Whereas just 10 percent of people lived in cities in 1900, now more than half the global population is urban and that proportion continues to grow. Cities occupy less than 3% of the Earth’s land surface, but generate 78% of global CO2 emissions and consume 76% of wood used for industrial purposes.
Urban dwellers depend on the productive and assimilative capacities of ecosystems well beyond their city boundaries — “ecological footprints” tens to hundreds of times the area occupied by a city — to produce the flows of energy, material goods, and nonmaterial services (including waste absorption) that sustain human well-being and quality of life [Grimm 2008: 756].
The social and environmental costs of building and servicing the world’s wealthiest cities since the colonial period to the present has been enormous:
Although exacerbated by recent globalization trends, centuries ago the demands of European consumers led to deforestation of colonial lands and, more recently, demand for beef from countries of the Western Hemisphere has transformed New World tropical rainforests into grazing land [Grimm 2008: 756].
Because cities so radically transform landscapes, creating new and less functional ecosystems in the process, they were “shunned” by ecologists during the 20th Century, “with the result that ecological knowledge contributed little to solving urban environmental problems” [Grimm 2008: 756]. However, even though cities contribute disproportionately to the current ecological crisis, they are by the same token increasingly seen as a necessary part of the solution, both in terms of mitigating their effects, and withstanding and adapting to severe weather.
The field of “urban ecology” seeks to better understand the processes and patterns of urban ecosystems, with an eye toward boosting the ecosystem services within and around cities upon which urban dwellers depend. An observable pattern is the cycling of resources through a city. “The concept of urban metabolism analogizes a city to an organism that takes in food and other required resources and releases wastes to the environment” [Grimm 2008: 757]. Unlike natural ecosystems, though, which constantly recycle resources, urban ecosystems notoriously recycle little to nothing, and are therefore reliant on fresh extraction for the provision of new resources, while letting waste products accumulate as pollution.
Through an urban ecology lens, this discrepancy between natural and urban resource metabolism is duly noted and practical solutions proposed:
Cities are hot spots of accumulation of N [nitrogen], P [phosphorus], and metals and, consequently, harbor a pool of material resources. Could high-nutrient, treated wastewater substitute for commercial N fertilizers to supply crops and lawns with nitrogen, for example? [Grimm 2008: 757]
A small (but growing) proportion of the copper extracted globally is recycled, yet increasing the reuse and recycling of copper and other metals would do much to stem the rapid rise in demand from sources increasingly difficult to extract. Such reuse also would alleviate problems of metal accumulation in soils [Grimm 2008: 757].
Another tendency of urban ecosystems is to generate surplus heat, creating an urban heat island (UHI), due to reduced vegetation cover (thus, reduced cooling effects of evapotranspiration) and increased surface area absorbing solar energy (buildings, roads, etc.). This UHI effect in turn increases the use of air conditioning by 3-8% in the US, the additional energy use for which represents a positive feedback, which increases global warming. By contrast, increasing vegetation cover in cities reduces the UHI effect, while also removing greenhouse gases from the atmosphere, thus representing a negative feedback, which reduces global warming.
Another area of analysis in urban ecology involves water management (including channelization of streams and sewers, for example). The design of urban water systems is typically devoid of the ecosystem service provided by the waterways that urban systems replace, making cities vulnerable to flooding, drought and excessive pollution.
Among the most important modifications that affect streams in urban areas is increased impervious cover, which changes hydrology and funnels accumulated pollutants from buildings, roadways, and parking lots into streams [Grimm 2008: 759].
Successful, ecologically based designs of novel urban aquatic ecosystems are becoming more common and exemplify stream-floodplain protection, retrofitting of neighborhood stormwater flow paths, and use of low-impact stormwater/water capture systems as creative solutions to urban stormwater management [Grimm 2008: 759].
Grimm, Nancy, et al., 2008, Global change and the ecology of cities, Science 319, https://science.sciencemag.org/content/319/5864/756.