If you are reading this, chances are that you live in a city – one, perhaps, on its way to becoming a megacity with a population that exceeds 10 million or more. If not, you and most of the world’s population soon will be, according to global population demographics projections.

“When we think of global change, images of melting ice caps and pasture replacing tropic rainforest come to mind,” Arizona State University ecologist Nancy Grimm says. “What drives these changes? In fact, much of the current environmental impact originates in cities, and with demographic transition to city life the urban footprint is likely to continue to grow.”

Urban challenges face communities worldwide, with solutions lagging behind. Grimm and her colleagues promote a global perspective of urban development. Their analyses capture some of the commonalities that will face future city planners and societies, viewing cities as both drivers of and responders to environmental change. The authors chart the socio-ecological challenges and changes ahead for all cities, but particularly those in rapidly developing regions, like China and India.

These changes range from land use and cover, urban waste discharge and urban heat island effects to global climate change, hydrosystems, biodiversity and biogeochemical cycles. In all, the authors demonstrate that cities are substantive ecosystems in their own right, replete with complex human-environmental interactions and far-reaching impacts.

“Cities, and the people in them, will ultimately determine the global biodiversity and ecosystem functioning,” says Wu. “Sustainable urbanization is an unavoidable path to regional and global sustainability.”

Cities as ecosystems

For a decade, Grimm, Redman and more than a dozen co-principal investigators have pioneered urban studies in one of the first long-term ecological research (LTER) projects designed about urban environments. One of two urban long-term projects funded by the National Science Foundation (NSF) – the other is the Baltimore Ecosystem Study in Maryland – CAP LTER researchers have examined the living and non-living components of a city with participation from city planners, engineers, sociologists and other scientists, revealing the dynamic nature of this “ascendant ecosystem.”

“Urban areas are hot spots that drive environmental change,” says John Briggs. “They are complex, adaptive socioecological systems, centers of production and consumption, in which the delivery of the ecosystems services link society and ecosystems at multiple levels.”

Phoenix’s rapid growth provides a platform for CAP LTER researchers, as an evolving “before” and “after” laboratory. Phoenix is the fifth largest city in the U.S., with a metro area population or more than 4 million. Phoenix’s growth is emblematic of the U.S. West in general, which is expected to experience the largest percentages of population increases in the next 20 years.

“Phoenix, and cities in general, are microcosms for the kinds of changes that are happening globally,” notes Grimm. “In biogeochemical cycles, for example, they show symptoms of the imbalances in nitrogen, carbon dioxide, ozone and other chemicals that they help to create globally.”

Life on the edge

Cities literally are proving to be a hotbed for environmental research. Studies by urban ecologists reveal that city centers are physically hotter. Known as the heat island effect, urban and suburban temperatures are “2 to 10 degree F (1 to 6 degree C) hotter than nearby rural areas,” according to the Environmental Protection Agency. This rise in temperatures translates into “increases in peak energy demand, air conditioning costs, air pollution levels and heat-related illness and mortality.”

Just a one-degree rise in temperature can bump up residential water use 290 gallons per month on average for a single-family unit. However, knowledge about heat island effects also has meant innovation and the rise of new and greener technologies, such as roofing materials with a high solar reflectance and recycled rubber/asphalt composites to pave roadways.

But not all the challenges that occur in the city stay in the city. Grimm says rural landscapes at a city’s edge show changes in soils, built structures, human settlements, the diversity of plant and animal species and further impacts on fringe ecosystems. The authors invoke future thinking about cities and their effects as expressed by urban planner and policy expert Robert Lang, of Virginia Polytechnic Institute and State University. Lang believes that a city’s “footprint” has ballooned so that “cities are no longer independent, but represent a limited number of dominant megapolitan regions across the globe, with coalitions of urban centers built up in the intervening areas.”

“What we see is that landscapes, virtually anywhere in the world, will experience the impact of the growth and operation of nearby and long distance cities,” Redman says. “We need to understand the complexity of impacts that rapid global urbanization has both within urban boundaries and across landscapes at increasing distance.”

How can so many environmental challenges and changes be considered in any unified way? One recent approach has been to view urban systems as organic units: organisms that take up resources and produce wastes. Though controversial, such an integrated perspective can be useful for interpreting such things as biogeochemical cycles in cities and to analyze their regional or global effects. For example, cities are point sources for carbon dioxide and other greenhouse gases, and anthropogenic nutrient deposition. Fall out from cities can come in the form of urban aerosols, including atmospheric nitrogen, such as that wafted from fast-food joints or manicured lawns.

