The city as an ecosystem

An urban ecosystem is an ecosystem found within a built-up settlement, where human activity strongly shapes conditions. Like any ecosystem, it has two broad kinds of components:

• Biotic components are the living parts of an ecosystem. These include humans, plants, animals, fungi and microorganisms.
• Abiotic components are the non-living physical and chemical parts of an ecosystem, such as soil, water, air, climate and topography.

The two interact constantly. Street trees affect shade, temperature and water interception, while buildings alter wind and light. People create habitats, introduce species and generate pollution. Gardens, waterways, parks, roadside verges, brownfield sites and small cultivated plots can all form urban habitats.

Urban ecosystems aren’t ecological blanks. Even densely built cities support organisms, though people heavily modify their habitats, species composition and environmental conditions.

Urban and rural settlements

An urban area is a built-up area with a high population density, buildings and infrastructure. Cities, towns and suburbs all count as urban areas.

In urban areas, buildings and people are packed closely together for residential, cultural, productive, trade and social purposes. The physical networks and facilities that support society are known as infrastructure. These include roads, public transport, electricity supply, water pipes, sewers, communications and waste services.

A rural area is a settlement or landscape where population density is relatively low and buildings are dispersed. There is no universal numerical boundary between rural and urban areas. Instead, they sit on a continuum, and national authorities apply different population and density thresholds.

A suburb can still be urban, even if its population density is lower than that of the city centre. The key is that it belongs to a built-up settlement where population and infrastructure are concentrated.

Components, storages and flows

An urban system is the interconnected system of buildings, microclimate, transport, goods and services, power/energy, water/sewage supply, humans, plants and animals. These components don’t operate entirely alone. Transport, for instance, affects energy demand, air quality, access to work and where housing is located.

Like a natural ecosystem, a city is an open system with:

• inputs, including energy, water, food, raw materials, manufactured goods, finance and people;
• storages, including buildings, reservoirs, infrastructure, green spaces, stocks of goods and the resident population;
• processes, including transport, production, consumption, construction, water treatment and recycling;
• outputs, including products, services, wastewater, solid waste, atmospheric emissions and exported energy or materials.

In a systems flow diagram, boxes represent storages and arrows represent flows. The direction of an arrow shows where the flow goes; its width may indicate relative magnitude. A clear system boundary is essential. Without one, it’s impossible to decide whether a movement is internal or counts as an input or output.

Efficiency, sustainability and resilience

Urban efficiency describes the relationship between the useful services a city delivers and the resources it consumes or wastes. Compact infrastructure, efficient buildings and mass transport may deliver those services with lower energy and material use per person.

Urban sustainability is the capacity of an urban system to meet present needs while maintaining environmental, social and economic conditions that allow future needs to be met. Urban resilience is the capacity of an urban system to withstand disturbance, adapt and continue its essential functions.

Waste and pollution point to inefficient flows: potentially useful materials or energy leave the system and cause environmental harm. Reuse, repair, recycling and resource recovery form feedback loops, reducing both inputs and outputs.

Urban systems resemble natural ecosystems because both have interacting biotic and abiotic components, storages, flows and feedbacks. However, the comparison has limits. Cities depend heavily on imported resources, human institutions and engineered infrastructure, and often export concentrated waste far beyond their boundaries.

Population and land-use change

Urbanization is the population shift from rural to urban areas. This affects where people live and reshapes the physical landscape.

As urbanization takes place, land becomes more built-up and industrialized, with dense, continuous human settlement and infrastructure. Farmland or natural ecosystems may give way to housing, factories, commercial areas and transport networks.

Urbanization isn't simply the growth of one city's population. The urban share may rise because of rural–urban migration, natural population increase within urban areas, expanding urban boundaries or the reclassification of formerly rural settlements. It covers both an increasing proportion of people living in urban areas and the conversion of land.

Why people move

Rural–urban migration is the internal movement of people from rural settlements to urban settlements in the same country. Most rural–urban migration is internal, though international migrants may settle in cities too.

A push factor is a disadvantage in the place of origin that encourages someone to leave. Insecure employment, low farm income and land shortage can all push people away. Other examples include weak healthcare or education, conflict, discrimination, drought, and inadequate water or electricity services.

A pull factor is an actual or perceived advantage that attracts a migrant to a destination. Possible urban pulls include employment, higher wages, education and healthcare. Safety, utilities and social opportunities may also attract people. The word “perceived” matters. Advertised or expected opportunities don’t always match reality, so migrants may face unemployment, expensive housing or inadequate services after they arrive.

Migration may be voluntary migration, where a person has meaningful choice about moving. It may instead be forced migration, where threats such as conflict, persecution, eviction or disaster leave little realistic choice. These categories can overlap. Severe poverty or environmental degradation may restrict a person’s choices without removing them completely.

Rural–urban migration has helped make the global urban population larger than the rural population, and the urban proportion continues to rise. Within a country, it commonly leads to rapid growth in major cities, while some rural areas develop declining or ageing populations.

