution to return nitrogen to more natural levels conducive to maintaining native prairie habitat because the cattle remove nitrogen sources from the ecosystem while grazing.

Air Quality and the Urban Heat Island Effect

Anyone who has walked or ridden a bike on a hot sidewalk or roadway and stopped to cool off under a shady tree already knows something about urban ecology: cities are warm places. Replacing cool, moist, natural vegetation with dry buildings, roads, and other urban constructs translates to higher air temperatures, called the urban heat island effect. The effect is most intense on hot summer days. For example, cities with a million or more people typically average 1.8 to 5.4 degrees F (minus 16.8 to minus 14.8 degrees C) warmer than nearby rural areas during the day, with peak intensities often reaching 18 to 27 degrees F (minus 7.8 to minus 2.8 degrees C) higher. Cities are warmer at night, too, as the heat stored during the day is released to the night air. Even a desert city that has more cooling plants than the surrounding landscape is likely to be warmer because the impervious and dark-colored surfaces still store a great deal of heat.

Interestingly, the urban heat island effect may offer extra opportunities to offset warmer temperatures due to climate change. Because lack of vegetation makes cities warmer, “regreening” can reduce temperatures. In the most urban areas this can be accomplished by increasing street and parking lot trees, commercial and industrial landscaping, and green roofs (ecoroofs). Dark, hard surfaces absorb more heat than light ones, so lightening the color of roadways and rooftops will help. These activities can especially target the neighborhoods and areas that are hottest in the summer, when they would have the most effect in reducing air temperature. In terms of climate change there's a double added bonus: fewer air conditioners running reduces energy use, plus trees and vegetation store carbon.

Urbanization also changes air quality through emissions from industry, power plants, motor vehicles, wood-burning stoves, and a myriad of other causes. These activities increase pollutants (which can also be nutrients) such as nitrous oxides, which are produced during combustion and harm human health; ozone, which is protective high in the atmosphere but hard on the respiratory tract near the ground; heavy metals, such as highly toxic mercury from industrial emissions; and particulate matter that may harm the heart and lungs. Combustion also produces carbon dioxide, a key greenhouse gas. A haze of pollution over metropolitan areas is often visible from far away, and it is carried by wind currents to other areas.

Water Quality and Quantity

In cities, the amount and timing of water delivery are critical. The hydrologic cycle relates to the occurrence, pattern, timing, and distribution of water and its relationship with the environment. Impervious surfaces, combined with loss of natural soils and vegetation to slow and capture water, interrupt the hydrologic cycle, alter stream structure, increase urban runoff, and degrade the chemical profile of the water that flows through streams. These changes to water storage and delivery harm the environment in a variety of ways, and are cumulative within watersheds. The cumulative effects are products of an altered hydrologic cycle, or altered hydrology.

Water quality responds predictably to changes in land cover, typically declining as vegetation is replaced with impervious surfaces. In metropolitan areas, hydrology is most altered from the central city, with the impact declining as land cover becomes more permeable. However, numerous studies demonstrate that even low levels of imperviousness, in the range of 5 to 10 percent, are enough to damage stream channels and water quality. The following are some of the common effects of altered hydrology due to urbanization:

• Streams become “flashier”—higher flows during storms, but less water in the dry season; some streams that had year-round water prior to development dry up.
• Stream channels widen and deepen to accommodate the higher flows, damaging stream banks, stripping vegetation, and increasing sediments in the water and on the stream bed.
• Deepening the stream channel can cut into the groundwater table, partially draining it. Groundwater is typically what keeps a stream flowing during dry periods.
• Impervious surfaces stop water from percolating down to the groundwater, so the water table drops.
• Disconnection with large floodplains.
• Locally, flooding becomes more common and severe, particularly in heavily urbanized areas, because too much water enters the stream too quickly and it overruns the banks.
• Water temperature rises because impervious surfaces are warm, whereas trees and vegetation shade slow and cool stormwater. Unnaturally warm water is one of the most ubiquitous water quality problems in cities and has contributed substantially to the decline of salmon and other cold-water organisms.

In older developed areas, hydrology may reach a different but stable condition. Streams and other water bodies will not necessarily support the same plants and animals, but the new system can support longer-lived species or those that require predictable environments. The new system may have more generalist species and more individual organisms, at the expense of specialists such as cold, clean water specialists or those that need a rocky stream bottom.

