Footprints that never disappear

The beauty of Pueblo la Plata for this kind of work lies in its protected status, notes Hoski Schaafsma, a paleobotanist at Arizona State who is working with Spielmann at the site. "We can look at the complete package - a society that has gone through its conception, florescence, and collapse" without the "noise" of additional occupation before or after the pueblo's heyday, he says.

The legacy can be dramatic. He notes other sites he's studied in which plots identified as ancient fields hosted only one species of plant, while the surrounding landscape averaged 28. "That's a huge reduction in species diversity a thousand years after these fields were abandoned," he says.

Legacies from past human occupation continue to shape Phoenix as well, note researchers with the Central Arizona-Phoenix Long-Term Ecological Research (LTER) group at Arizona State University. The effort is one of two urban LTERs the federal government funded beginning in 1997, part of a 26-facility network that runs from the subtropics to the Arctic and Antarctica. The LTERs allow scientists to conduct landscape-scale experiments. Data from these experiments can provide crucial insights that scientists can fold into projections of how the environment is likely to respond to and affect changes such as global warming.

Land use before urban Phoenix sprang up "is really important for shaping the ecological conditions you see today" in the city, notes Dr. Grimm, codirector of the LTER. Studies of residential landscaping have shown, for example, that typical lawn-and-garden landscapes tend to be found where the land once was used for farming. Homes built on former desert have to settle for plants that are accustomed to arid climates.

Already the work has yielded some surprises, according to ecologist Stan Faeth. For example, in an effort to reduce water use, developers landscape new housing tracts with drought-tolerant plants, gravel, and cacti - so-called xeriscaping.

In looking at biodiversity among insects, particularly spiders, "we found that xeriscaped yards that you'd think would mimic desert sites or desert remnants within the city are more like industrial sites in species composition and dominance," Dr. Faeth says, adding that it's a warning that xeriscaped yards that look like desert don't necessarily help bolster biodiversity among species native to the region.

One challenge is that xeriscaping often is done with plants from Australia, Africa, and South America, adds Dr. Schaafsma. "No one is putting in Sonoran Desert plants.... One of the real messages that's starting to come out of studies of ecology in urban landscapes is that if systems haven't evolved together, if something new is introduced, it doesn't resonate with the local fauna. Even though to our eyes these imported plants perform the same function, spiders, birds, and pollinators avoid them, even if they have big beautiful flowers."

Availability of water remains a dominant focus for the LTER, although unlike other major cities in the west, Phoenix appears to have enough to meet its needs for several decades, researchers say. But it's critical to get a better handle on the factors controlling the region's supplies - information that water managers can use to parcel out this precious resource.

"We're experiencing a number of climatic uncertainties," notes Patricia Gober, an Arizona State University geography professor who is the codirector of a new center focusing on how desert cities respond to resource uncertainties in the face of changing climate. Global warming is expected to cut the region's scant precipitation by 5 percent in the next 50 years. The region has been in the throes of a drought for nearly a decade. The presence of the city alone has raised nighttime summer temperatures by an average of 12 degrees F. - the urban heat-island effect.

Yet archaeologist Charles Redman, codirector of the LTER, notes that since the dawn of civilization, water has been the key resource, around which people organized as communities, allowing political, social, and economic creativity to sprout along with beans and grain.

"As awful as people think Phoenix is and as wrong as it is to be in the desert, we are here," he says. "It may be a house of cards. That's what we're all trying to look at. Is it a house of cards, a Ponzi scheme" in the face of tenuous resources? "Or is it the most creative environment in the world" capable of a more sustainable future?

"It's a mix of both," he adds. "We want to make sure that the results are more positive than negative."

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