Deforestation and slash-and-burn practices change the ecosystems in rainforests and make them less resilient. As a result, these are considered devastated and impoverished. But to a certain extent, even such managed tropical forests are still home to thriving flora and fauna, as a study has now shown. These forests can recover and continue to serve as a CO2 sink. In order to protect biodiversity and the climate, it is therefore worthwhile not only to protect untouched, intact rainforests, but also areas that have already been modified. At a certain point, however, the forests are actually lost.
The earth’s tropical rainforests are huge CO2 sinks and therefore important for maintaining the climate. They are also home to thousands of animal and plant species. Their destruction and degradation through slash-and-burn, deforestation or fragmentation therefore harms biodiversity and the climate. But at what point is deforestation too much? What is the limit at which a rainforest cannot regenerate itself?
Threshold values using Malaysia as an example
To find out, a research team led by Robert Ewers from Imperial College London has now evaluated data from 127 studies on the state of flora and fauna in a rainforest in Malaysia. They document the development of 590 plants, 88 mammals, 161 birds, nine reptiles, 42 amphibians, 26 fish and 635 invertebrates such as insects over a period of eleven years. The area in the state of Sabah on Borneo includes various forestry zones: from untouched rainforest to protected buffer zones along rivers and partially cultivated areas to areas that have been almost completely converted into oil plantations.
The analysis showed that human intervention in the tropical forest always has ecological consequences – no matter how small the area cleared is. But an important limit seems to be “only” 29 percent biomass loss: If fewer trees are cut down in the forests, the biodiversity in them is largely preserved. “We found that many of these ‘devastated’ forests are home to a surprising biodiversity,” says co-author Will Pearse from Imperial College London. If the forests are left alone below this limit, they can usually recover, as the team found.
According to the data, another important threshold is 68 percent biomass loss. If this point is exceeded, many plants and animals die out or are displaced by invasive species. This weakens the ecosystem considerably. Comprehensive protection measures are needed to preserve biodiversity in these areas, say the researchers. These rainforests cannot regenerate on their own. “Changes in biodiversity are faster below 30 percent and above 70 percent biomass loss. This suggests that any improvement in the habitat in these areas would lead to dramatic changes in biodiversity,” senior author Cristina Banks-Leite, also from Imperial College London, summarizes the results.
The exact figures for the threshold values can vary from region to region, as the team reports. However, the figures determined for Malaysia as an example provide an orientation and rule of thumb for the first time as to how much support the rainforests in Southeast Asia need for regeneration, depending on the extent of their previous management. “We now have a framework to assess where these change points could be in other ecosystems,” says Ewers.
Managed forests can also be ecologically valuable
The study thus provides new clues for governments and decision-makers as to which forests are still “worth saving” – or where which protection or reforestation measures should be most sensibly implemented. “In nature conservation, there is a tendency to see pristine forests as the only ones worth investing in – that forests that have already been partially cleared are not worth considering,” says Ewers. “What we have shown is that the potential ‘nature reserve’ is much larger than we thought. Although pristine forests are shrinking worldwide, this does not mean that all hope is lost. There are other forests that we can protect to preserve biodiversity.”
The findings also expand the possibilities for a combination of forestry and nature conservation: “A certain amount of deforestation may always be necessary. Although these forests are not as ideal as pristine forests, above a certain threshold they can still survive as functioning ecosystems,” says Pearse. “We have shown that flora and fauna can survive and even thrive in managed and partially deforested forests within certain limits. These modified forests are worth preserving because of their biodiversity and their function as a carbon sink,” adds co-author Roger Kitching from Griffith University in Australia. The researchers now want to use the data to create a virtual image of the tropical forest in Malaysia in order to be able to understand and predict natural developments even better.
Source: Imperial College London; Article: Nature, doi: 10.1038/s41586-024-07657-w