Urban ecology is the scientific study of the relation of living organisms with each other and their surroundings in the context of an urban environment. The urban environment refers to environments dominated by high-density residential and commercial buildings, paved surfaces, and other urban-related factors that create a unique landscape dissimilar to most previously studied environments in the field of ecology. The goal of urban ecology is to achieve a balance between human culture and the natural environment.
Urban ecology is a recent field of study compared to ecology as a whole. The methods and studies of urban ecology are similar to and comprise a subset of ecology. The study of urban ecology carries increasing importance because more than 50% of the world's population today lives in urban areas. At the same time, it is estimated that within the next forty years, two-thirds of the world's population will be living in expanding urban centers. The ecological processes in the urban environment are comparable to those outside the urban context. However, the types of urban habitats and the species that inhabit them are poorly documented. Often, explanations for phenomena examined in the urban setting as well as predicting changes because of urbanization are the center for scientific research.
Ecology has historically focused on "pristine" natural environments, but by the 1970s many ecologists began to turn their interest towards ecological interactions taking place in, and caused by urban environments. Jean-Marie Pelt's 1977 book The Re-Naturalized Human, Brian Davis' 1978 publication Urbanization and the diversity of insects, and Sukopp et al.'s 1979 article "The soil, flora and vegetation of Berlin's wastelands" are some of the first publications to recognize the importance of urban ecology as a separate and distinct form of ecology the same way one might see landscape ecology as different from population ecology. Forman and Godron's 1986 book Landscape Ecology first distinguished urban settings and landscapes from other landscapes by dividing all landscapes into five broad types. These types were divided by the intensity of human influence ranging from pristine natural environments to urban centers.
Urban ecology is recognized as a diverse and complex concept which differs in application between North America and Europe. The European concept of urban ecology examines the biota of urban areas, the North American concept has traditionally examined the social sciences of the urban landscape, as well as the ecosystem fluxes and processes, and the Latin American concept examines the effect of human activity on the biodiversity and fluxes of urban ecosystems. The world's first urban ecology laboratories were founded, for temperate ecosystems, in 1999 (Urban Ecology Research Laboratory, University of Washington), and for tropical ecosystems, in 2008 (Laboratory of Urban Ecology, Universidad Estatal a Distancia of Costa Rica.
Since urban ecology is a subfield of ecology, many of the techniques are similar to that of ecology. Ecological study techniques have been developed over centuries, but many of the techniques use for urban ecology are more recently developed. Methods used for studying urban ecology involve chemical and biochemical techniques, temperature recording, heat mapping remote sensing, and long-term ecological research sites.
Chemical techniques may be used to determine pollutant concentrations and their effects. Tests can be as simple as dipping a manufactured test strip, as in the case of pH testing, or be more complex, as in the case of examining the spatial and temporal variation of heavy metal contamination due to industrial runoff. In that particular study, livers of birds from many regions of the North Sea were ground up and mercury was extracted. Additionally, mercury bound in feathers was extracted from both live birds and from museum specimens to test for mercury levels across many decades. Through these two different measurements, researchers were able to make a complex picture of the spread of mercury due to industrial runoff both spatially and temporally.
Other chemical techniques include tests for nitrates, phosphates, sulfates, etc. which are commonly associated with urban pollutants such as fertilizer and industrial byproducts. These biochemical fluxes are studied in the atmosphere (e.g. greenhouse gasses), aquatic ecosystems and soil vegetation. Broad reaching effects of these biochemical fluxes can be seen in various aspects of both the urban and surrounding rural ecosystems.
Temperature data can be used for various kinds of studies. An important aspect of temperature data is the ability to correlate temperature with various factors that may be affecting or occurring in the environment. Oftentimes, temperature data is collected long-term by the Office of Oceanic and Atmospheric Research (OAR), and made available to the scientific community through the National Oceanic and Atmospheric Administration (NOAA). Data can be overlaid with maps of terrain, urban features, and other spatial areas to create heat maps. These heat maps can be used to view trends and distribution over time and space.
Remote sensing is the technique in which data is collected from distant locations through the use of satellite imaging, radar, and aerial photographs. In urban ecology, remote sensing is used to collect data about terrain, weather patterns, light, and vegetation. One application of remote sensing for urban ecology is to detect the productivity of an area by measuring the photosynthetic wavelengths of emitted light. Satellite images can also be used to detect differences in temperature and landscape diversity to detect the effects of urbanization.
