IB ESS SL8.2 Urban Systems & Urban PlanningPaper 1 & 2~13 min read
What Makes an Area Urban
A city is not just a lot of buildings in one place. It pulls in food, water, energy and people, uses them up, and pushes out waste, sewage and heat. Once you start seeing a city as a system with inputs and outputs, the rest of this topic gets a lot easier.
📘 What you need to know
An urban area is built up: high population density and a large concentration of buildings and infrastructure.
Urban areas are centres for homes, work, trade, culture and services. Cities, towns and suburbs all count.
Rural areas have lower population densities and settlements that are spread out, usually around farming and natural land.
Urban ecosystems have biotic parts (plants, animals, people, microbes) and abiotic parts (soil, water, air, climate, buildings).
Gardens, parks, derelict land, railway edges, cemeteries and even landfill sites are all urban habitats.
An urban system is a network of connected parts: buildings, transport, energy, water, sewage, people, plants and animals.
Cities create their own microclimate and are usually warmer than the countryside around them.
Linear metabolism cities take, use and dispose. Circular metabolism cities reduce, reuse and recycle.
A systems flow diagram shows inputs, processes, outputs and feedback.
Efficiency, sustainability and resilience are three different ideas. Do not mix them up.
What counts as an urban area
An urban area is a built-up environment. Two things give it away: a high population density and a large concentration of buildings and man-made infrastructure.
Urban areas act as centres for residential, cultural, economic, trade and social activity. That is why people go there. Cities, towns and suburbs are all urban areas, even though they are very different sizes.
DefinitionUrban area — a built-up area with a high population density and a large concentration of buildings and infrastructure, which acts as a centre for homes, work, trade and services.
Rural areas are the opposite. They have lower population densities and settlements that are more dispersed, usually focused on agriculture and natural landscapes. Compare London — dense population, heavy infrastructure, cultural hubs — with the Lake District, where you get scattered villages and an economy built on farming and tourism.
Both panels cover the same area of ground. The difference is how many people and buildings are squeezed into it.
Examiners like you to compare urban and rural areas using three things: population density, amount of infrastructure, and the type of ecosystems present. Learn those three and you can answer almost any comparison question on this.
Urban ecosystems
An urban ecosystem is the environment you get inside a city or town, where human activity meets plants, animals and climate. Cities are still ecosystems — just very heavily modified ones.
Like any ecosystem, an urban one has two sets of components:
Biotic (living): plants in parks, gardens and street trees; birds, insects and mammals; humans; and microorganisms that break down waste and enrich soil.
Abiotic (non-living): soil, water, air, climate — plus something no natural ecosystem has, urban infrastructure such as buildings, roads and bridges.
The bit students forget. Buildings and roads are abiotic components of the urban ecosystem. They are not just a backdrop. They change where water goes, how much heat is stored and where wildlife can live.
Types of urban ecosystem
Green space in a city is not only parks. Wildlife takes whatever it can get, and some of the richest habitats are the ones nobody planned.
Habitat
What lives there and why it matters
Residential gardens
Homes for plants, birds, insects and small mammals. Also improve air quality and take the edge off the urban heat island
Industrial sites
Factories and warehouses. Often a source of pollution, though many are now being redeveloped with green space
Inner-city derelict land
Abandoned land that nature reclaims. Biodiversity often builds up quickly, and the land is useful for regeneration projects
Parks and open spaces
Habitat for animals, better air quality, and somewhere for people to relax
Traffic corridors
The strips beside roads and railways. Hardy species survive the pollution and disturbance. Planners can turn them into green corridors that join habitats up
Cemeteries
Quiet, undisturbed and full of trees and grass, so they act as biodiversity hotspots
Waste disposal areas
Landfill and treatment plants pollute, but they also feed certain birds, insects and bacteria that thrive on waste
Forests, fields and water bodies
Patches of woodland, lakes and rivers inside the city. Important habitat, and they help control city temperature
Green corridors are worth remembering by name. A park on its own is an island; a corridor along a railway line joins the islands together so species can actually move between them. That single point turns a 2-mark answer into a 3-mark one.
A city as a system
An urban system is a network of interconnected parts that work together to keep life going in a city or town. That includes buildings, transport, power and energy supply, water supply, sewage systems, plants and animals, and the people themselves.
Because it is a system, you can draw it as a systems flow diagram with four parts.
Feedback is the part students leave out. It is any action that sends information back into the system and improves it.
Part of the system
Examples
Inputs — what the city needs
Energy (electricity and fuel), water, food, goods such as building materials, and labour (the workforce)
Processes — what the city does with them
Buildings, transport systems, services such as healthcare and education, infrastructure such as roads, power plants and water supply
Outputs — what comes back out
Solid domestic waste (rubbish and recycling), air and water pollution, sewage, waste heat
Feedback — what improves the system
Urban planning decisions such as building more green space, switching to renewable energy, and waste management schemes
The parts you might forget
Buildings and infrastructure are the core: residential, commercial and industrial space, joined up by roads, bridges and utilities.
Transport moves people and goods. Good transport cuts congestion and air pollution — the London Underground carries huge numbers of people who would otherwise be driving.