For example, freshwater mussels can be good indicators of system stability because they are relatively slow-growing and adults are sedentary, while juveniles travel by attaching to specific host fish species. Some species can live more than 150 years. If only young mussels are present, the system is probably still changing, whereas older mussels have probably been there a while, indicating some level of stability. In North America, nearly three-quarters of all freshwater mussel species are imperiled and about 35 went extinct in the twentieth century. Altered hydrology is undoubtedly a contributor to these species' declines.

Habitat Changes

At the local or site level, human habitat management leads to loss of structural complexity. Structural complexity and the total amount of vegetation are well-known contributors to wildlife species richness in forested areas. Humans tend to like a parklike setting with trees and grass, all green and alive. Dead trees and fallen branches are removed and leaves are raked. Unfortunately, this “clean” habitat fails to meet the needs of many wildlife species. Most birds feed on insects, seeds, and berries from shrubs, which also provide a great deal of cover and nesting habitat. Salamanders, centipedes, and salmon rely on dead wood, on the ground or in the water. Standing dead trees, or snags, are required nesting or roosting habitat for scores of birds, mammals, reptiles, and amphibians. Woodpeckers, swallows, bats, and bluebirds nest in snags and control pest insects. Leaves on the ground attract insects and their predators, such as towhees and thrushes. Sparrows, voles, bugs, snakes, lizards, and amphibians make use of rocks, brush, and wood piles. Leaving or adding some of these features to a yard will make it more natural and attract more native wildlife species. These elements also help build healthy soil.

At a larger scale, habitat fragmentation is the process of breaking apart large areas of natural habitat into multiple smaller disconnected patches. The term is generally used in the context of forested areas, but also applies to other habitat types such as wetland, shrub, or grassland. Fragmentation is widely recognized as an overarching threat to wildlife and ecosystem health and is closely linked to habitat loss, loss of habitat connectivity, and invasive species. Habitat fragmentation is characteristic of urban ecosystems.

Three basic characteristics—habitat patch size, isolation or connectedness, and size—heavily influence wildlife diversity in fragmented urban landscapes. Native habitat loss and conversion are first and foremost, with metropolitan areas retaining only a small fraction of the original habitat. The remaining natural areas are often converted to other habitat types, sometimes as a result of altered disturbance regimes. For example, in many cities native prairie or grasslands are converted to shrub or forest habitat due to fire suppression and invasive species such as Himalayan blackberry. Around the urban fringe, agriculture often replaces more natural habitats. The combination of habitat loss and changes threatens native species, particularly those that specialize on specific habitats.

When habitat is fragmented the patches become increasingly isolated. Two patches that are close together typically contain more species than two that are further apart, and if there are connecting corridors to other patches, even more species are present. In completely isolated habitats animals are essentially trapped or in danger if they leave the habitat patch. Isolated patches lose species over time, and without connection to other habitats, the species cannot come back.

Identifying important wildlife movement corridors and providing viable connectivity between remaining habitat patches can help reduce many of the ecological impacts of habitat fragmentation. Urban areas often protect streams through zoning and regulation and streams tend to connect habitat patches, therefore stream corridors sometimes offer the best options for wildlife movement. In addition, the amount and placement of a few key landscape features, such as trees and shrubs, significantly influence the types of wildlife that can survive in urban areas. Studies show that landscaping and street trees increase wildlife connectivity in measurable ways.

Patch size is another critical factor, because larger habitats support more species per acre than smaller patches. Species requiring a large habitat patch (area-sensitive) may become locally extinct in fragmented habitats. In larger patches, area- and disturbance-sensitive species can find refuge in the middle of such patches away from disturbance, and the habitat quality may be better away from the patch's edge. Some studies suggest certain thresholds, depending on species or geographic area, at which area-sensitive species begin to appear. For example, a lower threshold, but one that may be particularly important in urban areas with smaller habitat patches, may be around 30 acres in naturally forested areas.

Large habitat patches benefit many of the region's sensitive species, but small habitat patches increase the mobility of wildlife in a landscape. Urban areas with trees and shrubs scattered throughout, combined with larger natural areas connected by corridors, are likely to hold more species and more animals than large patches and corridors embedded within an entirely urban matrix. Backyards, street trees, right-of-ways, and green roofs can all provide valuable opportunities to increase permeability. For these more urban solutions, an emphasis on native plants will help maintain native animal diversity. Numerous studies link native wildlife, bees, and butterflies to native plants.