Long-term ecological research (LTER) sites are research sites funded by the government that have collected reliable long-term data over an extended period of time in order to identify long-term climatic or ecological trends. These sites provide long-term temporal and spatial data such as average temperature, rainfall and other ecological processes. The main purpose of LTERs for urban ecologists is the collection of vast amounts of data over long periods of time. These long-term data sets can then be analyzed to find trends relating to the effects of the urban environment on various ecological processes, such as species diversity and abundance over time. Another example is the examination of temperature trends that are accompanied with the growth of urban centers.
Humans are the driving force behind urban ecology and influence the environment in a variety of ways, such as modifying land surfaces and waterways, introducing foreign species, and altering biogeochemical cycles. Some of these effects are more apparent, such as the reversal of the Chicago River to accommodate the growing pollution levels and trade on the river. Other effects can be more gradual such as the change in global climate due to urbanization.
Humans place high demand on land not only to build urban centers, but also to build surrounding suburban areas for housing. Land is also allocated for agriculture to sustain the growing population of the city. Expanding cities and suburban areas necessitate corresponding deforestation to meet the land-use and resource requirements of urbanization. Key examples of this are Deforestation in the United States and Brazil.
Along with manipulation of land to suit human needs, natural water resources such as rivers and streams are also modified in urban establishments. Modification can come in the form of dams, artificial canals, and even the reversal of rivers. Reversing the flow of the Chicago River is a major example of urban environmental modification. Urban areas in natural desert settings often bring in water from far areas to maintain the human population and will likely have effects on the local desert climate. Modification of aquatic systems in urban areas also results in decreased stream diversity and increased pollution.
Both local shipping and long-distance trade are required to meet the resource demands important in maintaining urban areas. Carbon dioxide emissions from the transport of goods also contribute to accumulating greenhouse gases and nutrient deposits in the soil and air of urban environments. In addition, shipping facilitates the unintentional spread of living organisms, and introduces them to environments that they would not naturally inhabit. Introduced or alien species are populations of organisms living in a range in which they did not naturally evolve due to intentional or inadvertent human activity. Increased transportation between urban centers furthers the incidental movement of animal and plant species. Alien species often have no natural predators and pose a substantial threat to the dynamics of existing ecological populations in the new environment where they are introduced. Such invasive species are numerous and include house sparrows, ring-necked pheasants, European starlings, brown rats, Asian carp, American bullfrogs, emerald ash borer, kudzu vines, and zebra mussels among numerous others, most notably domesticated animals. In Australia, it has been found that removing Lantana (L. camara, an alien species) from urban green spaces can surprisingly have negative impacts on bird diversity locally, as it provides refugia for species like the superb fairy (Malurus cyaneus) and silvereye (Zosterops lateralis), in the absence of native plant equivalents . Although, there seems to be a density threshold in which too much Lantana (thus homogeneity in vegetation cover) can lead to a decrease in bird species richness or abundance .
Urbanization results in a large demand for chemical use by industry, construction, agriculture, and energy providing services. Such demands have a substantial impact on biogeochemical cycles, resulting in phenomena such as acid rain, eutrophication, and global warming. Furthermore, natural biogeochemical cycles in the urban environment can be impeded due to impermeable surfaces that prevent nutrients from returning to the soil, water, and atmosphere.
Demand for fertilizers to meet agricultural needs exerted by expanding urban centers can alter chemical composition of soil. Such effects often result in abnormally high concentrations of compounds including sulfur, phosphorus, nitrogen, and heavy metals. In addition, nitrogen and phosphorus used in fertilizers have caused severe problems in the form of agricultural runoff, which alters the concentration of these compounds in local rivers and streams, often resulting in adverse effects on native species. A well-known effect of agricultural runoff is the phenomenon of eutrophication. When the fertilizer chemicals from agricultural runoff reach the ocean, an algal bloom results, then rapidly dies off. The dead algae biomass is decomposed by bacteria that also consume large quantities of oxygen, which they obtain from the water, creating a "dead zone" without oxygen for fish or other organisms. A classic example is the dead zone in the Gulf of Mexico due to agricultural runoff into the Mississippi River.
