Urban ecology is the study of how living things interact with each other and their environment in cities. A city environment includes many houses and businesses, lots of paved roads, and other features that make it different from natural areas. The purpose of urban ecology is to find ways for people and nature to coexist peacefully.

Urban ecology is a newer area of study compared to general ecology. Most research in this field focuses on easier-to-study animals like mammals and birds [source needed]. To learn more, scientists should study all types of animals, including insects and fish, in cities. This study should also include suburban areas, which have a mix of buildings and natural spaces. The methods used in urban ecology are part of the larger field of ecology.

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

History

In the past, ecology mainly studied natural environments. However, by the 1970s, many ecologists began focusing on how urban areas affect ecosystems and the interactions within them. In the 1800s, scientists like Malthus, De Candolle, Lyell, and Darwin discovered that competition for resources influences population growth and contributes to species extinction. This idea became the foundation of evolutionary ecology. Early works, such as Jean-Marie Pelt’s 1977 book The Re-Naturalized Human, Brian Davis’ 1978 article Urbanization and the diversity of insects, and Herbert Sukopp et al.’s 1979 study “The soil, flora and vegetation of Berlin’s wastelands,” were among the first to highlight urban ecology as a unique field, similar to how landscape ecology differs from population ecology. In 1986, Forman and Godron’s book Landscape Ecology categorized all landscapes into five types based on how much human activity influenced them, from untouched natural areas to heavily developed cities.

Early ecologists defined ecology as the study of how living things interact with their environment. Over time, urban ecology became recognized as a complex field 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 urban areas, as well as how ecosystems function. In Latin America, it focuses on how human activities affect biodiversity and ecosystem processes in cities. A major growth in urban ecology research began in the 1990s when the U.S. National Science Foundation funded two long-term urban ecological research centers, encouraging more study in this area.

Today, urban ecology is a growing field with more research centers being established. The Urban Ecology Research Laboratory (UERL) at the University of Washington, founded in 2001, studies urban landscapes, ecosystem functions, and how land use changes over time. The Urban Ecology Laboratory (LEU) at Costa Rica’s Distance University, created in 2008, is the first center dedicated to researching tropical urban ecosystems. Research there includes studying biodiversity, how climate change affects cities and nearby areas, especially tropical highlands, and how human activities influence urban environments.

Methods

Urban ecology is a part of the larger field of ecology, so many methods used in both areas are similar. Scientists have used many techniques to study ecology for a long time, but some methods used in urban ecology are newer. Techniques for studying urban ecology include chemical and biochemical methods, temperature recording, heat mapping, remote sensing, and long-term ecological research sites.

Chemical methods help scientists find the amounts of pollutants and their effects. Some tests are simple, like using a test strip to check the pH of water. Other tests are more complex, such as studying how heavy metals like mercury spread in the environment. In one study, scientists examined the livers of birds from the North Sea and tested for mercury. They also checked mercury levels in feathers from both living birds and birds stored in museums. These tests helped scientists understand how mercury pollution spread over time and across different areas.

Other chemical methods test for substances like nitrates, phosphates, and sulfates, which are often found in urban areas due to things like fertilizers 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. Scientists can link temperature changes 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 over time and space.

Remote sensing is a method where scientists collect data from faraway places using satellites, radar, and aerial photos. In urban ecology, this technique helps study land shapes, weather, light, and plants. For example, scientists use satellite images to measure the light plants emit during photosynthesis to see how productive an area is. These images also show temperature differences and landscape changes caused by urban growth.

Long-term ecological research (LTER) sites are areas where scientists collect reliable data over many years to study long-term changes in the environment. These sites provide data on things like temperature, rainfall, and other ecological processes. For urban ecologists, LTER sites are important because they offer large amounts of data collected over time. This data helps scientists find trends, such as how urban areas affect species diversity or temperature changes. Two active urban LTER sites are the Central Arizona-Phoenix (CAP) site, started in 1997 at Arizona State University, and the Minneapolis-St. Paul Metropolitan Area (MSP) site. The Baltimore Ecosystem Study (BES) was an urban LTER site started in 1998 but is no longer supported by the National Science Foundation as of 2021.