Studies by Sharon Hall, an ecologist with ASU’s College of Liberal Arts and Sciences, and Grimm find that fertilized and irrigated lawns release more nitrous oxide, a potent greenhouse gas, than the native desert soils that preceded them. Also, lawns support a more sustained, year-round production of nitrogen oxide than desert soils, which contributes to tropospheric ozone production and regional increases in photochemical smog.

“Global emissions of nitrous oxide (N2O) and nitric oxide (NO) have increased dramatically during the last century, primarily due to human activity associated with agriculture and fossil fuel combustion,” notes Hall. “We are just now discovering how urban centers figure into this equation, and how cities such as Phoenix impact surrounding landscapes, as well as contribute to larger regional or global climate."

Studies over the last 10 years by Wu and his students using geospatial analysis and computer modeling have shown that the Phoenix urban landscape has become geometrically more complex, but ecologically more fragmented. Also, urbanization-induced increases in temperature, CO2, and nitrogen deposition will significantly affect the productivity, carbon and nitrogen cycling, and a suite of biogeochemical processes of the native ecosystems, resulting in altered ecosystem functioning and services.

Selection and the city

Biodiversity studies in cities are equally revealing. Urban environments alter species compositions, biomass, distributions and ecosystem function. Studies by CAP-LTER and other groups show that plant types and habitat patches are, somewhat counter intuitively, increased by human activity relative to wild areas and involve a socio-economic component. Wealthier neighborhoods plant more exotics and show increases in yard-to-yard heterogeneity.

Co-author, Faeth, has found that numbers of birds and arthropods like grass hoppers, jump within city boundaries – though at the cost of a diversity of types. In addition, urban-dwelling species often flourish at the expense of indigenous species, the long-term effects of which may be reflected in altered life-history traits and, potentially, evolution. Thus, Faeth notes, cities are ecological and evolutionary arenas that create novel environments, with selective pressures that change flora and fauna, including human “fauna,” and that these will become more prevalent worldwide. The article points out that, worldwide, cities alter the behaviors, physiologies, disease patterns, population densities, morphologies and genetics of city-dwelling organisms.

“Cities create novel biological communities and these communities, no matter how ‘unnatural’ they are, are the ones that most humans know, and in the future, will experience,” Faeth says.

“Knowing how cities function, how the ‘ecosystem services’ they provide can be enhanced through planning and urban design, gives us a chance to improve the quality of life and the environment for animal, plant and human inhabitants of cities,” Grimm says. “Although every city and its surrounding environment are different, ecological studies of those differences, and participation of ecologists in decision making, can create solutions that apply across many situations.”

The NSF became an active partner in long-term urban study in 1997 with the launch of the central Arizona and Baltimore LTER programs. Since then, NSF has expanded support for urban systems research through a wide range of directorates, reflecting the complex questions at hand, encompassing biological sciences, geosciences, social, behavioral, and economic studies, and environmental research and education programs.

“Agglomerations of people in cities increasingly dominate environmental change globally, but are clearly understudied from an ecological standpoint,” notes Henry Gholz, of NSF’s Division of Environmental Biology. “This hampers our abilities to scale ecological information and make informed predictions of, or policies regarding, future global ecological states.”

Grim future?

Urban ecological study may be multi-faceted and complex, yet it offers pivotal insight in how to navigate a sustainable urban future. As soon-to-be dominant ecosystems, cities, harbor a wealth of ideas and creative accomplishments, as they have over centuries of urban living, Grimm and her colleagues say. Moreover, increasing public understanding that cities are more than miles of roadways, steel and glass means that urban ecosystems can be managed and that costs to citizens and environments can be understood and balanced.

“The relatively young and highly interdisciplinary field of urban ecology has demonstrated how well-designed cities can actually have less overall impact on the environment than equivalent dispersed rural populations,” says Jonathan Fink, director of ASU’s Global Institute of Sustainability. “The kind of counter-intuitive research results described in Grimm’s paper show how an ecological perspective can help urban planners and engineers find ways for society to live more harmoniously with nature.”

“Global Change and the Ecology of Cities,” Science, Feb. 8, 2008. Grimm’s co-authors include ecologists John Briggs, Stan Faeth, and Jianguo (Jingle) Wu of ASU’s School of Life Sciences; archaeologist Charles Redman, director of the ASU School of Sustainability; as well as researchers, Nancy Golubiewski from New Zealand Centre for Ecological Economics and Xuemei Bai of CSIRO Sustainable Ecosystems in Australia.