Just as pollutants and alterations in the biogeochemical cycle alter river and ocean ecosystems, they exert likewise effects in the air. Some stems from the accumulation of chemicals and pollution and often manifests in urban settings, which has a great impact on local plants and animals. Because urban centers are often considered point sources for pollution, unsurprisingly local plants have adapted to withstand such conditions.
Urban environments and outlying areas have been found to exhibit unique local temperatures, precipitation, and other characteristic activity due to a variety of factors such as pollution and altered geochemical cycles. Some examples of the urban effects on climate are urban heat island, oasis effect, greenhouse gases, and acid rain. This further stirs the debate as to whether urban areas should be considered a unique biome. Despite common trends among all urban centers, the surrounding local environment heavily influences much of the climate. One such example of regional differences can be seen through the urban heat island and oasis effect.
The urban heat island is a phenomenon in which central regions of urban centers exhibit higher mean temperatures than surrounding urban areas. Much of this effect can be attributed to low city albedo, the reflecting power of a surface, and the increased surface area of buildings to absorb solar radiation. Concrete, cement, and metal surfaces in urban areas tend to absorb heat energy rather than reflect it, contributing to higher urban temperatures. Brazel et al. found that the urban heat island effect demonstrates a positive correlation with population density in the city of Baltimore. The heat island effect has corresponding ecological consequences on resident species. However, this effect has only been seen in temperate climates.
Greenhouse gas emissions include those of carbon dioxide and methane from the combustion of fossil fuels to supply energy needed by vast urban metropolises. Other greenhouse gases include water vapor, and nitrous oxide. Increases in greenhouse gases due to urban transport, construction, industry and other demands have been correlated strongly with increase in temperature. Sources of methane are agricultural dairy cows  and landfills.
Processes related to urban areas result in the emission of numerous pollutants, which change corresponding nutrient cycles of carbon, sulfur, nitrogen, and other elements. Ecosystems in and around the urban center are especially influenced by these point sources of pollution. High sulfur dioxide concentrations resulting from the industrial demands of urbanization cause rainwater to become more acidic. Such an effect has been found to have a significant influence on locally affected populations, especially in aquatic environments. Wastes from urban centers, especially large urban centers in developed nations, can drive biogeochemical cycles on a global scale.
The urban environment has been classified as an anthropogenic biome, which is characterized by the predominance of certain species and climate trends such as urban heat island across many urban areas. Examples of species characteristic of many urban environments include, cats, dogs, mosquitoes, rats, flies, and pigeons, which are all generalists. Many of these are dependent on human activity and have adapted accordingly to the niche created by urban centers.
Research thus far indicates that, on a small scale, urbanization often increases the biodiversity of non-native species while reducing that of native species. This normally results in an overall reduction in species richness and increase in total biomass and species abundance. Urbanization also reduces diversity on a large scale.
Urban stream syndrome is a consistently observed trait of urbanization characterized by high nutrient and contaminant concentration, altered stream morphology, increased dominance of dominant species, and decreased biodiversity The two primary causes of urban stream syndrome are storm water runoff and wastewater treatment plant effluent.
Diversity is normally reduced at intermediate-low levels of urbanization but is always reduced at high levels of urbanization. These effects have been observed in vertebrates and invertebrates while plant species tend to increase with intermediate-low levels of urbanization but these general trends do not apply to all organisms within those groups. For example, McKinney’s (2006) review did not include the effects of urbanization on fishes and of the 58 studies on invertebrates, 52 included insects while only 10 included spiders. There is also a geographical bias as most of the studies either took place in North America or Europe.
The effects of urbanization also depend on the type and range of resources used by the organism. Generalist species, those that use a wide range of resources and can thrive under a large range of living conditions, are likely to survive in uniform environments. Specialist species, those that use a narrow range of resources and can only cope with a narrow range of living conditions, are unlikely to cope with uniform environments. There will likely be a variable effect on these two groups of organisms as urbanization alters habitat uniformity. Surprisingly, endangered plant species have been reported to occur throughout a wide range of urban ecosystems, many of them being novel ecosystems.
