A city looks like the opposite of nature. It is not. A city takes in energy, water and food, transforms them, and pushes out waste and heat — which is exactly what an ecosystem does. Treating a city as a system, with inputs, processes and outputs, is the idea the whole of this topic is built on.
📚 What you need to know
An urban area is built-up, with high population density and a concentration of buildings and infrastructure.
Rural areas have lower density, scattered settlement, and land used mainly for farming or nature.
An urban ecosystem has biotic components (plants, animals, people, microorganisms) and abiotic ones (soil, water, air, climate, buildings).
An urban system has inputs (energy, water, food, goods, labour), processes, outputs (waste, sewage, pollution, heat) and feedback.
Cities create their own microclimate. The urban heat island makes them warmer than the countryside around them.
Efficiency, sustainability and resilience are three different things and are frequently confused.
Urban against rural
There is no single global definition of “urban” — each country sets its own threshold, which is one reason urbanisation figures should be handled carefully. What every definition has in common is density and built-up land.
Feature
Urban area
Rural area
Population density
High, often thousands per square kilometre
Low, often under a hundred per square kilometre
Land cover
Mostly buildings, roads and hard surfaces
Mostly fields, forest, water or open ground
Settlement pattern
Continuous and connected
Scattered villages and isolated homes
Main economic activity
Services, industry, trade, administration
Farming, forestry, fishing, tourism
Infrastructure
Dense networks of transport, water, power and waste
Sparse networks, longer distances to services
The boundary between the two is blurry, and that is a legitimate exam point. A commuter village with 2 000 people, fibre broadband and no farms is rural on paper but urban in how it actually works. Definitions like this are useful, not exact.
Urban ecosystems
An urban ecosystem is the community of living things in a city together with the non-living surroundings they interact with. It behaves like any other ecosystem, except that one species — us — controls most of the energy and material flows.
Biotic components
Plants in parks, gardens, street trees, verges and cracks in the pavement.
Animals, especially birds, insects, rodents and mammals such as foxes that do well around people.
Humans, whose building, gardening and commuting shape everything else.
Microorganisms in soil, drains and treatment works, breaking down waste.
Abiotic components
Soil, often compacted, sealed under tarmac or contaminated by past industry.
Water in rivers, canals, drains and supply pipes.
Air, frequently carrying nitrogen oxides and particulates from traffic.
Climate, modified by the city itself into a distinct microclimate.
Built structures, which act as physical habitat: a tower block ledge is a cliff to a peregrine falcon.
Habitats hiding in plain sight
Cities contain more ecosystem types than people expect, and some of the richest are the ones nobody planned.
Gardens and allotments — collectively a huge habitat for insects, birds and small mammals.
Parks and open spaces — habitat, recreation, air filtering and cooling all at once.
Derelict land — abandoned sites often develop surprising biodiversity as nature reclaims them.
Cemeteries — quiet, old, undisturbed grassland and mature trees; often local biodiversity hotspots.
Road and rail verges — long, thin corridors of hardy species that can link habitats if planned as green corridors.
Rivers, canals and ponds — aquatic habitat plus a cooling effect on hot days.
Landfill and treatment works — polluting, but also feeding grounds for gulls, insects and bacteria.
Why green corridors matter. Isolated parks are habitat islands, and small isolated populations lose genetic diversity and die out. Linking them along railway lines, rivers and verges lets species move between them, which is why corridors do far more for biodiversity than the same area of scattered green space.
The city as a system
This is the framework examiners want. Every city takes in resources, does things with them, and produces outputs. Draw it as a flow diagram and most questions in this topic become answerable.
Learn the four labels — inputs, processes, outputs, feedback. Almost any “explain how an urban system works” question can be answered from this diagram.
🧩 The four parts of an urban system
Inputs. Energy (electricity and fuel), water, food, goods and building materials, and labour.
Processes. What the city does with them: housing people, moving them around, manufacturing, providing healthcare and education.
Outputs. Solid waste, sewage, air and water pollution, waste heat and greenhouse gases.
Feedback. Decisions that change the system: recycling schemes, planning rules, switching to renewables, congestion charges.
Cities make their own weather
Because a city is a system with a huge energy input, it heats up. Dark roofs and roads absorb sunlight; concrete and brick store it and release it slowly at night; there is little vegetation to cool the air through evaporation; and engines, air conditioners and industry add heat directly. The result is the urban heat island: city centres run several degrees warmer than the countryside nearby, and the gap is usually largest at night.
The park dip is the important part. It shows the heat island is a design problem, not an unavoidable fact about cities.
WORKED EXAMPLE
Explain, using the concept of an urban system, why improving a city’s public transport can also improve its air quality and its residents’ health.
Step 1: identify the input being changed
Better public transport reduces the energy input per journey, because one bus or train replaces many car trips.
Step 2: follow it to the outputs
Less fuel burned means smaller outputs of nitrogen oxides, particulates and carbon dioxide, and less waste heat.
Step 3: follow it further
Cleaner air lowers rates of asthma and heart and lung disease, and more walking to stops raises physical activity.
One change to an input improves several outputs at onceThis is the point of systems thinking: parts of a city are connected, so a change rarely stays in its own box.
Three words students mix up
Term
Question it answers
Example
Efficiency
How much output do we get per unit of input?
Insulating homes so less energy is needed for the same warmth
Sustainability
Can we keep doing this without harming future generations?
Powering the city from renewables rather than depleting fossil fuels
Resilience
Can we absorb a shock and recover from it?
Flood defences and emergency plans after a hurricane, as in New Orleans after 2005
A city can be efficient and still unsustainable — burning coal very efficiently is still burning coal. It can be sustainable and still fragile, if everything depends on a single water source. Keep the three ideas separate and you will pick up marks most candidates lose.
WORKED EXAMPLE
A city upgrades its street lighting to LEDs, cutting lighting electricity use by 60%. Is this a change to efficiency, sustainability or resilience? Justify your answer.
Step 1: what has changed
The same amount of light is produced from far less electricity.
Output per unit of input has risen, so this is efficiency.Step 2: does it affect the others?
It helps sustainability indirectly, because lower demand means fewer emissions, but it does not change where the electricity comes from.
Step 3: resilience
Barely affected. The city is no better prepared for a flood or a heatwave than before.
Primarily efficiency, with a small sustainability benefit“Primarily X, with some effect on Y” is a strong way to answer this style of question.
💡 Exam tip
Practise drawing the inputs, processes, outputs and feedback diagram from memory. It is quick and it earns marks.
Define urban and rural using density and land use, not just “cities and countryside”.
Give both biotic and abiotic examples if a question mentions urban ecosystems.
Have at least three surprising urban habitats ready — cemeteries, derelict land and rail verges are the memorable ones.
Explain the heat island with causes: dark surfaces, thermal mass, little vegetation, waste heat from engines and cooling.
Keep efficiency, sustainability and resilience apart, and say which one a question is actually about.
⚠ Common mix-ups
Saying cities have no ecosystems. They have many, some of them unusually rich.
Listing buildings as biotic. Buildings are abiotic, even though they provide habitat.
Confusing outputs with processes. Transport is a process; the exhaust fumes are an output.
Forgetting feedback. Any diagram without a feedback loop is incomplete.
Explaining the heat island only with “there are more people”. The causes are surfaces, materials, lack of vegetation and waste heat.
Using efficient and sustainable as synonyms. They answer different questions.
Up next: Why Cities Grow — urbanisation, rural-urban migration, and the moment more of humanity started living in cities than outside them.
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