By Erik Stokstad | Science |

After Europeans brought smallpox and other highly infectious diseases to the Amazon in the 15th and 16th centuries, millions of native people died and much of their civilization was wiped out. But it didn’t disappear entirely. Left behind was a verdant, leafy legacy in the untold numbers of palms and other trees that had been cultivated across the Amazon. Now, researchers report that Pre-Columbian peoples had a significant impact on Amazonian forest diversity by making their favorite species much more common.

The findings “contribute to an emerging consensus that Pre-Colombians altered most of the Amazon,” says Joe Wright, an ecologist with the Smithsonian Tropical Research Institute in Panama City, who was not involved in the research.

Pre-Columbian people began domesticating plants on the edges of the vast Amazonian forests at least 8000 years ago, and their descendants continue to cultivate many species today. Archaeologists have long known that certain domesticated plants—palm trees for example—are often found around ancient sites in the Amazon, such as earthen mounds and the fertile soils known as terra preta, a relic of past agriculture.

To get a better idea of Amazonian forest diversity, ecologist Hans ter Steege of the Naturalis Biodiversity Center in Leiden, the Netherlands, and colleagues had previously assembled a massive database of biological surveys done by many researchers. They reported in 2013 that certain tree species were particularly common, or “hyperdominant,” in Amazonian forests; half of all trees across the entire region belong to just 227 species. Many of these tree species are heavily used by local people like the Yanomami of northern Brazil, mainly for food. That raised the question of whether some of these abundant and widespread species might have originally been planted by people.

The team made a list of woody species that had evidence of domestication, like larger fruit. Most of these 85 species can also survive in the wild without the care of farmers, unlike modern crops, putting them in a state of “incipient” domestication. The next step was for Ter Steege, Carolina Levis, a Ph.D. student at Brazil’s National Institute for Amazonian Research and Wageningen University in the Netherlands, and colleagues to compare the abundance and richness of these species, relative to other species, in 1091 research plots in the Amazon basin and the neighboring Guiana shield. Twenty of the 85 domesticated species, they found, were hyperdominant.

To see whether humans had a hand in this dominance, the team checked the abundance and richness of trees around places with no evidence of past occupation and also near 3348 archaeological sites, such as rock art or earthen mounds. Common domesticated species, such as the Brazil nut tree, were more abundant near places where people once lived, the team reports today in Science. Nearby forests also typically contained more kinds of domesticated species than elsewhere. The richness and abundance was particularly striking in the eastern and southwestern Amazon. In parts of the Amazonian forest within Bolivia, for example, domesticated species accounted for up to 61% of tree diversity. “I was actually a bit stunned,” Ter Steege says. “The effect of Pre-Columbian people is much more pronounced than many of us believed.”

Wright says the relationship between domesticated species and archaeological sites is “very convincing.” There is a strong case for the ancient cultivation of forests in the southern and eastern parts of the Amazon, agrees Mark Bush, a biogeographer at the Florida Institute of Technology in Melbourne, who specializes in the paleo-ecology of tropical forests. But he’s not convinced by the evidence elsewhere in the region. One problem, he says, is that the new research can’t determine when these domesticated species became common; some of the cultivation could have been more recent. It would be a “huge overreach” to claim that all the Amazon is dominated by domesticated species, he says.