The edge of a natural habitat patch is an ecotone, or a place where two types of habitat (for example, forest and urban) meet. Larger patches have less edge habitat than smaller patches, and patch shape also influences the amount of edge habitat. A long, narrow habitat patch has relatively more edge than a round patch of the same size. Edge effects are the changes that occur at the edge of a habitat area, and fragmented urban habitats have a lot of edge. Because ecotones host species from each habitat type, the number of species is often higher than inside a habitat area, and some species such as turtles and amphibians require more than one habitat type. Elk require forest for cover plus fields and shrubby areas for feeding. Fifty years ago, when world population was quite a bit lower and more natural habitat remained, biologists counted edge effects as a plus.

However, too much of a good thing can be bad, particularly in urban ecosystems where roads, buildings, cars, foot traffic, cats, and dogs can be quite hostile for native wildlife. If a deer steps out of a forest onto a busy roadway, wildlife-vehicle collisions, economic damage and injuries, and human and wildlife mortality may occur. Other well-studied negative edge effects include jays, crows, and small predators stealing bird eggs and nestlings along edges, where such high-protein food is poorly hidden; loss of disturbance-sensitive species near roads and heavy-use trails; and increased problems with invasive species because seeds are carried along edges by birds, car tires, wind, and other means.

Invasive species are typically nonnative plants or animals that negatively affect habitat and biodiversity, often with serious economic consequences. The estimated costs of damage by and control of invasive species in the United States is estimated at more than $120 billion per year. Many nonnative species are not invasive, and many landscaped areas include such species. Invasive species typically share certain traits, including the following:

• Fast growth and rapid reproduction
• High dispersal ability, for example, wind-carried seeds or rhizomes carried downstream
• Generalist species that are able to tolerate or adapt to a wide range of conditions
• Early successional species—those that colonize an area after disturbance such as fire, such as grasses and thistle
• Association with humans
• Lack of natural controls such as predators, competitors, and disease

Infamous examples of invasive species examples in the United States include Japanese kudzu and English ivy, which can take down an entire forest. On the other hand, rhododendrons native to the northwestern United States are an invasive problem in Europe. There are literally thousands of other examples throughout the world.

Fish and Wildlife

In general, species best adapted to urban environments are those not limited to a single habitat type, those with populations easily maintained by outside recruitment, and those that can exploit the more urban habitats. For example, in the western United States habitat generalists or edge-loving species such as scrub jays, American robins, and European starlings are abundant, and chimney-nesting species are increasing. European starlings are particularly harmful to native cavity-nesting birds along streams because they nest early and often, and they aggressively remove other species to occupy nest cavities. However, they can be controlled by increasing tree cover and the width of riparian forests.

Predators are at the top of the food web in essentially all ecosystems. Urban areas see an increase in small and medium-sized predators such as cats, raccoons, and coyotes, and a loss of top predators such as cougar, bear, and wolves to control the smaller predators. Smaller predators prey on smaller animals to the detriment of many birds, small mammals, and reptiles. Birds that nest on or near the ground tend to decline rapidly in newly urbanized areas. Backyard bird feeders and other supplemental feeding may increase birds but also favor smaller predators.

Long-distance migratory bird species typically decline in urban areas across the Northern Hemisphere. The reasons are unclear; studies in the Netherlands linked disturbance from road noise to bird declines, and studies elsewhere show that some bird and frog species change the pitch of their song to be heard over road noise. There are probably other urban-related reasons as well. A disproportionately high number of neotropical migratory birds (those that nest in the United States and Canada, but travel south of the United States-Mexico border to winter) are habitat specialists, area-sensitive, or both. Many rely on shrubs and complex vegetation structure, although grassland species, which tend to require large habitat areas, are also rapidly declining.

Many migratory birds are sensitive to human disturbance. In fact, some so-called area-sensitive species probably need to avoid disturbance more than they need large habitat patches. There is little doubt, however, that human disturbance from roads, trails, industry, housing, and other development harms wildlife through noise, sound, light, and human and pet impacts. The species most sensitive to these impacts die, fail to reproduce, or leave.

