How "virgin" is virgin rainforest?

When you think of the remotest and wildest places on Earth, aside from the polar regions, tropical rainforests are likely to come to mind straight away. However, a paper that I have recently read questions whether ‘virgin’ rainforest is actually as pure as we have previously thought.

They argue that our three major rainforests are actually secondary rainforests, and the areas have been deforested once before. The forests investigated are the Amazon basin, the Indo-Malay basin and the lower Congo basin. To test their hypothesis three geographers from the University of Oxford collated many case studies of the areas. Their objective was to look at evidence that humans have previously diminished the rainforests, with the forests subsequently regaining their biodiversity up to its current status.

An item of evidence put forward are the presence of fertile “terra preta” soils as standard in rainforests today. The formation of these soils is known as being due to prehistoric burning and subsequent agricultural activity. Additionally there is plentiful archaeological confirmation of substantial past settlements, which indicate formation of intensive agricultural land and parkland. Included in this evidence are stone tools, pottery fragments and palm oil nuts. Palaeoecological findings also firmly support the terra preta soils’ burning hypothesis, because distinct charcoal horizons are found.

Using this evidence it is thought that these three rainforest regions (and possibly others) supported huge networks of prehistoric humans who were actively changing the landscape around them, followed by population crashes in the areas. The time at which these activities were occurring varies between the regions, as illustrated in the figure above.

As this paper is based on a multitude of studies and has many points of evidence, I believe it puts forward a good case for our present day rainforests being secondary. This is an important realisation, because it shows these areas are not as fragile as we once thought, and are clearly well adapted for bouncing back providing enough forest is left for that to be viable. Further palaeoecological work could now be done to see the rate at which the rainforests recovered, and whether biodiversity is as high as it was in the primary rainforests.

This may help to put into perspective just how much of a risk to rainforests we humans are being presently. Could it be that the depletion of natural resources that comes as the rainforests are being chopped down, for example wood and medicinal plants, may not be as irreversible as once thought?