Urban ecology is the study of how living things interact with each other and their environment in cities. An urban environment includes areas with many buildings where people live and work, roads and sidewalks made of hard materials, and other features that make city landscapes different. The purpose of urban ecology is to find ways for humans and nature to coexist peacefully.

Urban ecology is a newer area of study compared to general ecology. Most research in this field so far has focused on studying mammals and birds because they are easier to observe. To learn more, scientists should study all types of animals in cities, including insects and fish. This research should also include suburban areas, which have a mix of natural spaces and developed land. The methods used in urban ecology are part of the larger field of ecology.

Urban ecology is becoming more important because over half of the world’s people now live in cities. It is also predicted that in the next 40 years, two-thirds of the world’s population will live in growing urban areas. The natural processes that happen in cities are similar to those in non-urban areas. However, scientists know little about the different types of habitats in cities and the animals that live there. More research is needed to better understand urban ecosystems.

History

Historically, ecology has studied natural environments, but by the 1970s, many ecologists started focusing on ecological interactions in urban areas. In the 1800s, scientists like Malthus, De Candolle, Lyell, and Darwin discovered that competition for resources affects population growth and leads to extinction. This idea became the foundation of evolutionary ecology. Books and articles from the 1970s, such as Jean-Marie Pelt’s The Re-Naturalized Human (1977), Brian Davis’ Urbanization and the diversity of insects (1978), and Herbert Sukopp et al.’s article “The soil, flora and vegetation of Berlin’s wastelands” (1979), were among the first to recognize urban ecology as a unique field, similar to how landscape ecology differs from population ecology. In 1986, Forman and Godron’s book Landscape Ecology separated urban areas and landscapes into five categories based on how much human activity influenced them, from untouched natural environments to cities.

Early ecologists defined ecology as the study of living things and their environments. Over time, urban ecology became a complex topic with different focuses in North America, Europe, and Latin America. In Europe, urban ecology examines the plants and animals in cities. In North America, it has traditionally studied the social aspects of cities and the processes within urban ecosystems. In Latin America, it focuses on how human actions affect biodiversity and ecosystems in cities. A new wave of urban ecology research began in the 1990s when the U.S. National Science Foundation funded two long-term ecological research centers, which helped advance the study of urban ecology.

Today, urban ecology is growing quickly, with more research centers being created. The Urban Ecology Research Laboratory (UERL) at the University of Washington, founded in 2001, studies urban landscapes, ecosystem functions, land cover changes, and ways cities can adapt to challenges. The Urban Ecology Laboratory (LEU) at the Costa Rican Distance University, established in 2008, is the first center dedicated to studying tropical urban ecosystems. Research there includes biodiversity, effects of climate change on cities and nearby areas, especially tropical highlands, and how human activities interact with urban environments.

Methods

Urban ecology is a part of ecology, so many of the methods used are similar to those in ecology. Ecological study methods have been developed over many years, but some methods used in urban ecology are newer. Techniques for studying urban ecology include chemical and biochemical tests, temperature recording, heat mapping, remote sensing, and long-term ecological research sites.

Chemical methods can be used to find pollutant levels and their effects. Tests can be simple, like using a test strip to check pH, or more complex, such as studying how heavy metals spread from industrial runoff. In one study, scientists examined the livers of birds from different parts of the North Sea to measure mercury levels. They also tested mercury in bird feathers from both living birds and museum samples to compare mercury levels over many years. These tests helped scientists understand how mercury spreads from industrial runoff over time and across different areas.

Other chemical methods include testing for nitrates, phosphates, and sulfates, which are often found in urban pollutants like fertilizer and industrial waste. These chemical changes are studied in the air, water, and soil. Their effects can be seen in both urban and nearby rural areas.

Temperature data is used in many studies. It helps scientists connect temperature to other environmental factors. Long-term temperature data is often collected by the Office of Oceanic and Atmospheric Research (OAR) and shared by the National Oceanic and Atmospheric Administration (NOAA). This data can be combined with maps of land, cities, and other areas to create heat maps. These maps show temperature trends and patterns over time and space.

Remote sensing is a method where data is collected from faraway places using satellites, radar, and aerial photos. In urban ecology, it is used to study land, weather, light, and plants. One use of remote sensing is measuring the light emitted by plants to determine how productive an area is. Satellite images can also show temperature differences and landscape variety to study the effects of urban growth.

