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About UNEP
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United Nations Environment Programme
Division of Technology, Industry and Economics
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Freshwater Management Series No. 7

Phytotechnologies

A Technical Approach in Environmental Management

II. Global Issues Requiring Innovative Sollutions >

D. Cities as Sustainable Ecosystems

Asian street corner

Cities are pollution sources and people living in them utilize resources and generate waste. Due to inadequate systems and poor planning, cities are disproportionately driving global warming, deforestation, and increasing water scarcity. The world’s cities take up just two percent of the Earth’s surface, yet account for roughly 78 percent of the carbon emissions from human activities, 76 percent of industrial wood use, and 60 percent of the water tapped for use by people.

Cities import resources and export pollutants, but have limited carrying capacities. If the carrying capacity of a city is eroded, it becomes increasingly difficult, if not impossible, to achieve sustainable development goals. For example, the trucking of garbage to landfills outside of a city becomes increasingly costly, the further from the city the landfills are located. Also, the importation of fresh water to replenish a city's depleted aquifers becomes increasingly costly, the greater the distance the water must be piped. One of the challenges for the future will be our ability to reform urban systems so that they mimic the metabolism of nature. Rather than devouring water, food, energy, and processed goods, and then belching out the remains as pollutants, cities need to align their consumption with realistic needs, produce more of their own food and energy, and put much more of their wastes to use.

Western riverfront city with a ferris wheel

The concept of “Cities as Sustainable Ecosystems”, or CASE, provides a framework for examining and understanding the interactions of urban activity and the environment and how these can be transformed into a sustainable relationship. CASE is the multidisciplinary study of urban and economic systems and their linkages with natural systems. CASE provides a conceptual framework upon which understanding and reasoned improvement of current practices can be based. One area where phytotechnologies can contribute to the objective of CASE involves biodiversity in cities. In preserving biodiversity, cities can exercise two strategies – they can enhance and reinforce biodiversity throughout their domain, and they can promote the conservation of threatened species.

Town buildings with trees

The presence of nature within the city can take many forms. Natural systems tend to persist in the urban setting where strong geological features (i.e., rivers and geological contours) and land unsuitable for building exist. The initial concepts instrumental in guiding the construction of the city are also important factors, given that the density of buildings and infrastructure leaves either more or less room for nature. In certain cities, zoning regulations call for plenty of open space, which is usually developed as green space. There are typically two types of green space within cities – the natural systems, in which human intervention has been minimised, and which are composed of a high proportion of indigenous species; and the cultivated biological areas deliberately created by humans to embellish the city, usually characterized by introduced species or horticultural plantings requiring human intervention. The distinction between these two types of green space is an oversimplification. In reality, there is considerable overlap between spaces deliberately developed by humans making use of indigenous species (i.e., the concept of the “natural” garden) and introduced species adapting to the city and spreading without outside intervention. This overlap is important as cities continue to evolve in ways that integrate the natural and built environment. This is an area where phytotechnologies can play a beneficial role.

The ecology and biodiversity in a city go hand in hand. In natural spaces comprised of different types of vegetation, a natural urban ecosystem can function and varied urban wildlife can flourish. Many horticultural species (e.g. conifers or many-branched shrubs) are often well adapted to the microclimate of the city, and can provide shelter for urban wildlife. Tall trees, particularly if planted in belts, can have a positive impact on the local environment and improve air and water quality. Urban biological diversity also plays an important educational role in providing opportunities for observing wildlife. It sensitizes humans to the green spaces and natural systems which balance the “inorganic” reality of buildings and urban infrastructure.

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