Home > Feature Archive > Urban Ecology

Urban ecology study witnessing the birth of a “designer ecosystem”

When ASU’s Central Arizona-Phoenix Long Term Ecological Research Project (CAP LTER) was funded by the National Science Foundation in 1997, more than 50 scientists signed on to do the multidisciplinary research, knowing they were embarking on something unusual: the first long-term ecological study of “a human-dominated ecosystem” – in other words, a city.

Seven years later, the first phase of the research has been completed, and the NSF has renewed the project with a second grant of $4.9 million for six more years of study, indicating the agency’s satisfaction with the researchers’ accomplishments.

But the long-term study has made more than just a good start; it’s produced results that may transform the study of ecology.

The project scientists increasingly are convinced that they are looking at a new kind of ecosystem – one that is radically different from the native desert that surrounds it and driven in part by forces unlike those usually studied by ecologists.

“It’s not what people generally think. They think there’s either nature or there are cities,” says Charles Redman, director of ASU’s Center for Environmental Studies and one of the project’s principal investigators. “That’s what this is all about – there is nature in the city. The city is part of nature.”

The development of urban LTERs was considered a major leap forward in the field of ecology because they included human culture as a driver of – and responder to – the ecosystem being studied, and because the research would include studies far outside traditional ecology or even the biosciences, climatology and earth sciences: sociology, anthropology, engineering and economics.

Phoenix was chosen as one of two urban LTER sites (Baltimore is the other) because it is a fast-growing desert city, with an archeological record for the area going back more than 2,000 years. The city has gone from a small farming center to a major metropolitan area in little more than a century, with the major growth occurring after World War II.

“It’s an ideal situation, because the development of the city has literally been happening as we watch,” says ASU Life Sciences Professor Nancy Grimm, CAP LTER’s other principal investigator. “We’ve made an assault on understanding the structure and function of the urban ecosystem on numerous fronts. From air quality to birds and bugs and plants to water quality and usage, to landscaping choices, climate, economics, zoning and pets, … we are considering everything that is part of the ecosystem.”

One of the revelations is the special nature of the urban biota, which has a distinctive mixture of native and exotic plants and animals but differs even further in the dynamics of how these organisms interact.

“We have been defining the Phoenix urban ecosystem – which, it turns out, functions very much like an ecosystem, with bugs and birds and plants – but in different sets of relationships, with much different abundances,” Redman says. “The abundance of organisms is higher overall. If you just measure it, it is richer in town than in the desert.”

The scientists note that the urban environment differs from the surrounding desert in that it has an ample, year-round supply of basic life resources, such as water, that in the native environment is seasonal or highly localized. This leads to a larger and more consistent supply of growing plants, and ultimately to a greater number of animals, such as birds.

The larger populations of birds that thrive in the city’s oasis are not the same as those in the desert, but are less diverse in species, with “generalist” native and non-native species (such as mourning doves, grackles and English sparrows) being favored over many of the more specialized birds found in the natural desert.

A more important detail to the researchers is something that is practically invisible: a major shift in the environment’s food web – what ecologists call the “trophic structure.”

“We’ve been monitoring and assessing the nature of trophic structure of Phoenix’s wildlife – the big animals, the smaller animals that they eat and the plants that they eat,” Redman says. “You’re spreading water all over the environment, and this is one of the prime reasons for the greater abundances of birds. This also has consequences for many, many other things. A fun thing is that this high abundance of birds, in turn, means that a variety of insects are kept at very low abundance.”

The group found that the insect population, which is controlled in the native desert mainly by seasonal scarcity of vegetation, is kept in check in the city by a larger population of birds. Bird populations are large because of ample water and the absence of natural predators, especially hawks. In turn, the lack of predation leads the city birds to spend more time hunting nutrient-rich insects instead of seeds – a behavior that would be risky around predators.

Other behavioral and trophic shifts also appear to follow. Particularly intriguing is the further implication that populations and trophic structures vary somewhat within the city, depending on the economic status of the neighborhood.

While the team’s interest is in doing fundamental ecology research, there also are some important applied science issues behind the project.

“What we really want to know is whether we can have a sustainable urban ecosystem in this kind of environment and setting,” Grimm says. “What are the elements of it that are warning signs of vulnerability, of some kind of event that could cause collapse? How do our institutions and the systems we have set up stand up against various kinds of stress? Is the urban ecosystem resilient?

“People in ecology are beginning to talk about ‘designer ecosystems’ – systems that have been heavily influenced by humans. What we’re doing is pioneering this.”

By James Hathaway. Hathaway, with the College of Liberal Arts & Sciences, can be reached at (480) 965-6375 or (hathaway@asu.edu).