A new day dawns, and morning light filters through the towering stems of an ocotillo, casting striped shadows on the ground. Although the thorny plant looks a bit lackluster, it provides an adequate perch for a zealously chirping curve-billed thrasher. An abundance of black widow spiders lurk unseen in the cracks and crevices below.
By all appearances, this is a typical morning in Arizona’s Sonoran Desert. In some ways it is, but in many ways it isn’t.
Many residential yards in Arizona are designed with xeric landscaping, which requires little water and looks like the wild desert surrounding the city. Ecologically speaking, however, these yards are “functionally not at all like the desert,” according to Nancy Grimm, a professor in the School of Life Sciences at Arizona State University.
That ocotillo, for instance, is drowning. Homeowners tend to shower too much water on their yards even when they mimic an arid landscape. Desert birds like the curve-billed thrasher seem to exhibit more aggressive territorial behavior in the city than those living out in the desert. What about the black widows? To homeowners’ distress and exterminators’ delight, their populations are denser in urban areas than in rural ones, nourished by the insects that swarm to the oasis of the city.
Less visible changes are happening, too. Biogeochemical cycles in the soil, water and air are all different between residential yards and wild Sonoran sites.
These are just a few of the findings from ASU’s Central Arizona-Phoenix Long-Term Ecological Research (CAP LTER) program, funded by the National Science Foundation (NSF) since 1997. The NSF supports 26 LTER sites across the United States, but only two focus on urban environments: the Baltimore Ecosystem Study and CAP LTER.
"Phoenix, and cities in general, are microcosms for the kinds of changes that are happening globally," notes Grimm, the CAP LTER principal investigator and project director. "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.”
Human activities create these changes, which in turn affect humans. As people adapt to the altered environment, they change the natural world further. This circular relationship within the urban socio-ecosystem is the basis of CAP LTER’s overarching research question: How do the services provided by evolving urban ecosystems affect human outcomes and behavior, and how does human action (responses) alter patterns of ecosystem structure and function, and ultimately, urban sustainability, in a dynamic environment? Sociologists, geographers, anthropologists, ecologists, biologists, climatologists, engineers and others all contribute their expertise to finding answers.
“It’s an imperative to understand how we make cities sustainable, because like it or not, that’s where most people are going to be living.”
Designer ecosystems are one of the major research themes in CAP LTER. Residential landscapes, like the xeriscape described above, are an example of these. Others include storm retention basins and artificial lakes.
ASU PhD student Libby Larson, working with Grimm, identified about 1,000 artificial lakes in the Phoenix metropolitan area using remote sensing imagery, GIS data and state impoundment-permit information. These lakes provide habitat and water for many animals, and could help rid the city of excess nitrogen.
They also boost home values. Waterfront properties are valued $31,271 higher, on average, than similar houses further from the water, according to research from Joshua Abbott, an associate professor in ASU’s School of Sustainability, and his colleague H. Allen Klaiber at The Ohio State University.
In their zeal to maximize the number of waterfront properties they can sell, developers often create unnaturally shaped lakes with many narrow inlets. However, this may have ecological effects that can lead to algae overgrowth and fish death.
Findings like these reveal the motivations behind people’s environmental modifications and the impacts of their choices. Studying the urban ecosystem is particularly compelling in a desert city like Phoenix, which presents unique challenges.
“There’s this idea, which we dispute, that there shouldn't be any cities in the desert, that it's a crazy place to have a city because there's not enough water here,” Grimm says.
Yet arid lands take up over 40 percent of the Earth’s land surface, according to the United Nations, and account for some of the highest population growth rates in the world. Those people aren’t going anywhere. Even if they could, where would they go?
“If you're going to put 8 billion or 10 billion people on the planet, what's the best distribution of them?” Grimm asks. “Should they be evenly spread out? Or should they be in cities? Where should those cities be?”
Understanding Phoenix can offer insights to other urbanizing areas of the world. The city’s rapid growth means that its transition from desert to farmland to city has been relatively recent, providing an evolving "before" and "after" laboratory. This makes it particularly useful for studying land-use change, land-cover change and land legacies.
It is also useful for studying water resources. Arizona’s population is more concentrated in urban areas than most other states, because people need to live where water is accessible. Harnessing and redirecting so much water has dramatically altered the natural environment of the region, from the water system to the climate.
CAP LTER research provides a rich array of data on the long-term impacts—on both humans and nature—of urban living in an arid region. Cities are becoming dominant ecosystems, housing most of the human population and affecting the world far beyond their boundaries. The more we understand about how these ecosystems change over time, the better prepared we’ll be to make changes beneficial to all—humans, ocotillos and even black widows.