Urban ecosystems
Cities are not ecological deserts. They contain functioning ecosystems, and like any ecosystem, urban ecosystems are built from two categories of components: biotic and abiotic.
An urban ecosystem is the assemblage of living organisms and the physical environment within an urban area, interacting as a system shaped heavily by human activity.
Biotic components are all living organisms present in the city:
- Plants: street trees, shrubs, grass verges, window boxes, and park vegetation. They produce oxygen, intercept rainfall, and moderate local temperature.
- Animals: birds, insects, small mammals (foxes, raccoons), domestic pets, and soil invertebrates.
- Humans: the dominant species, and the one that most actively reshapes the system.
- Microorganisms: bacteria and fungi that decompose organic matter and cycle nutrients through urban soils.
Abiotic components are the physical and chemical conditions:
- Soil: often compacted, contaminated, or sealed beneath impervious surfaces, limiting plant growth and water infiltration.
- Water: rivers, canals, ponds, reservoirs, and engineered drainage networks.
- Air: modified by vehicle emissions, industrial activity, and the heat released by buildings.
- Climate: urban areas tend to run warmer than surrounding countryside because concrete and asphalt absorb solar energy and release it slowly: the urban heat island effect.
- Topography: slope and elevation govern drainage patterns, wind flow, and how the built form is distributed across the landscape.
The urban heat island effect is not just a comfort issue. Higher temperatures accelerate ozone formation at ground level and increase energy demand for cooling, creating a feedback that amplifies air quality problems.
What counts as an urban area
An urban area is a built-up area with a high population density, buildings and infrastructure, serving residential, cultural, productive, trade and social purposes.
The key contrast is with rural areas, which have low population density and dispersed settlements. Rural land is dominated by agriculture, forestry, and open space rather than continuous built fabric.
Urban areas are classified into three types:
- Cities: large centres with substantial populations, complex governance, and major infrastructure.
- Towns: smaller than cities but still possess significant infrastructure and economic activity.
- Suburbs: lower-density residential zones on the periphery of cities, still classified as urban.
The boundary between these categories is not always sharp, and different countries use different population thresholds. What matters for ESS purposes is the contrast in density, land use, and ecological character between urban and rural systems.