Long-term ecological research (LTER) sites are places where scientists collect reliable data over many years to study long-term climate or ecological changes. These sites provide data on temperature, rainfall, and other ecological processes over time. For urban ecologists, LTERs help collect large amounts of data over long periods. This data can be used to find trends in how urban areas affect ecological processes, such as the number and variety of species over time. For example, scientists study how temperature changes relate to the growth of cities. Two active urban LTERs are the Central Arizona-Phoenix (CAP), started in 1997 at Arizona State University, and the Minneapolis-St. Paul Metropolitan Area (MSP). The Baltimore Ecosystem Study (BES) was an urban LTER funded since 1998 but is no longer supported by the National Science Foundation as of 2021.

Urban effects on the environment

Urban ecology is shaped by human activity, which affects the environment in many ways. One major example is urbanization, which connects social, economic, and environmental processes. Six main areas are important to study: air pollution, ecosystems, land use, biogeochemical cycles, water pollution, solid waste management, and climate. Urbanization began with people moving to cities, which led to environmental changes like increased carbon emissions, higher energy use, and harm to natural ecosystems. These effects were mostly negative. However, today, people are starting to see urbanization as a chance to solve problems rather than just a challenge. Cities have many people who are wealthy, educated, and creative, and they are using science to improve urban policies and ideas. A combined approach that looks at how different systems work together in cities can help solve these issues at a low cost. These solutions are broad, work in many ways, and consider the social, economic, and cultural aspects of cities. They also take into account chemical, physical, and ecological factors that define urban areas, including lifestyle choices connected to city culture. Even though cities are using these opportunities, the results of research ideas are still unclear.

People use land not only to build cities but also to create suburban areas for housing. Land is also used for farming to support growing city populations. As cities and suburbs expand, forests are often cut down to meet land and resource needs. Examples of this include deforestation in the United States and Europe.

In cities, natural water sources like rivers and streams are changed to meet human needs. Changes can include building dams, creating artificial canals, or even reversing river flows. A major example is the reversal of the Chicago River. In desert areas, cities often bring water from far away to support people, which can affect local climates. Changing water systems in cities can also reduce the variety of life in streams and increase pollution.

Both local shipping and long-distance trade are needed to supply cities with resources. Transporting goods releases carbon dioxide, which adds to greenhouse gases and pollution in urban areas. Shipping also accidentally spreads living organisms to places where they do not naturally live. These non-native species often lack natural predators and can harm local ecosystems. Invasive species, which are successful at spreading quickly and adapting to new environments, include animals like house sparrows, brown rats, and plants like kudzu vines. Brown rats are common in cities like New York, where they damage infrastructure, harm native species, and spread diseases. A study found that many New York rats originally came from Great Britain. In Australia, removing an invasive plant called Lantana from urban areas can harm bird populations that rely on it for shelter. However, too much Lantana can reduce bird diversity by making the environment too uniform.

Some urban animals help humans. Studies show that having pets can reduce stress, loneliness, and anxiety. Some animals also control pests that harm humans. Urban species can also help with farming, transportation, and protection.

Other urban animals harm humans. Pests like rats can spread germs through their waste or skin, which can cause diseases such as salmonella, meningitis, and Lyme disease. Some people are allergic to insects like bees and wasps, and exposure can cause serious reactions like rashes.

According to Seth Magle, rare attacks by wildlife in cities can affect how people view animals. Media coverage of these rare events makes people think such interactions are more common than they are, which can reduce tolerance for urban wildlife.

Urbanization increases the use of chemicals in industries, construction, agriculture, and energy production. This affects natural cycles, causing problems like acid rain, eutrophication, and global warming. In cities, impermeable surfaces like concrete stop nutrients from returning to soil, water, and air, disrupting natural processes.

Expanding cities increase the need for fertilizers, which change soil chemistry by adding high levels of sulfur, phosphorus, nitrogen, and heavy metals. Fertilizers also cause runoff, which pollutes rivers and streams. This runoff leads to eutrophication, where excess nutrients cause algae to grow rapidly. When the algae die, bacteria consume oxygen, creating "dead zones" where aquatic life cannot survive. A well-known example is the dead zone in the Gulf of Mexico, caused by fertilizer runoff from the Mississippi River.

Pollution and changes in natural cycles also affect air quality. Chemicals and pollution in urban areas impact plants and animals. Since cities are major sources of pollution, local plants have adapted to survive in these conditions.

Urban effects on climate

Urban areas and nearby regions often have different temperatures, rainfall, and other environmental conditions because of factors like pollution and changes in natural chemical cycles. Examples of how cities affect climate include the urban heat island, oasis effect, greenhouse gases, and acid rain. This has led to discussions about whether cities should be considered a separate type of ecosystem. While cities share some common climate patterns, the local environment around them greatly influences their climate. For example, the urban heat island and oasis effect show how different regions can experience unique climate changes.