A study of 463 bird species reported that urban species share dietary traits. Specifically, urban species were larger, consumed more vertebrates and carrion, and fed more frequently on the ground or aerially, and also had broader diets than non‐urban species.
The urban environment can decrease diversity through habitat removal and species homogenization—the increasing similarity between two previously distinct biological communities. Habitat degradation and habitat fragmentation reduces the amount of suitable habitat by urban development and separates suitable patches by inhospitable terrain such as roads, neighborhoods, and open parks. Although this replacement of suitable habitat with unsuitable habitat will result in extinctions of native species, some shelter may be artificially created and promote the survival of non-native species (e.g. house sparrow and house mice nests). Urbanization promotes species homogenization through the extinction of native endemic species and the introduction of non-native species that already have a widespread abundance. Changes to the habitat may promote both the extinction of native endemic species and the introduction of non-native species. The effects of habitat change will likely be similar in all urban environments as urban environments are all built to cater to the needs of humans.
Wildlife in cities are more susceptible to suffering ill effects from exposure to toxicants (such as heavy metals and pesticides). In China, fish that were exposed to industrial wastewater had poorer body condition; being exposed to toxicants can increase susceptibility to infection. Humans have the potential to induce patchy food distribution, which can promote animal aggregation by attracting a high number of animals to common food sources; “this aggregation may increase the spread of parasites transmitted through close contact; parasite deposition on soil, water, or artificial feeders; and stress through inter‐ and intraspecific competition.” The results of a study performed by Maureen Murray (et. al.), in which a phylogenetic meta-analysis of 516 comparisons of overall wildlife condition reported in 106 studies was performed, confirmed these results; “our meta‐analysis suggests an overall negative relationship between urbanization and wildlife health, mainly driven by considerably higher toxicant loads and greater parasite abundance, greater parasite diversity, and/or greater likelihood of infection by parasites transmitted through close contact.”
The urban environment can also increase diversity in a number of ways. Many foreign organisms are introduced and dispersed naturally or artificially in urban areas. Artificial introductions may be intentional, where organisms have some form of human use, or accidental, where organisms attach themselves to transportation vehicles. Humans provide food sources (e.g. birdfeeder seeds, trash, garden compost) and reduce the numbers of large natural predators in urban environments, allowing large populations to be supported where food and predation would normally limit the population size. There are a variety of different habitats available within the urban environment as a result of differences in land use allowing for more species to be supported than by more uniform habitats.
Cities should be planned and constructed in such a way that minimizes the urban effects on the surrounding environment (urban heat island, precipitation, etc.) as well as optimizing ecological activity. For example, increasing the albedo, or reflective power, of surfaces in urban areas, can minimize urban heat island, resulting in a lower magnitude of the urban heat island effect in urban areas. By minimizing these abnormal temperature trends and others, ecological activity would likely be improved in the urban setting.
Urbanization has indeed had a profound effect on the environment, on both local and global scales. Difficulties in actively constructing habitat corridor and returning biogeochemical cycles to normal raise the question as to whether such goals are feasible. However, some groups are working to return areas of land affected by the urban landscape to a more natural state. This includes using landscape architecture to model natural systems and restore rivers to pre-urban states.
It is becoming increasingly critical that conservation action be enabled within urban landscapes. Space in cities is limited; urban infill threatens the existence of green spaces. Green spaces that are in close proximity to cities are also vulnerable to urban sprawl. It is common that urban development comes at the cost of valuable land that could host wildlife species. Natural and financial resources are limited; a larger focus must be placed on conservation opportunities that factor in feasibility and maximization of expected benefits. Since the securing of land as a protected area is a luxury that cannot be extensively implemented, alternative approaches must be explored in order to prevent mass extinction of species.
The need to pursue conservation outcomes in urban environments is most pronounced for species whose global distribution is contained within a human-modified landscape. The fact is that many threatened wildlife species are prevalent among land types that were not originally intended for conservation. Of Australia’s 39 urban-restricted threatened species, 11 species occur at roadsides, 10 species occur in private lands, 5 species occur in military lands, 4 species in schools, 4 species in golf courses, 4 species at utility easements (such as railways), 3 species at airports and 1 species at hospitals. The spiked rice flower species Pimelea spicata persists mainly at a golf course, while the guinea-flower hibbertia puberula glabrescens is known mainly from the grounds of an airport. Unconventional landscapes as such are the ones that must be prioritized. The goal in the management of these areas is to bring about a “win-win” situation where conservation efforts are practiced while not compromising the original use of the space. While being near to large human populations can pose risks to endangered species inhabiting urban environments, such closeness can prove to be an advantage as long as the human community is conscious and engaged in local conservation efforts.