Deurbanization

Deurbanization is the movement of people away from large urban areas towards smaller settlements or rural areas. High housing costs, congestion and pollution may encourage it. Remote working, retirement or a desire for more space can have the same effect. A country may continue to urbanize overall even while particular large cities lose residents, so the scale being discussed is crucial.

Outward movement within the urban area

Suburbanization is due to the movement of people from dense central urban areas to lower-density peripheral areas. Several factors can encourage this shift: better transport, rising incomes, demand for larger homes and gardens, or dissatisfaction with crowded centres.

Low-density housing uses more land per person, so suburbanization can contribute to urban sprawl—the outward spread of an urban area across a large area of formerly rural or undeveloped land. The two terms are closely related, but they aren't interchangeable. Suburbanization describes population movement and growth at the periphery, while sprawl refers to an extensive, low-density pattern of land use.

Households may gain more living space and access to private green space. Across the urban system, however, costs can include longer journeys, greater car dependency and expensive utility extensions. Habitats and farmland may also become fragmented.

Land and ecosystems

As towns and cities expand, agricultural land, forests and other natural ecosystems are replaced by buildings and infrastructure. Food-producing land may shrink, habitats disappear and biodiversity falls. The habitat that remains can also become fragmented into isolated patches. Flat, well-drained land suits farming and construction, so urban growth often competes directly with agriculture.

Water quality and river flow

Runoff from roads can wash oil, metals, tyre particles and litter into waterways. Industry, sewage leaks and poorly managed waste add more pollutants. As a result, water quality may decline because of increased toxicity, nutrient enrichment, suspended material and pathogenic contamination.

Concrete, roofs and roads are impermeable surfaces—surfaces through which water cannot readily infiltrate. They reduce infiltration and groundwater recharge, while drains carry water rapidly into channels. After rainfall, peak discharge rises sooner, increasing flood risk and stream erosion. Dry-period flow may decline where groundwater recharge has been reduced.

Air pollution

Traffic, industry, construction, power generation and household fuel use all release air pollutants. In sprawling settlements, greater car dependency can increase emissions. These pollutants may cause respiratory and cardiovascular disease, damage vegetation and reduce visibility.

The effects are linked. Replacing vegetation with roads removes habitat, increases runoff and encourages transport emissions at the same time.

Planning for competing needs

Urban planning is the process of deciding how urban land and buildings can best be used. It coordinates the location of housing, commerce, industry, transport, services, recreation and protected spaces.

The aim is to meet the physical, domestic, environmental, commercial, industrial, financial and health needs of everyone with a stake in the community. A stakeholder is a person, group or organization that affects, or is affected by, a decision. Residents and businesses may want different outcomes, as may landowners, local authorities, transport providers, health services and environmental groups.

Trade-offs are unavoidable. Building housing near employment may cut travel, but residents still need protection from industrial noise and pollution. A park can provide cooling, drainage and recreation, yet the same land may be needed for affordable housing. Consultation can bring these conflicts to light, though participation only counts if different groups can genuinely influence decisions.

What sustainable planning must combine

Modern planning asks whether an urban system can stay socially liveable, environmentally responsible and economically workable over time. The main factors are closely connected:

• housing that is both affordable and good quality;
• integrated public transport, supported by safe walking or cycling routes;
• green spaces that people can access;
• security, education and employment;
• use of renewable resources;
• waste reuse and recycling;
• energy-efficient buildings and infrastructure;
• genuine involvement from the community;
• green buildings.

Doing well in just one area doesn’t necessarily make a plan sustainable. For example, an energy-efficient district that excludes lower-income residents still fails to address the social side of sustainability.

Example: reducing car use in Copenhagen

Copenhagen has developed extensive cycle routes, pedestrian-friendly spaces and public transport. At the same time, it uses policies that make driving less attractive. By reducing dependence on private vehicles, this approach can cut energy use, traffic congestion, local air pollution and greenhouse-gas emissions. It can also increase everyday physical activity.

Infrastructure and policy support each other. A painted cycle symbol achieves little when routes are unsafe or disconnected. There are limitations, including construction costs and political resistance to restrictions on cars. Access is also unequal for people whose mobility or type of journey makes cycling difficult.

Investigating urban development through maps

To investigate a city’s development, compare maps from at least two dates. Keep the scale, extent and orientation the same. Check for changes in the built-up boundary, road network, settlement density, green space, waterways and land-use zones. Area comparisons are more reliable when GIS layers or georeferenced historical maps are used.

Focus on direction and pattern rather than simply stating that “the city became larger”. Development might follow transport corridors, fill gaps inside the existing boundary or leap over a protected belt. Maps provide evidence of spatial change, but they can’t establish the cause on their own. Planning records, population data or transport history may be needed to explain the pattern.

A holistic approach

Ecological urban planning is a more holistic approach that treats the urban system as an ecosystem, understanding the complex relationships between its biotic and abiotic components. Here, “holistic” means looking at the connected system instead of tackling each problem separately.

Changing a roof, for example, affects more than a building’s appearance. It can alter building temperature, energy demand, rainwater runoff, habitat availability and human well-being. Ecological planning looks at these feedbacks, the ecosystem services involved and the consequences for the whole urban system.