The urban heat island is a situation where the center of a city is much warmer than the surrounding areas. This happens because surfaces like concrete, cement, and metal in cities absorb more heat instead of reflecting it. Research in Baltimore found that cities with more people tend to have stronger heat island effects. This increased heat can affect the plants and animals living there, but these effects are mainly seen in temperate climates.

Greenhouse gases help keep Earth warm enough for humans to live by trapping heat from the sun. In 1896, a scientist named Svante Arrhenius discovered that burning fossil fuels releases carbon dioxide, the most common and harmful greenhouse gas. In the 20th century, James E. Hansen found that the greenhouse effect is making Earth's climate worse.

Carbon dioxide makes up three-fourths of all greenhouse gas emissions and comes from burning coal, oil, gas, wood, and other materials. Methane, another greenhouse gas, is released from landfills, natural gas, and petroleum industries. Nitrous oxide, which makes up about 6% of emissions, comes from fertilizers, manure, and burning agricultural waste. Fluorinated gases, which account for 2% of emissions, are found in refrigerants and solvents. Too much of these gases causes problems like global warming, health issues from pollution, and the loss or movement of animal species. These problems can be reduced by replacing fossil fuels with renewable energy sources.

Pollution from cities changes the natural cycles of carbon, sulfur, nitrogen, and other elements. Ecosystems near cities are especially affected by these pollution sources. High levels of sulfur dioxide from industrial activity make rainwater more acidic, which harms aquatic life. Waste from large cities in developed countries can affect chemical cycles worldwide.

Urban areas are considered a human-made biome, which is defined by specific species and climate patterns like the urban heat island. Common animals in cities include cats, dogs, mosquitoes, rats, flies, and pigeons. These animals are generalists that rely on human activity and have adapted to city life. However, many wild species are also found in cities, showing that urban areas support a wide variety of life. The relationship between cities and wildlife diversity is more complex than previously thought. This change in understanding comes from studying more cities in different parts of the world, which showed that earlier ideas were biased toward cities in temperate, developed countries.

Biodiversity and urbanization

Studies in temperate regions show that urbanization can increase the number of non-native species in small areas but often decreases the number of native species. This usually leads to fewer total species and more total living things, like plants and animals. In developed countries, urbanization reduces diversity on a large scale, but in tropical and subtropical cities, high biodiversity can remain if small natural areas are kept within cities. Even homes and neighborhoods near busy city centers can support over 1,000 types of large living things, many of which are native. These urban areas can also support complex interactions between living things. However, urbanization often harms interactions that happen in natural areas.

Urban stream syndrome is a common effect of urbanization. It includes high levels of nutrients and pollutants in streams, changes in stream shapes, more dominance by a few species, and lower biodiversity. The main causes are stormwater runoff and wastewater from treatment plants.

Biodiversity usually decreases when urbanization is at low to medium levels but always decreases at high levels. This has been seen in animals and insects, though plants often increase at low to medium levels. These trends are not the same for all species. For example, one study did not look at fish, and most studies on insects focused on bugs rather than spiders. Most research has been done in North America or Europe.

The effects of urbanization depend on the resources an organism needs. Generalist species, which can survive in many conditions and use many types of food, are more likely to live in uniform environments. Specialist species, which need specific conditions and limited food sources, are less likely to survive in such environments. As urbanization changes habitat uniformity, its effects on these groups may vary. Some endangered plant species have been found in many urban areas, even in new types of ecosystems.

A study of 463 bird species found that urban birds share similar eating habits. They tend to be larger, eat more vertebrates and dead animals, and forage more on the ground or in the air. They also have more varied diets than non-urban birds. These changes are part of a process called synurbization, where wildlife adapts to city life.

Urban areas can reduce diversity by destroying habitats and making different ecosystems more similar. Habitat loss and fragmentation, caused by roads, neighborhoods, and parks, reduce the amount of suitable habitat. This can lead to the extinction of native species, but artificial shelters may help non-native species survive, like house sparrows and mice. Urbanization also makes ecosystems more similar by removing native species and introducing non-native ones. This can affect parasite communities, such as in California cities, where some bird parasites decline while others increase.

Wildlife in cities is more likely to suffer from harmful substances like heavy metals and pesticides. In China, fish exposed to industrial waste had worse health. Humans can also create uneven food sources, which may cause animals to gather in groups. This can spread parasites and increase competition between animals. A study of 106 studies found that urbanization is linked to worse wildlife health, mainly due to higher levels of toxins and more parasites.

Urban areas can also increase diversity. Many non-native species are introduced through human activity or naturally. Humans provide food sources, like bird feeders and trash, and reduce large predators, allowing more animals to live in cities. Urban areas have varied habitats, like parks and gardens, which support more species than uniform environments.