Urban effects on the environment

Urban areas are shaped by human activities, which affect the environment in many ways. One major example is urbanization, the process of cities growing and changing. This process connects social, economic, and environmental changes. There are six main areas to consider: air pollution, ecosystems, land use, chemical cycles, water pollution, waste management, and climate. Urbanization started with people moving to cities, which led to problems like more carbon emissions, higher energy use, and harm to nature. However, today, people are focusing more on solutions than challenges. Many cities have people who are wealthy, educated, and creative, and they are using science to improve city planning. Cities have complex systems that work together, and five key features can help solve problems in a cost-effective way. These solutions consider the social, economic, and cultural needs of cities, as well as their natural and chemical processes. They also take into account how people live and how their choices affect the environment. However, even with these efforts, it is unclear how well these solutions will work in the long term.

People use land not only to build cities but also for suburbs and farmland to support growing populations. As cities expand, forests are often cut down to meet land needs. Examples of this include deforestation in the United States and Europe.

In cities, natural water systems like rivers and streams are changed to meet human needs. This can include building dams, canals, or even reversing river flows. A famous example is the Chicago River, which was redirected. In desert areas, water is brought from far away to support people, which can affect local climates. Changing water systems also harms water diversity and increases pollution.

Transporting goods, both locally and over long distances, is needed to support cities. This process adds carbon dioxide to the air, contributing to greenhouse gases and pollution. Shipping also accidentally moves living organisms to new areas. These non-native species can disrupt local ecosystems because they often lack natural predators. Invasive species, like house sparrows, rats, and zebra mussels, can spread quickly and harm native plants and animals. For example, brown rats in New York City live in streets and subways and can carry diseases that affect humans. Studies show some rats in NYC originally came from Great Britain. In Australia, removing an invasive plant called Lantana can harm bird populations because it provides shelter for some species. However, too much Lantana can reduce bird diversity.

Some urban animals help humans by reducing stress, controlling pests, or aiding in agriculture and transportation. However, other animals can spread diseases or cause allergies. For example, pests like rats can carry germs that cause illnesses such as salmonella or Lyme disease. People may also have allergic reactions to bees or wasps.

According to Seth Magle, rare attacks by urban wildlife are sometimes reported in the media, making people believe such events happen more often than they do. This can lower public acceptance of wildlife in cities.

Urbanization increases the use of chemicals in industries, construction, agriculture, and energy production. This affects natural chemical cycles, leading to problems like acid rain, eutrophication, and global warming. In cities, impermeable surfaces, such as roads, stop nutrients from returning to soil and water.

Expanding cities increase the need for fertilizers, which can change soil chemistry by adding too much sulfur, nitrogen, and phosphorus. These chemicals can run off into rivers and streams, causing eutrophication. This happens when excess nutrients lead to algae blooms, which then die and use up oxygen in the water, creating "dead zones" where marine 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 chemical changes also affect air quality, harming plants and animals. Cities are often major sources of pollution, and some plants have adapted to survive in these conditions.

Urban effects on climate

Urban areas and nearby regions often have different temperatures, rainfall, and other climate features 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 have varying impacts.

The urban heat island is when the center of a city is much warmer than surrounding areas. This happens because surfaces like concrete, cement, and metal absorb more heat than they reflect. Research in Baltimore found that the heat island effect becomes stronger as city populations grow. This higher temperature can affect plants and animals living in the area, though this effect is mainly seen in temperate climates.

Greenhouse gases help keep Earth warm enough for life by trapping heat from the sun. In 1896, scientist Svante Arrhenius discovered that burning fossil fuels, like coal and oil, releases carbon dioxide, the most common and harmful greenhouse gas. In the 20th century, scientist James E. Hansen showed 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 fuels like coal, oil, and wood. Methane, another greenhouse gas, is released from landfills, natural gas, and oil industries. Nitrous oxide, which makes up about 6% of emissions, comes from fertilizers, animal waste, and burning crops. Fluorinated gases, responsible for 2% of emissions, are found in refrigerants and solvents. Too many greenhouse gases cause problems like global warming, pollution-related health issues, and changes in wildlife habitats. These issues could be reduced by replacing fossil fuels with renewable energy sources.

