Many of the world’s largest and most biodiverse forests, from the Amazon to Borneo, are being cleared for timber and to make way for agricultural land. Yet not even these forests can withstand deforestation beyond a tipping point, scientists warn.

Once forests have been cleared in bits and pieces down to half in size or so the remaining forest disappears very quickly, says Tomasz Stepinski, a professor of geography who studied deforestation patterns over the past decades by help of high-resolution satellite images from the European Space Agency.

“[O]nce forest share drops below 50% the remainder of the transit is rapid,” Stepinski and a colleague write in a new study. “This suggests that possible conservation policy is to protect mesoscale tracts of land before the forest share drops below 50%.

What the experts have found is that mixed landscapes of forest and agricultural land do not stay mixed for long. Rather, mixed blocks often become homogeneous over time regardless of the landscape type.

“I think it’s very intuitive. It corresponds to the different climatic zones. The Earth before people was certainly like that. You had forests and mountains and wetlands and deserts,” Stepinski explains. “You would expect people would create more fragmentation, but as it turns out, people never stop. They convert the entire block on a large scale.”

Based on data from satellite images, the scientist says that 22% of Earth’s habitable surface was altered in just over two decades, between 1992 and 2015, largely through converting forest to agricultural land. Over the past few decades vast swathes of rainforest have been cleared from South America to Southeast Asia and in many places deforestation is proceeding apace.

The new findings suggest that forests cleared in patches can easily be pushed beyond a point of no return by human hands. “If you are cutting forest, you have the infrastructure to finish it. It’s so much easier to cut the rest,” Stepinski observes. In addition, he adds, “the forest is more vulnerable to change when there has been a disturbance.”

The solution then is to protect large enough areas of forests from further harm and leave them in relative isolation from development.

“You’d be hard pressed to find land managers who wouldn’t be strongly in favor of protecting larger tracts because they’re more resilient to a variety of challenges, including invasive species and climate change,” Martin McCallister, a project manager for the Edge of Appalachia Nature Preserve in southern Ohio, comments. “Once a [forest] gets fragmented by roads, it’s easier to extract resources. It’s also easier for invasive species and pests to get a foothold,” he elucidates.

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