Raining on theories of Amazonian rainforest populations

From: http://www.usatoday.com/tech/science/columnist/vergano/2007-02-25-amazonia_x.htm

Feb 25, 2007

One setting for this recasting of pre-Columbian history has been the Amazon rainforest, the vast 1.2 billion-acre basin drained by that river. In 2003, a Science magazine study led by the University of Florida's Michael Heckenberger uncovered remains of at least 19 towns in Brazil's Xingu region, deep in the rainforest and dating as far back as the year 1200 AD. That was surprising because people were not thought to have lived in the region in large numbers, given its poor soils. The finding, together with archaeologists uncovering earthworks, irrigation ruins and soils containing ancient charcoal — the signature of slash and burn farming — led some scholars to revise their estimate of the Amazon's ancient population. Previously thought to be roughly 1 million people, it grew upwards to 11 million, spread out over the forest when Columbus arrived.

But in one of those inevitable ebbs and flows of science, some researchers suggest those high numbers, and the notion that people were widely spread across Amazonia, may be overstating the case. In a just-released paper in the journal Philosophical Transactions of the Royal Society B, a team led by Mark Bush of the Florida Institute of Technology in Melbourne, reports on the implications of charcoal and pollen records at two lake regions in the eastern and western Amazon. "In particular, we ask 'Did human occupation result in widespread landscape modification at these two widely separated Amazonian sites?'," write the researchers.

In their survey, the team looked at western lakes, Gentry, Vargas, Parker and Werth, near Puerto Maldonado, Peru, and eastern ones, Lakes Geral, Santa Maria and Saracuri, near Prainha, Brazil. Today people live in homes mostly near a central lake in each region — Gentry and Geral. From 1990 to 2001, the team took core samples from the center of each lake, part of efforts to reconstruct the past climate of the region, Bush says.

By looking at the chemistry of pollen and sand flowing into the lakes over decades and piling up upon their bottoms, researchers can tell what sort of weather the regions enjoyed in the past. The study used this climate data, which tells the tale of climate and land use in the region dating past 10,000 BC, to examine the archaeological question of how the Amazon was settled.

If Amazonian settlements were widely dispersed, the team expected it would find signs of crop pollens, corn and manioc cropping up in every lake, Bush says. Charcoal should show up too, as it only appears in the wet rainforest in appreciable amounts when people deliberately burn trees to clear cropland.

At Lake Gentry, the team reports signs of these crops appearing around 3,700 years ago and charcoal use around 8,000 years ago. A similar pattern emerged at Lake Geral. But the side lakes, where few people live today, show only sporadic signs of people farming nearby. "In our experience of studying more than 30 lake records from Amazonia, a general rule of thumb has emerged: If the lake has modern use, has a road to it or has a settlement beside it, there is a very good probability of finding a long record (thousands of years) of disturbance. If, on the other hand, the lake is remote from modern society, it probably has a history lacking the characteristic signature of human activity," the team concludes.

Bush suggests some lakes and river regions likely had the conditions to support comfortable living in the Amazon — a local dry season for example. And others didn't. Bush thinks that rather than being widely dispersed, people living in the Amazon most likely clustered near the good places, and that overall population numbers were likely not as high as the top estimates of pre-Columbian people. He suggests this message matters today, because some foresters have called for exploiting the Amazon, arguing it was widely settled before and recovered, and thus could withstand another clear-cutting today. "I really think we need to argue against that being a reasonable idea," he says.

"The Brazilian Amazon contains roughly 40% of the world's remaining tropical forest. It is also among the most threatened with rapid degradation due to agro-pastoral expansion and other development," Heckenberger notes, in a related paper in the same journal. "Comparing pre-Columbian population scales or technologies with contemporary agribusiness and frontier development is like comparing a 17th Century grist mill with a modern hydrodam."