With the ever-increasing demands for resources necessitated by urbanization, recent campaigns to move toward sustainable energy and resource consumption, such as LEED certification of buildings, Energy Star certified appliances, and zero emission vehicles, have gained momentum. Sustainability reflects techniques and consumption ensuring reasonably low resource use as a component of urban ecology. Techniques such as carbon recapture may also be used to sequester carbon compounds produced in urban centers rather continually emitting more of the greenhouse gas.
Green Infrastructure Implementation
Urban areas can be converted to areas that are more conducive to hosting wildlife through the application of green infrastructure. Although the opportunities of green infrastructure (GI) to benefit human populations have been recognized, there are also opportunities to conserve wildlife diversity. Green infrastructure has the potential to support wildlife robustness by providing a more suitable habitat than conventional, “grey” infrastructure as well as aid in stormwater management and air purification. GI can be defined as features that were engineered with natural elements or natural features. This natural constitution helps prevent wildlife exposure to man-made toxicants. Although research on the benefits of GI on biodiversity has increased exponentially in the last decade, these effects have rarely been quantified. In a study performed by Alessandro Filazzola (et. al.), 1,883 published manuscripts were examined and meta-analyzed in reference to 33 relevant studies in order to determine the effect of GI on wildlife. Although there was variability in the findings, it was determined that the implementation of GI improved biodiversity compared to conventional infrastructure. In some cases, GI even preserved comparable measures of biodiversity to natural components.
In land-use planning, urban green space is open-space areas reserved for parks and other "green spaces", including plant life, water features -also referred to as blue spaces- and other kinds of natural environment. Most urban open spaces are green spaces, but occasionally include other kinds of open areas. The landscape of urban open spaces can range from playing fields to highly maintained environments to relatively natural landscapes.
Generally considered open to the public, urban green spaces are sometimes privately owned, such as higher education campuses, neighborhood/community parks/gardens, and institutional or corporate grounds. Areas outside city boundaries, such as state and national parks as well as open space in the countryside, are not considered urban open space. Streets, piazzas, plazas and urban squares are not always defined as urban open space in land use planning. Urban green spaces have wide reaching positive impacts on the health of individuals and communities near the green space.Urban greening policies are important for revitalizing communities, reducing financial burdens of healthcare and increasing quality of life. Most policies focus on community benefits, and reducing negative effects of urban development, such as surface runoff and the urban heat island effect. Historically, access to green space has favored wealthier, and more privelaged communities, thus recent focus in urban greening has increasingly focused on environmental justice concerns, and community engagement in the greening process. In particular, in cities with economic decline, such as the Rust Belt in the United States, urban greening has broad community revitalization impacts.
Increasing Wildlife Habitat Connectivity
The implementation of wildlife corridors throughout urban areas (and in between wildlife areas) would promote wildlife habitat connectivity. Habitat connectivity is critical for ecosystem health and wildlife conservation yet is being compromised by increasing urbanization. Urban development has caused green spaces to become increasingly fragmented and has caused adverse effects in genetic variation within species, population abundance and species richness. Urban green spaces that are linked by ecosystem corridors have higher ecosystem health and resilience to global environmental change. Employment of corridors can form an ecosystem network that facilitates movement and dispersal. However, planning these networks requires a comprehensive spatial plan.
One approach is to target “shrinking” cities (such as Detroit, Michigan, USA) that have an abundance of vacant lots and land that could be repurposed into greenways to provide ecosystem services (although even cities with growing populations typically have vacant land as well). However, even cities with high vacancy rates sometimes can present social and environmental challenges. For instance, vacant land that stands on polluted soils may contain heavy metals or construction debris; this must be addressed before the repurposing. Once land has been repurposed for ecosystem services, avenues must be pursued that could allow this land to contribute to structural or functional connectivity.
