United Nations Environment Programme

Cities are ecosystems

Urban green governance increases the quality of life and protects vital services
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Biodiversity must play an essential role in the sustainability of 21st century cities, not just as a bioindicator, but because of its contribution to urban quality of life, and provision of ecosystem services.

Cities are often perceived as monuments of human disregard for the natural world, the very antithesis of nature. But urban biodiversity has become a sustainability indicator and the importance of urban green governance is increasingly apparent.

Biodiversity must play an essential role in the sustainability of 21st century cities, not just as a bioindicator, but because of its contribution to urban quality of life, and provision of ecosystem services. Such services are regulating (purifying air and water or mitigating floods); provisioning (including supplying food, water, or medicines) and cultural (covering aesthetic, spiritual, recreational, and intellectual benefits). Indeed it is more accurate to affirm that cities are ecosystems than that cities have ecosystems.

Urban ecosystems consist of three subsystems: green (all living matter in natural soil), grey (built-up areas) and blue (coastal zones, rivers, standing water, and fountains). Each can be divided further into specific biotopes – living spaces that provide suitable conditions for the development of certain living organisms – or localized elements, such as trees (in the green system), sidewalks (grey) and ponds (blue). All three are of equal importance. Walls and buildings, for example, are as much part of the urban ecosystem as are forested areas. Jerusalem's Wailing Wall, for example, is an important and ancient nesting site for swifts, while the caper bush grows wild in stone walls throughout Rome and Amman.

Cities can ‘naturalise’ to increase the presence and resilience of a diversity of species through initiatives aimed at blending nature more broadly and deeply into urban life. This includes creating feeding, breeding and sheltering sites in green, grey and blue areas. Establishing parks and gardens is a common tactic, but activities can also include creating green roofs, walls, facades and balconies.

Barcelona's Jardí Terradellas, for example, is a ‘green wall’ structure that is home to extensive bird life. This approach includes creating natural connectors that criss-cross a city, and link to natural areas (or ‘re-charge nodules’) outside it. It is a key tool for promoting a city’s biodiversity objectives, relying on urban green areas as entryways for flora and fauna from beyond the city system. The result is an expansion in the number or area of ecosystems within a city that can function autonomously, without human input. Such a city makes valuable green areas available for citizens, while also providing and promoting urban biodiversity services.

Birds are among the most relevant indicators of these processes. Due to their high mobility, they use street trees and avenues as corridors that connect re-charge nodules with urban and peri-urban areas and provide permeability to the urban system.

Animals living in urban ecosystems face less pressure from natural predators than those living in peri-urban and surrounding natural areas. As a result, urban animals show reduced stress levels and a decrease in ‘alert distance’, or the point at which an animal begins to exhibit alert behaviours in response to an approaching human.

Urban fauna can be classified in relation to its origin in the urban system. Drawn fauna are those those species that are linked symbiotically to human activities, taking advantage of available resources and materials flows without causing either negative or positive effects (such as sparrows). Induced fauna exist as a result of human activities and installations that have favoured the presence of certain species originally from other habitats, and even other continents, (such as parrots in Rome, or grey heron in Barcelona). By contrast, some fauna continue to live in more natural, longstanding green habitats (such as squirrels in Hyde Park, London and common starling in Central Park, New York).

Urban fauna doesn’t only produce beneficial effects and is often perceived negatively, through being scary or unpleasant. In some cases, it may carry disease and cause disturbance or damage to urban infrastructure and amenities. Examples include monkeys in New Delhi, baboons in Durban, and raccoons and coyotes in some US cities.

Still, urban biodiversity is a strong indicator of human well-being. It serves as a tool for monitoring global change and measuring a city's efforts to harmonise its activities with nature. In recent decades, several urban biodiversity indexes and indicators have been developed and used on the ground. Such indexes bring together parameters such as biodiversity, ecosystem services and management practices. They are useful tools for both policymakers and urban planners confronting issues such as climate change, and can help guide the development of urban masterplans. They can also make cities aware of important gaps in information about their biodiversity, and help them become acquainted with the ecological and cultural value of each species.

Urban biodiversity is an indicator of urban health. Cities that are more species-diverse are more resilient and produce enhanced ecosystem services, such as enabling city dwellers to enjoy the beauty of different seasonal effects. A deeper understanding of the importance of urban biodiversity can lead to improvements in the relationship between humans and the planet, meaning sustainable cities provide hope for the future.