Footprints that never disappear
Ancient dig and modern Phoenix show how mankind has permanently changed the environment
AGUA FRIA NATIONAL MONUMENT, ARIZ. — Separated by 40 miles and nearly a millennium, ancient Pueblo la Plata and modern Phoenix seem to have little in common.
Sitting atop Perry Mesa in Agua Fria National Monument, Pueblo la Plata was seldom home to more than 50 people at a time during its 200-year history, starting around AD 1200. No organized community appears to have occupied this area before or since.
To the south, Greater Phoenix's 3.2 million inhabitants sprawl across 2,000 square miles of what once was an oasis in the Sonoran desert. Home to the sixth-largest city in the United States, the region is expected to double its population by the middle of the century. It has been continuously inhabited since at least AD 500.
For all their odd-couple appearance, the two settlements form bookends in ambitious efforts by archaeologists, ecologists, and others to investigate the long-lasting effects of human habitation and what can be done to make it more sustainable in arid regions in the future. It also raises an intriguing question: If humans leave their mark on an environment even centuries after they've left, is any place on Earth really pristine?
The answers are likely to resonate far beyond central Arizona.
"A lot of the urbanization globally will take place in desert regions," says Nancy Grimm, an ecologist at Arizona State University. Lessons gleaned here could well help cities around the world.
As ancient Southwestern pueblos go, Pueblo la Plata is not exactly postcard material. Except for the remains of rectangular rooms built upon a small rise in the otherwise flat terrain and the small potsherds strewn about, the landscape looks natural.
But Katherine Spielmann, an archaeology professor at Arizona State University, points to a "doughnut" of parched, rock-free soil that surrounds the pueblo. Beyond it, grasses and other small shrubs dominate, with rocks strewn everywhere. In some areas, small piles of basaltic rock seem to sprout along with a handful of agave plants.
Each is a sign of the pueblo's human inhabitants. The doughnut is the most likely site for rocks used to build the pueblo and a defensive wall that seals off a "prow" in the mesa where two canyons meet. The small piles of dark rock were placed around imported agave plants to provide them with extra warmth, corral moisture, and so extend their growing season. Some of the plants may be the original imports, Dr. Spielmann speculates.
"A grassland ecologist would come [here] and they'd see evidence of [modern-day] cattle grazing, and that's all they'd see," Dr. Spielmann says. By subtracting the effects of grazing, they could walk away feeling that they knew what a pristine grassland would be like. "Our hypothesis is that the landscape has been modified for so long that you can't understand the ecology of the area without understanding what prehistoric people did."
It's a point ecologists are beginning to recognize elsewhere. Scientists at the Harvard Forest Long-Term Ecological Research station have tracked the lasting effect of Colonial-era farming on the nature of forests that sprang up in abandoned fields following the Industrial Revolution. Elsewhere, Katherine Willis, a plant ecologist at Oxford University in Britain, notes that in three of the world's largest "undisturbed" blocks of tropical rain forest, large tracts were farmed as far back as 8,000 years ago. She and two colleagues noted in an article in the journal Science last April that the ancients' burning and fertilization techniques so enriched the soil that today these long-abandoned tracts are some of the most productive "natural" sections of the rain forest.
Here at Pueblo la Plata, serious field exploration only began in April. Even basic information such as the identity of the inhabitants remains a mystery. Research teams laid two 900-meter lines and at regular intervals surveyed artifacts, plants, small mammals, and the ever-present rocks. Similar data gathered along a third "transect" on a nearby, uninhabited section of the mesa will provide a basis to determine how these ancient inhabitants changed the ecology. So will comparisons of small-mammal bones in their ancient trash piles with small mammals found today.
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."