Urban habitats, such as backyards and parks, can reveal new species. In 2025, scientists discovered several new species in cities, including a skink in Chengdu, China, a fish in Beijing, and a beetle in Washington, D.C. Other new species were found in cities like Shenzhen, Mexico City, Los Angeles, Brussels, and São Paulo. Urban areas have also revealed new vertebrates, like a frog in New York City and a spider in Bangladesh.

Urbanization changes not only which species live in cities but also how they behave. Cities often favor traits like less fear of humans, reliance on human food, and changes in daily activity patterns.

Ways to improve urban ecology: civil engineering and sustainability

Cities should be planned and built to reduce their impact on the environment, such as the urban heat island effect and changes in rainfall patterns. This can be done by increasing the reflective power of surfaces, which helps lower temperatures in cities. Reducing these temperature changes can improve natural activities in urban areas.

Urbanization has greatly affected the environment, both locally and globally. It is difficult to create natural paths for animals or restore natural cycles, which makes it unclear if these goals can be achieved. However, some groups are working to make areas affected by cities more natural again. They use landscape design to copy natural systems and restore rivers to their original condition.

It is important to protect nature within cities because space is limited. Building new areas in cities can reduce green spaces, and nearby natural areas are also at risk from city expansion. Often, city development takes over land that could support wildlife. Since natural and financial resources are limited, efforts to protect nature must focus on practical solutions that provide the most benefit. Because it is not always possible to set aside land as protected areas, other ways to protect wildlife must be explored. Borgström et al. 2006 noted that urban areas can have challenges where the best actions depend on the size of the species. For example, small species like soil microbes can live in small spaces like gardens, but larger or more mobile species, such as pollinators, need larger or connected areas to survive.

Protecting wildlife in cities is especially important for species that only live in human-modified areas. Many endangered animals are found in places not originally meant for conservation, such as roadsides, private lands, military areas, schools, golf courses, and airports. For example, the spiked rice flower lives mainly on golf courses, and the guinea-flower is found mostly on airport grounds. These unusual areas must be prioritized for conservation. The goal is to find ways to protect wildlife while keeping the original use of the space. While being close to people can harm some species, it can also help if communities are involved in conservation efforts.

Bringing back species to cities can help restore lost biodiversity, but certain rules must be followed to avoid problems:

• No animals that can harm children will be reintroduced to cities.
• Species that threaten human health, pets, crops, or property will not be brought back.
• Reintroduction will not happen if it causes harm to the animals, such as stress from capture.
• Animals that carry diseases will not be reintroduced.
• Animals whose genes could harm other species in the city will not be reintroduced.
• Reintroduction will only occur if scientific evidence shows the animals can survive long-term. If resources are not enough for long-term care, reintroduction will not be done.
• Reintroduced animals will get food and medical help as needed.
• Reintroduction will be tested in both experimental and control areas, with monitoring to address problems if they arise.
• Reintroduction will happen in multiple places and repeated over years to handle unexpected events.
• People in affected areas must be involved in decisions and educated to support conservation, though final choices must be based on scientific evidence.

As cities grow, efforts to use resources more sustainably, such as LEED-certified buildings, Energy Star appliances, and zero-emission vehicles, have become more common. Sustainability includes using resources wisely as part of urban planning. Methods like capturing carbon or using renewable energy (such as solar, wind, or geothermal power) can also reduce pollution.

Cities can support wildlife by using green infrastructure, which includes natural features like parks, wetlands, and green roofs. While green infrastructure benefits people, it can also help protect wildlife by providing better habitats than traditional, man-made structures. It also helps manage rainwater and clean the air. Green infrastructure uses natural elements to reduce exposure to harmful chemicals. Although research on its benefits has grown, studies have shown that green infrastructure improves biodiversity compared to traditional infrastructure. In some cases, it even supports biodiversity levels similar to natural areas.

In urban planning, green spaces are open areas with plants, water features, and natural environments, such as parks. These spaces can range from well-maintained fields to more natural, less managed areas. Some green spaces are not publicly accessible, like school fields, university campuses, or corporate areas. Areas outside cities, such as national parks, are not considered urban green spaces. Boulevards, plazas, and city squares are not included in this category.

Summary

Urbanization causes many effects on biodiversity, chemical processes in the environment, water movement, and climate, among other areas. These effects are not completely understood because urban ecology is a newer area of study, and more research is needed. Studies outside the United States and Europe are limited. Many studies show similar impacts of urbanization on biodiversity and how species interact, but the exact causes of these effects are still unclear. Urban ecology is an important and very relevant area of study within ecology. More research must be done to better understand how human cities affect the environment.