Example: biophilic design

Biophilic design is an approach to architecture and planning that deliberately brings features of nature into built environments. Living roofs and walls can create habitat and intercept rainfall. Along with water features and natural light, they may also cool buildings, filter some pollutants and support mental well-being.

These designs involve trade-offs. A green roof needs structural support, suitable plants, water and maintenance. If species are poorly chosen, they may fail or become invasive. Water features can consume water or create pollution risks. The strongest designs suit vegetation and water use to the local climate, while connecting isolated features to a wider habitat network.

Other ecological approaches include urban ecology, urban farming, resilience planning and regenerative architecture. All share one central idea: urban nature and infrastructure should work together rather than occupy entirely separate spaces.

Three linked principles

Urban compactness concentrates development at relatively high density inside a limited urban footprint. By reducing outward expansion, it can place enough users near public transport and local services to make them viable.

Mixed land use places compatible residential, commercial, educational, recreational and employment uses close to each other. With shorter distances between them, walking, cycling and public transport become practical options.

Social mix practice provides housing and shared services in the same area for people from different income and social groups. The aim is inclusion, rather than simply placing unequal groups side by side without equal access to opportunities.

Used together, these principles can reduce urban sprawl, car dependency and energy consumption. They can make frequent public transport viable, improve access to work and services, and support greater social equality. Compactness isn’t automatically sustainable, though. Excessive density without green space, affordable housing or adequate services can lead to congestion, heat exposure and poor living conditions.

Access and environmental justice

Accessibility describes how easily people can reach destinations, services and opportunities. Distance, time, cost, mobility needs and transport quality all affect it; the existence of a facility somewhere in the city isn’t enough.

Environmental justice concerns the fair distribution of environmental benefits and burdens, along with fair participation in environmental decisions. Unequal access to parks and tree cover is therefore an environmental-justice issue. Wealthier districts may benefit from cooling, cleaner air, recreation and better health, while disadvantaged communities face heat and pollution with little green space.

Planners should assess who can use green areas, whether people can reach them safely and whether redevelopment could displace existing residents. Equal area on a map doesn’t necessarily provide equitable access.

From linear throughput to circular flows

A circular economy is an economic model that keeps products and materials in use through reduction, durability, repair, reuse, remanufacture and recycling while regenerating natural systems. In a city, it replaces as much of the linear “take–make–dispose” flow as possible with loops.

Urban organic waste can be turned into compost or biogas, while treated wastewater can be reused. Recoverable construction materials can go into new buildings, and sharing or repair services can reduce demand for new products. Recycling alone doesn't create circularity—avoiding unnecessary resource use comes first.

Doughnut economics is a model that aims to meet essential human needs within an ecological ceiling. Its inner social foundation covers needs such as housing, health, education, income and political voice. The outer boundary represents environmental limits that should not be exceeded.

Example: Amsterdam's circular strategy

Amsterdam has used circular-economy and doughnut thinking to shape urban decisions. This includes greater reuse of construction materials, lower use of virgin resources and consideration of both social needs and environmental pressures in policy. The model prompts planners to ask two questions at once: does a proposal support a good life for residents, and does it remain within ecological limits?

The value of this approach is systemic. Waste, housing, jobs, energy and material supply are treated as connected. However, measuring ecological limits at the city level is difficult. Other limitations include reliance on supply chains outside the city boundary and possible rebound effects if efficiency lowers costs but total consumption continues to grow.

Buildings with lower impacts

Green architecture minimizes harmful effects of construction projects on human health and the environment, and aims to safeguard air, water and earth by choosing environmentally friendly building materials and construction practices. Rather than looking only at appearance, it considers how a building is constructed, operated and maintained, as well as its eventual reuse or demolition.

Useful strategies include:

• using passive orientation, shading, insulation and natural ventilation to lower heating or cooling demand;
• installing efficient appliances, lighting and water systems;
• generating renewable energy;
• reusing existing structures and making efficient use of space;
• choosing responsibly sourced, recycled or low-impact materials;
• adding vegetation for shade and habitat, with some interception of pollutants;
• capturing rainwater and using permeable surfaces to reduce runoff;
• designing for repair, adaptability and material recovery.

The local climate shapes which strategies work best. Hot climates suit shading and cross-ventilation, whereas colder conditions may place more emphasis on insulation and solar gain. Vernacular architecture is building design based on local materials, climate and established cultural practices. Raised floors in flood-prone regions and wind-driven ventilation, for example, can provide low-energy solutions.

Evaluation

Green design can cut operating energy, pollution and water use. It can also reduce demand for virgin materials while improving indoor health and comfort. Reusing an existing building may avoid much of the energy and material demand linked to demolition and replacement.

However, limitations remain. Specialist materials or technologies can be expensive or unavailable, while some designs need skilled maintenance. A building labelled green may still have high embodied impacts due to material extraction, manufacturing and transport. Its benefits therefore need to be judged across the whole life cycle and within the local context.

Reimagining cities through green architecture can contribute to a more sustainable future, but buildings can't achieve this alone. The strongest benefits come when green architecture is coordinated with compact planning, clean energy and public transport, alongside circular material flows and fair access to healthy housing.