Land-Use Planning: Portland, Oregon

The ecological problems brought about by urbanization are cumulative, but occur at a variety of spatial scales. So do the solutions. This section describes some effective tools used in the Portland, Oregon, metropolitan area to reduce the impacts of urbanization on wildlife, habitat, and water quality.

Comprehensive land-use planning with nature in mind can reduce negative impacts. Comprehensive planning helps a community identify goals and aspirations, always including development, housing, jobs, services, and transportation but often pertaining to nature as well.

In 1977 the Portland, Oregon, metropolitan area implemented the first Urban Growth Boundary (UGB) in the United States, designed to protect high-value farm and forest land from urban encroachment. A UGB motivates efficient use and redevelopment of urban lands and thoughtfully planned infrastructure such as roads and sewers. The Portland metropolitan area's current population is about 2.3 million people. An elected regional government, Metro, serves to bring these governments and citizens together to plan major transportation projects, UGB expansions, and other projects to meet the needs of the population. Every six years, the UGB is reviewed and adjusted based on the needs of growth forecasts; the Portland UGB has been expanded about three dozen times, and a system for designating urban and rural reserves was established in 2007.

The region faces federal Clean Water Act and Endangered Species Act issues due to water quality and salmon declines. In 2005, Metro Council passed an ordinance requiring increased habitat protection along the region's streams. The regulation is implemented by the local jurisdictions. Regulation was limited to the most important water-related habitat, but the ordinance also called for a review of cities' development codes to identify and remove barriers to nature-friendly development practices and proposed voluntary measures including natural area acquisition, restoration, and environmental education. Local jurisdictions have stepped up to the challenge and, despite a growing population, the region is looking more and more green.

Metro also has a substantial green spaces program through two citizen-funded bond measures. Metro has acquired and is restoring more than 12,000 acres of natural areas to date, with a focus on large, healthy, natural areas and connections between habitats. Some of the bond measure funds go to the cities and counties to meet more local habitat and park needs, and some funding goes to acquiring regional trail easements to promote nonvehicular travel.

The adjacent Portland and Vancouver, Washington, metropolitan areas provide homes for more than three hundred native wildlife species. These animals must be able to navigate the intricate network of roads, parking lots, backyards, and barriers to survive and thrive. The region is expecting significant population growth in coming decades—about 470,000 to 725,000 more people between 2015 and 2035. Further, anticipated changes in temperature and weather patterns will impact habitat and wildlife in ways that are not yet known.

The Portland–Vancouver metropolitan region teamed up to create The Intertwine, a collaborative organization designed to help improve and connect the region's system of parks, trails, and natural areas. The Intertwine created a voluntary biodiversity plan that outlines existing conditions, habitat types, and major ideas, concepts, and challenges to conserving the region's nature. It provides a way for people to look at larger-scale issues such as habitat connectivity, but it is specific enough to identify specific habitat areas where preservation and restoration would most benefit wildlife and water quality. It represents shared knowledge so that the entire region can work toward some of the same goals.

There are many other environmental efforts in the Portland–Vancouver metropolitan region, including highly successful recycling programs, incentives for green roofs on businesses, financial and technical assistance for landowners to deal with stormwater on site, and a major backyard habitat certification program led by the local Audubon Society and a land trust. There is always more to be learned and more to do, but the region has made a good start toward a more sustainable urban ecosystem.

Bibliography

2014 Urban Growth Report: Investing in Our Communities, 2015–2035. Portland: Metro, 2015. Digital file.

Gartland, Lisa. Heat Islands: Understanding and Mitigating Heat in Urban Areas. New York: Routledge, 2010. Print.

Knox, Paul L, and Linda McCarthy. Urbanization: An Introduction to Urban Geography. 3rd ed. Boston: Pearson, 2011. Print.

Sanderson, David, Jerold S. Kayden, and Julia Leis, eds. Urban Disaster Resilience: New Dimensions from the International Practice in the Built Environment. New York: Routledge, 2016. Print.

Urban Design Associates. The Urban Design Handbook: Techniques and Working Methods. 2nd ed. New York: Norton, 2013. Print.

Derived from: "Urban and suburban systems." Biomes and Ecosystems (Online Edition). Salem Press. 2014.

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