Indigenous populations in some areas of Amazonia may have already been declining when Europeans arrived, according to new research published Thursday.

When the first Europeans arrived in South America, they brought with them new diseases, including smallpox, influenza, measles and the common cold, as well as violence through slavery and warfare, all of which led to the loss of an estimated 90% to 95% of the native population. This period, known as the Great Dying, led to the abandonment of much of the cultivated land in Amazonia — the basin of the Amazon River in northern South America — and rapid forest regrowth. 

However, this new research, published in Science, suggests that the arrival of Europeans exacerbated an already existing trend. Using fossil pollen records from 39 lake sites to track changes in forest cover and site abandonment across the region, the researchers found that forest regrowth had already begun in Amazonia several centuries before Europeans arrived, which suggests a depopulation of the Indigenous populations in the area. 

"We do not see a major ecological signal associated with [the Great Dying], and that's clearly an important message there," said Mark Bush, a professor at the Florida Institute of Technology and a co-author of the paper. "What our study does is, it says that there's another time, 400 or 500 years earlier, which actually is of more ecological significance in terms of changing forest area in the Amazon basin than the Great Dying was."

The researchers examined pollen records from between 1550 and 1750 in order to get a better understanding of the region before and after the first Europeans arrived. They found evidence of forest opening, burning and cultivating at 80% of the 39 lake sites studied, which signals that the areas were occupied by pre-Columbian natives. 

In a densely forested area, one would expect 95% to 98% of the pollens in the sediment to come from trees, Bush said, but the presence of pollen from grass and other crops is an indication of native inhabitants. The researchers also found charcoal in the fossil records, which shows people used forest burning to influence the ecosystem. 

The researchers wrote that the timing of forest regrowth at these sites can give insight into when the Indigenous peoples stopped cultivating the land.

The researchers especially took note of any presence of Cecropia, a short-lived and fast-growing genus of trees that produces easily identifiable pollen. Large amounts of this pollen in an area would mean that the area had recently been abandoned and was in the early stages of reforestation. As the forest matures, though, Cecropia disappears from the fossil record, making it an important marker for the true beginning of the Amazonia depopulation. 

At many sites, land abandonment and forest regrowth began approximately 300 to 600 years before the arrival of Europeans, according to the research.

"There's a whole discussion about how much pre-Columbian people impacted the Amazon and also whether there are still legacies of that activity in Amazonia," Bush said, "and this suggests that the regrowing forests are much older than previously envisaged, and therefore, they're not going to show that same level of legacy as you'd expect if it was only one or two generations since disturbance."

The team's observations are consistent with existing archaeological models that suggest stable or falling populations for centuries before Europeans arrived, Bush told The Academic Times. The timing of the reforestation in the lowlands coincides with the relocation of an estimated 25% of the Indigenous population from the Andes into the coastal lowlands between about 1000 and 1200, according to the paper. However, further research is needed to determine the reasons behind the site abandonment that occurred around 950 to 1350. Environmental change, pre-European pandemics and increased violence between Indigenous groups could have been contributing factors, the researchers wrote. 

"I'm hoping that people will get interested in this time, and the archaeologists will dig deep and give us some answers," Bush said. "The paleoecology can sort of point where archaeologists should look, but we can't necessarily answer those questions ourselves."

The study, "Widespread reforestation before European influence on Amazonia," published April 29 in Science, was authored by M.B. Bush and C.M. Åkesson, Florida Institute of Technology; M.N. Nascimento, Florida Institute of Technology and University of Amsterdam; S.Y. Maezumi, S.N. Huisman, and C.N.H. McMichael, University of Amsterdam; G.M. Cárdenes-Sandí, University of Costa Rica; H. Behling, University of Goettingen; A. Correa-Metrio, Universidad Nacional Autónoma de México; W. Church, Columbus State University; T. Kelly, Queen Mary University of London; and F.E. Mayle, University of Reading.