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
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
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. …