The protective barrier created by these tribal lands is enormoustwo and half times larger than the country of Costa Rica.
Here, study findings confirm what is visible to the naked eye from satellite maps.
"Where indigenous land in the Amazon starts is where the frontier stops," said Stephan Schwartzman, an anthropologist with New York-based nonprofit Environmental Defense and a co-author of the study.
The study did not examine how protected areas with human populationsincluding indigenous lands and national forestsmaintain aspects of environmental health other than standing forest.
In some parts of the Amazon, gold mining, selective logging, and illegal hunting have caused extensive damage. But these threats are less severe in many indigenous territories, where tribal groups guard against outside intrusions.
"Logging and hunting aren't trivial, but I'd far rather have these kinds of impacts than have lands with little or no forest at all," said William Laurance, a staff scientist with the Smithsonian Tropical Research Institute in Manaus, Brazil.
"Despite often being under staggering pressures, the indigenous lands are doing a pretty good job of protecting the Amazon," Laurance said.
By demonstrating such protection, the new study underscores the value of strategic alliances among conservationists, indigenous tribes, and other rural land users.
Such alliances have helped tribes like the Kayapo gain official recognition of reserve areas and aided them in enforcing bans on forest clearing and other illegal activities.
Study co-author Paul Lefebvre, a research associate at the Woods Hole Research Center, says a number of groups share a common interest in maintaining the forest.
"There are some who still argue that you need to put up fences and keep people out to protect the forest," Lefebvre said.
"What we're seeing now is that by keeping the people there, you can actually enhance protection."
After proceeding at a record rate from 2002 to 2004, Amazon deforestation slowed a bit in 2005.
While falling prices of commodities found in the region are thought to be partly responsible, new reserves and improved enforcement also played a role.
Last year the Brazilian government created nearly 20,000 square miles (51,800 square kilometers) of new sustainable-use reserves north of Kayapo lands.
These reserves create a formidable new barrier to frontier expansion, and alongside other reserves they form a nearly continuous protected zone over 90,000 square miles (233,000 square kilometers).
"The alliance between conservation and indigenous groups has now extended to 'smallholder' organizations," Nepstad said, referring to groups of small-scale farmers and forest users whose advocacy helped lead to the new protected areas.
"They are, in one respect, the Amazon's new conservationists," Nepstad said.
With additional new reserves planned over the coming years, more than 40 percent of the Brazilian Amazon forest will have some protected status.
"This is cause for optimism," Nepstad said, "but reserves will not be enough. We also must harness market pressures on large-scale ranchers and soybean farmers to improve their environmental performance."
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