Pollution from cities changes the natural cycles of elements like carbon, sulfur, and nitrogen. Ecosystems near cities are especially affected by these pollution sources. High levels of sulfur dioxide from industry make rainwater more acidic, harming aquatic life. Waste from large cities can influence chemical cycles worldwide.

Urban areas are classified as human-made biomes, shaped by human activity and climate patterns like the urban heat island. Common animals in cities include cats, dogs, rats, and pigeons, which adapt well to human environments. However, many wild species also live in cities, showing that urban areas support a wide range of life. Studies of cities in different parts of the world suggest that earlier ideas about urban wildlife were biased toward cities in developed, temperate regions.

Biodiversity and urbanization

Research in temperate regions shows that urbanization can increase the number of non-native species while decreasing native species in small areas. This often leads to fewer total species but more total living organisms. In developed countries, cities usually lose biodiversity, but in tropical and subtropical cities, high biodiversity can remain if small natural areas are kept within the city. Even homes near busy city centers can support over 1,000 types of large animals and plants, many of which are native. These areas can also support complex interactions between living things. However, urbanization can break up many natural interactions between species.

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

Biodiversity often decreases when urbanization is moderate or low but always decreases at high levels. This has been seen in animals and insects, though plants may increase slightly at lower urbanization levels. These patterns are not the same for all species. For example, a study by McKinney (2006) did not look at fish, and of 58 studies on insects, most focused on bugs while few studied spiders. Most research has been done in North America and Europe.

The effects of urbanization depend on how organisms use resources. Generalist species, which can use many resources and live in varied conditions, are more likely to survive in uniform environments. Specialist species, which need specific resources and conditions, struggle in such environments. As urbanization changes habitats, these two groups may be affected differently. Some endangered plant species have been found in cities, even in new types of ecosystems.

A study of 463 bird species found that urban birds share certain traits. They tend to be larger, eat more vertebrates and dead animals, forage on the ground or in the air, and 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 ecosystems more similar. Habitat loss and fragmentation, caused by roads, neighborhoods, and parks, reduce suitable living spaces. This can lead to the extinction of native species but may help non-native species survive, such as house sparrows and mice. Urbanization also increases similarity between ecosystems by removing native species and introducing non-native ones. This affects parasites too, like in California, where some bird parasites decline while others grow more common.

Wildlife in cities faces more harm from harmful chemicals like heavy metals and pesticides. In China, fish exposed to industrial waste had worse health. Humans can also create uneven food sources, leading animals to gather in groups, which may spread parasites and increase stress from competition. A study by Maureen Murray (et al.) found that urbanization is linked to poorer wildlife health, mainly due to higher levels of toxins and parasites.

Urban areas can also increase biodiversity by introducing new species. Humans may bring organisms intentionally, like plants for gardens, or accidentally, like insects on vehicles. Cities provide food sources (like bird feeders and trash) and reduce large predators, allowing more animals to thrive. Diverse land uses in cities, such as parks and gardens, support more species than uniform habitats.

Urban environments, like backyards and parks, can reveal new species. In 2025, several species were discovered in cities, including a lizard in Chengdu, China, a fish in Beijing, and a beetle in Washington, D.C. Other new species include a spider in Mexico City, flies in Los Angeles, and insects in cities across Europe and Brazil. Even vertebrates like a frog in New York City and a fish in Bangladesh have been found in urban areas.

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

Ways to improve urban ecology: civil engineering and sustainability

Cities should be planned and built in ways that reduce the negative effects they have on the environment, such as increased temperatures and changes in rainfall. For example, using materials that reflect more sunlight (called albedo) can help lower the temperature in cities, which reduces the urban heat island effect. By reducing these temperature changes and other environmental issues, nature can thrive better in urban areas.

