A city is an ecosystem too: An urban ecosystem is made of biotic and abiotic parts, like a forest or a lake.
The points to remember
- An urban area (a city or town) contains an urban ecosystem.
- Like every ecosystem it has biotic (living) and abiotic (non-living) components.
- Biotic: plants, animals and other forms of life such as fungi and bacteria.
- Abiotic: soil, water, air, climate and topography (the shape of the land).
- The components interact: trees shade and cool the air; polluted air slows plant growth.
Real example: in 2019 London became the world's first 'National Park City': almost half of the city is parks, gardens, rivers and other green or blue space.
Remember it as: Living things plus soil, water, air, climate, land.
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The living part of a city is more than people. Plants, animals and tiny decomposers live in every gap the city leaves.
Biotic components
- Plants: street trees, parks, gardens, green roofs, weeds in cracks.
- Animals: birds, foxes, bats, insects; many live close to people.
- Other life: fungi and bacteria break down waste in soil and water.
- Humans are the main species and decide what else can live there.
- Many species are generalists that eat almost anything; many are introduced from elsewhere.
Plants
- Street trees
- Parks and gardens
- Green roofs
Animals
- Foxes, birds, bats
- Insects
- Fish in rivers
Other life
- Fungi
- Bacteria
- Algae in ponds
Real example: London has about 8.4 million trees and about 10,000 red foxes. Foxes are generalists: they eat food waste, rats and fruit from gardens.
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The non-living part of a city is changed by buildings and roads. Each abiotic component is different from the countryside around it.
Abiotic components
- Soil: packed hard, sealed under roads, often polluted.
- Water: rivers in channels, fast runoff from roofs and roads, reservoirs.
- Air: fumes from traffic and industry; air pollution limits photosynthesis.
- Climate: warmer than the countryside, an urban heat island; parks and lakes are cooler.
- Topography: hills, valleys and coasts, often flattened or built up by people.
Real example: on still summer nights central London can be several degrees warmer than the countryside, because concrete and brick store the day's heat and release it at night: an urban heat island.
Link it to the plants: Asked what limits plant growth in a city? 'Air pollution limits photosynthesis' scores. 'It lowers productivity' alone does not: say it slows photosynthesis or plant growth.
A city is not one ecosystem but a patchwork of many. Each patch is a habitat with its own mix of living and non-living parts.
Ecosystems inside the city
- A city holds many ecosystem types: woodland, grassland, rivers, lakes, wetlands, parks, gardens.
- Name the type: 'woodland' or 'wetland', not just 'terrestrial' or 'freshwater'.
- Green spaces are often fragmented: small patches between buildings.
- A green belt of countryside may ring the city edge; rivers form green corridors.
- Corridors link patches, so plants and animals can move through the city.
Land ecosystems
- Woodland
- Grassland and meadow
- Parks and gardens
Water ecosystems
- Rivers and canals
- Lakes and ponds
- Wetlands and marsh
Real example: Walthamstow Wetlands in north-east London opened in 2017 on working reservoirs: wetland birds such as herons and ducks feed there. London's green spaces are patches, joined by the green corridor along the River Thames and ringed by a green belt.
Name the type: 'Terrestrial ecosystem' or 'freshwater ecosystem' is too vague. Write 'woodland', 'grassland', 'river' or 'wetland'.
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How like a natural ecosystem is a city? It has the same parts, but people bring in most of what keeps it going.
Urban and natural ecosystems compared
- Same as natural ecosystems: biotic and abiotic parts, food chains, energy flows, nutrient cycles.
- Different: most energy and food are imported from outside, not made by local producers.
- Different: few producers for the number of consumers, so food webs are simple.
- Different: humans control it: what grows, where water goes, what is removed.
- Different: many introduced species and much more waste and pollution.
Natural ecosystem
- Energy from local plants
- Many producers
- Wastes recycled on site
Urban ecosystem
- Energy and food imported
- Few producers
- Wastes exported or piled up
Real example: peregrine falcons nest on tall buildings in London and hunt the city's pigeons: a real food chain. But the pigeons live largely on food people drop, which was grown far away.
Remember it as: Same parts as nature; the fuel comes from outside.
How this comes up: Paper 2, Section B (a): outline the biotic and abiotic components of a named urban ecosystem [4].
Cities are often thought of as places without nature, yet they contain ecosystems.
Outline the biotic and abiotic components of a named urban ecosystem, such as London.
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