Cityscapes Are Landscapes, Too
Not all LTER sites are located in remote, rural areas. In 1997, NSF added two sites to the network specifically to examine human-dominated ecosystemsin other words, cities. One site is centered in Baltimore, Maryland, the other in Phoenix, Arizona.
The Central Arizona/Phoenix (CAP) site fans out to encompass nearly five million acres of Maricopa County. While much of the site's study area is urbanized, some portions are still agricultural field or desert, and there are also a few nature reserves. CAP researchers are in the early stages of laying the groundwork for long-term studies at the site. For one thing, they're busy identifying two hundred sampling sites that will encompass the city, the urban fringe, and enough spots on the very outer edge to ensure that some portion of the site will remain desert for the next thirty years.
"One of our exciting challenges will be to take those very standard common ecological measures that people use in the forest and desert and everywhere else, and say, well, is there an equivalent way to look at how the city operates?" says Charles Redman, Arizona State University archeologist and co-director of the CAP-LTER. To tackle that challenge, Redman and co-director Nancy Grimm work with a research team that includes ecologists, geographers, remote sensing specialists, sociologists, hydrologists, and urban planners.
As a framework for their foray into the ecology of a city, the researchers are adopting a popular and relatively new ecological perspective that recognizes that rather than being uniform, an ecosystem is patchy, rather like a quilt. For example, patches in a grassland might be recognized as areas that burned last year, areas that burned five years ago, and burned areas where bison are now grazing. Smaller patches exist within the larger patches: the bison might graze more heavily in some sections of the burned area than others, for example. There are patches of wildflowers, patches where bison have wallowed, and patches where manure piles have enriched the soil. Each time ecologists look closely at one type of patch, they can identify a mosaic of smaller patches that make up that larger patch. And if they can figure out what the patches are, how the patches change over time, and how the different types of patches affect one another, they might be able to figure out how the ecosystem functions as a whole.
Anyone who has flown over an urban area and looked at the gridlike mosaic below can imagine how easily cityscapes lend themselves to the hierarchical patch dynamics model. Still, it's a new approach where cities are concerned, says Jianguo Wu,
a landscape ecologist at Arizona State University. And the patches within cities are new to ecologists.
"You can see very large patchesthe built-up areas, the agricultural areas, the native desert areas," he says of the Phoenix site. "But if you zoom in, you see smaller patches. Walk into downtown Phoenix. There are trees, parking lots, concrete. They form a hierarchy of patches with different content, sizes, shapes, and other characteristics."
CAP researchers have gathered information about how land use in the Phoenix region changed from the early 1900s until today. The team has found that as the area became more urban, the patches became smaller and more regularly shaped. In the new millennium, the scientists want to see how this kind of more orderly patchiness affects ecological processes. For example, researchers would like to know how insects and other small animals move across the landscape, and how storm runoff carries away nutrients across the various patches, whether concrete or soil.
Grimm thinks that the patch dynamics model will help researchers integrate all the information they collect about the rapidly changing Phoenix metropolitan area. The model emphasizes linkages between different levels and types of patches such that researchers can design studies to ask: How might the actions of an individual eventually affect the ecology of a whole community-sized patch? If someone sells an undisturbed piece of desert property to a developer, for example, the ecosystem will change. What kind of development is builtwhether there is one house per acre or a series of closely packed townhomeswill differently affect the ecological processes in the adjacent patches of remaining desert.
"Once the land use changes, the ecology changes," says Wu, adding, "What is really important is the dynamicsthe impact of this patchiness on the ecological, physical, hydrological, and socioeconomic processes of the city."