Urbanization has greatly changed the environment, both locally and globally. Creating natural paths for animals and restoring natural processes, like how nutrients move through ecosystems, is difficult. However, some groups are trying to make urban areas more natural by designing landscapes that mimic natural systems and restoring rivers to their original condition before cities were built.

It is important to protect wildlife and nature within cities because space is limited. Building new structures in cities can reduce the amount of green areas, like parks and forests. Green spaces near cities are also at risk from expanding urban areas. Often, development takes over land that could support wildlife. Since natural and financial resources are limited, it is important to focus on conservation efforts that are practical and provide the most benefit. Because it is not always possible to set aside large areas as protected spaces, other methods must be used to protect species from extinction. Borgström et al. (2006) explain that urban ecosystems can have "scale mismatch," meaning the best way to protect species depends on their size. Small species, like soil microbes, can survive in small, isolated areas, but larger, more mobile species, like pollinators, need larger or connected spaces to live safely.

Protecting wildlife in cities is especially important for species that only live in human-altered environments. Many endangered species 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 (Pimelea spicata) is mainly found on golf courses, and the guinea-flower (Hibbertia puberula glabrescens) is mostly found on airport grounds. These unusual areas must be prioritized for conservation. The goal is to create a "win-win" situation where conservation efforts do not interfere with the original use of the space. While living near people can harm some species, it can also help if people are aware and support local conservation efforts.

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

• No animals that could 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 being captured.
• Animals that carry diseases will not be reintroduced.
• Species that could harm the genetic makeup of other animals in the city will not be reintroduced.
• Reintroduction will only occur if scientific evidence shows a good chance of long-term survival. If resources are not enough for long-term care, reintroduction will not happen.
• Reintroduced animals will get food and medical help if needed.
• Reintroduction will take place in both test and control areas to ensure reliable results. Monitoring will continue to check if changes are needed.
• Reintroduction will happen in multiple locations and repeated over several years to handle unexpected events.
• People in the affected areas must be involved in decisions and educated about conservation. Final choices must be based on scientific information.

As cities grow, they need more resources, which has led to efforts to use energy and resources more sustainably. Programs like LEED-certified buildings, Energy Star appliances, and zero-emission vehicles are becoming more common. Sustainability in cities includes using methods that reduce resource use, such as capturing carbon emissions instead of releasing them into the air. Using renewable energy sources like bioenergy, solar power, geothermal energy, and wind energy can also reduce greenhouse gas emissions.

Green infrastructure can help cities support wildlife by replacing traditional, man-made structures with natural features. Green infrastructure includes things like parks, green roofs, and wetlands. These features can help manage stormwater, clean the air, and provide better habitats for wildlife than traditional infrastructure. Green infrastructure is made with natural elements, which helps protect animals from harmful chemicals. While research on green infrastructure’s benefits for wildlife has grown, scientists are still working to measure these effects clearly. A study by Alessandro Filazzola and others looked at 33 studies and found that green infrastructure improves biodiversity compared to traditional infrastructure. In some cases, it even supports similar levels of biodiversity as natural areas.

Urban green spaces are open areas in cities set aside for parks, gardens, and other natural environments. These spaces include plants, water features (called blue spaces), and other natural areas. Most urban open spaces are green areas, though some may be other types of open areas. These spaces can range from highly maintained areas, like playing fields, to more natural, less managed landscapes.

Urban green spaces can also include areas not open to the public, such as university campuses, school sports fields, community gardens, and corporate buildings. Areas outside city limits, like national parks or rural lands, are not considered urban green spaces. Streets, plazas, and public squares are not classified as green spaces.

Summary

Urbanization causes many effects, both near and far away, on biodiversity, natural processes that move nutrients and chemicals in the environment, water movement and distribution, and climate, among other changes. Many of these effects are not fully understood because urban ecology is a new scientific field, and more research is needed. Studies on cities outside the United States and Europe are still limited. Observations about how urbanization affects biodiversity and how species interact are similar in many studies, but the exact reasons for these changes are not yet known. Urban ecology is an important and relevant area within the study of living things, and more research must be done to better understand how human cities affect the environment.