Power and energy can come from fossil fuels, nuclear power or renewables. Supply has to be both reliable and sustainable.
Water and sewage: clean water in for drinking and washing, wastewater out to be treated so it does not pollute or spread disease.
Microclimate: cities are warmer than the countryside around them because of all the human activity and hard surfaces. This is the urban heat island effect, and planners fight it with green spaces and cooler building materials.
Humans, plants and animals are part of the system too. Urban wildlife and green space give people places to relax and improve air quality and biodiversity.
Linear and circular metabolism
The word metabolism here just means how a city handles its resources. There are two models, and the difference is what happens to the outputs.
The only real difference is the dashed arrow on the right. Everything else about the two cities can look identical.
Linear metabolism cities follow a take, use, dispose approach. Resources such as energy and water are used once and then discarded as waste. The result is high consumption and high pollution.
Circular metabolism cities focus on recycling, reusing and reducing waste. The aim is a closed loop, where outputs become inputs again — recycled materials go back into building, and burning waste generates energy.
🧩
Remember it as a shape
Linear is a straight line: it starts at a mine and ends at a landfill. Circular is a ring: the end feeds back into the start. If you can draw the shape, you can explain the difference.
Efficiency, sustainability and resilience
These three words turn up constantly in this topic and they are easy to blur together. They mean different things.
Term
What it actually means
Quick test question
Urban efficiency
How well a city uses its resources — energy, water, transport. Efficient systems waste less, pollute less and improve quality of life
Is the city wasting anything?
Urban sustainability
Meeting the needs of people today without harming the ability of future generations to meet theirs. Renewable energy, less waste, more green space
Can it keep going forever?
Urban resilience
The ability to recover from shocks such as natural disasters, climate change or economic crises. Needs strong infrastructure, emergency services and preparedness plans
Can it recover from a shock?
Named example. After Hurricane Katrina in 2005, New Orleans in the USA rebuilt its flood defences and urban infrastructure so the city could stand up to future storms. That is resilience, not efficiency and not sustainability.
Worked examples
WE 1
Compare urban and rural areas
Outline two differences between an urban area and a rural area. (4 marks)
Difference 1: population density
Urban areas have a high population density, with people living close together in a small space. Rural areas have a much lower density and settlements that are spread out.
Difference 2: land use and infrastructure
Urban land is built up, with a large concentration of buildings, roads and other infrastructure. Rural land is mostly used for agriculture and natural landscapes, with far less infrastructure.
High density and heavy infrastructure = urban; low density and farmland = ruraltwo differences, so make two clearly separate points and compare both sides each time
WE 2
A city as a system
Explain how a city can be described as a system. (4 marks)
Point 1: it has inputs
A city takes in energy, water, food, goods and labour from outside its boundary.
Point 2: it has processes
Those inputs are used by buildings, transport systems, services and infrastructure in the daily running of the city.
Point 3: it has outputs
The processes produce solid waste, sewage, air and water pollution, and waste heat, which leave the city.
Point 4: it has feedback
Planning decisions such as adding green space, using renewable energy or improving recycling send information back and change how the system behaves.
Inputs to processes to outputs, with feedback altering the whole thingname all four parts — feedback is the one most answers miss
WE 3
Linear versus circular metabolism
Compare and contrast linear and circular metabolism in urban systems. (4 marks)
Similarity
Both describe how a city takes in resources and uses them, and both produce waste.
Difference 1: what happens to outputs
In a linear city, resources are used once and then discarded as waste. In a circular city, outputs are fed back in as inputs through recycling, reuse and energy from waste.
Difference 2: the consequences
Linear cities have higher consumption and higher pollution. Circular cities need fewer new resources and send less to landfill, so they are more sustainable.
Same starting point, but circular cities close the loopcompare and contrast means you must give at least one similarity as well as differences
💡 Exam tips
Be ready to compare urban and rural areas using population density, infrastructure and ecosystem types.
When you draw a systems flow diagram, label all four parts: inputs, processes, outputs, feedback.
Use the words biotic and abiotic when describing urban ecosystems, and remember buildings count as abiotic.
Urban systems are highly interconnected. If a question asks how improving transport helps, link it forward to congestion, air pollution and health.
Keep efficiency, sustainability and resilience apart, and have one named example ready for each.
Use the correct terms linear metabolism and circular metabolism rather than saying “wasteful” and “green”.
⚠ Common mistakes
Saying a city is not an ecosystem. It is one — a heavily modified one with biotic and abiotic components.
Forgetting feedback. A flow diagram with only inputs, processes and outputs is missing a mark.
Mixing up sustainability and resilience. Sustainability is about lasting; resilience is about recovering.
Thinking urban ecosystems are only parks. Derelict land, cemeteries and railway edges are all habitats.
Describing an urban area by size alone. It is density and infrastructure that matter, not how many people live there in total.
Saying circular cities produce no waste. They still do — they just reuse far more of it.
Up next: Why Cities Grow. Now that you can describe a city as a system, the next question is why more and more of the world’s population is choosing to live inside one.
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