Footprints That Never Disappear ; Ancient Dig and Modern Phoenix Show How Mankind Has Permanently Changed the Environment

Article excerpt

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.

Ancient change

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. …