The first indigenous human settlements in the Amazon Rainforest were based on a rotational form of agriculture known as swidden farming, or slash-and-burn cultivation. The indigenous Amazonians cleared a small, circular area of the heavy rainforest, typically between 2.5 to 5 acres, allowed it to dry and then burned the cleared and dried area. The burning transformed nutrients in the dried vegetation into a layer of fertilizer that enabled the growing of crops in soil that would have otherwise been poorly suited for agriculture.
Because the indigenous Amazonian settlements were small and the slash-and-burn farming was done on a rotational basis, the amount of environmental degradation was negligible. Unlike the modern-day deforestation taking place in the region, no irreversible damage to the environment took place.
As of 2015, there are about 800,000 indigenous inhabitants in the Amazon region and the total human population, which has increased significantly since the first European colonists began exploring the area, is more than 25 million. As a result of the heavy migration into the region, factors such as logging, oil and mineral extraction, cattle ranching and hydroelectric dams have affected the area's environment in ways that many conservationists consider irreversible.