Structural connectivity refers to parts of the landscape that are physically connected. Functional connectivity refers to species-specific tendencies that indicate interaction with other parts of the landscape. Throughout the City of Detroit, spatial patterns were detected that could promote structural connectivity. The research performed by Zhang “integrates landscape ecology and graph theory, spatial modeling, and landscape design to develop a methodology for planning multifunctional green infrastructure that fosters social-ecological sustainability and resilience”. Using a functional connectivity index, there was found to be a high correlation between these results (structural and functional connectivity), suggesting that the two metrics could be indicators of each other and could guide green space planning.
Although urban wildlife corridors could serve as a potential mitigation tool, it is important that they are constructed so as to facilitate wildlife movement without restriction. As humans may be perceived as a threat, the success of the corridors is dependent on human population density proximity to roads. In a study performed by Tempe Adams (et. al.), remote-sensor camera traps and data from GPS collars were utilized to assess whether or not the African elephant would use narrow urban wildlife corridors. The study was performed in three different urban-dominated land use types (in Botswana, South Africa) over a span of two years.
The results of the study indicated that elephants tended to move through unprotected areas more quickly, spending less time in those areas. Using vehicular traffic as a measure of human activity, the study indicated that elephant presence was higher during times when human activity was at a minimum. It was determined that “formal protection and designation of urban corridors by the relevant governing bodies would facilitate coexistence between people and wildlife at small spatial scales.” However, the only way this co-existence could be feasible is by creating structural connectivity (and thus promoting functional connectivity) by implementing proper wildlife corridors that facilitate easy movement between habitat patches. The usage of green infrastructure that is connected to natural habitats has been shown to reap greater biodiversity benefits than GI implemented in areas far from natural habitats. GI close to natural areas may also increase functional connectivity in natural environments.
In the United States, roadkill takes the lives of hundreds of thousands to hundreds of millions of mammals, birds and amphibians each year. Roadkill mortality has detrimental effects on the persistence probability, abundance and genetic diversity of wildlife populations (more so than reduced movement through habitat patches). Roadkill also has an effect on driver safety. If green areas cannot be reserved, the presence of wildlife habitats in close proximity to urban roads must be addressed. The optimal situation would be to avoid constructing roads next to these natural habitats, but other preventative measures can be pursued to reduce animal mortality. One way these effects could be mitigated is through implementation of wildlife fencing in prioritized areas. Many countries utilize underpasses and overpasses combined with wildlife fencing to reduce roadkill mortality in an attempt to restore habitat connectivity. It is unrealistic to try to fence entire road networks because of financial constraints. Therefore, areas in which the highest rates of mortality occur should be focused on.
Urban sprawl is one of many ways that Indigenous Peoples land is taken and developed, thus the intimate knowledge of the native area (ecology) is often lost due to the effects of colonization or because the land has been majorly altered. Urban development occurs around areas where Indigenous Peoples lived as these areas are easy for transport and the natural environmental is fruitful. When developing areas of urban land, consideration should go towards the intimate levels of knowledge held by Indigenous Peoples and the biocultural and linguistic diversity of the place. Urban ecology follows western science frameworks and compartmentalizes nature. Urban ecology has the opportunity to be viewed in an interconnected and holistic way, through "Two-Eyed Seeing" and be inclusive of the Traditional Ecological Knowledge held by the local Indigenous Peoples of the area.
Urban restoration ecology would be enriched by partnerships with the local Indigenous Peoples, if done in a respectful way that addresses the currently inequitable relationship. Non-indigenous people can support their local Indigenous communities by learning about the history of the land and ecosystems that is being restored or studied. Ecological restoration built with strong Indigenous partnerships benefits the Indigenous culture and identity, as well as all urban dwellers.
Urbanization results in a series of both local and far-reaching effects on biodiversity, biogeochemical cycles, hydrology, and climate, among many other stresses. Many of these effects are not fully understood, as urban ecology has only recently emerged as a scientific discipline and much more research remains to be done. Research on cities outside the US and Europe remains limited. Observations on the impact of urbanization on biodiversity and species interactions are consistent across many studies but definitive mechanisms have yet to be established. Urban ecology constitutes an important and highly relevant subfield of ecology, and further study must be pursued to more fully understand the effects of human urban areas on the environment.