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Madagascar: Early Human Settlers Set Forests On Fire 1,000 Years Ago

Feb 22, 2016 10:03 AM EST

Humans have no doubt irreversibly altered the natural environment. But contrary to common belief, people were impacting the Earth's landscape long before the Industrial Revolution. A new study suggests early settlers set Madagascar forests ablaze about 1,000 years ago to make room for pastures and grazing cattle. 

A team of scientists from Massachusetts Institute of Technology (MIT) and the University of Massachusetts at Amherst found evidence suggesting the permanent loss of large forest areas in Madagascar was a direct result of human settlers. Researchers came to this conclusion after examining the composition of two stalagmites from a cave in northwestern Madagascar, according to a news release.

Stalagmites form in caves as water percolates from the surface, through the soil. These finely layered pillars can be preserved for thousands of years, so their composition serves as a historical record of the environment above ground, researchers explained. Their analysis of Anjohibe Cave, a large cave system in northwestern Madagascar, revealed that the calcium carbonate composition of both stalagmites shifted dramatically from carbon isotope ratios typical of trees and shrubs, to those more consistent with grasslands, in a matter of only 100 years. 

Researchers say this transformation is much too sudden to be attributed to climate change or any natural occurring phenomenon, and was likely expedited by human activity. This theory was confirmed by consistent oxygen isotope levels found in the stalagmites from about the same time period. This indicates that rainfall, and therefore climate, remained relatively stable. 

"We went in expecting to just tell a climate change story, and were surprised to see a huge carbon isotope change in both stalagmites," David McGee, an Assistant Professor in the Department of Earth, Atmospheric and Planetary Sciences at MIT, explained in the release. "Both the speed at which this shift occurred and the fact that there's no real climate signal suggest human involvement."

Their finding also helps explain the loss of other large animals from about the same time period. This includes giant lemurs, pygmy hippos and giant tortoises.

"You'd think stalagmites in a cave are insensitive to what's going on in the landscape above them," McGee added. "But because they're basically fossilized groundwater deposits, precipitated in very regular layers, they're a fairly sensitive recorder of climate and ecosystem changes."

All plants absorb carbon dioxide for photosynthesis, but trees and shrubs prefer a fixed isotopic ratio of carbon-12, while grasses prefer a fixed isotopic ratio of carbon-13. It follows then the presence of either in the cave's stalagmites is a good indicator of what plants were growing above. In other words, researchers observed a dramatic shift in the ratio of carbon-12 to carbon-13 around 1,000 years ago in the stalagmites. 

"What we see in the record is that the change from carbon isotopes that look like forest, to isotopes that look like grassland, happens really rapidly, within a century, and it would be unusual for a forest to naturally completely turn into grassland that quickly," McGee concluded in MIT's release

Madagascar was first colonized about 3,000 years ago, and after years of hunting and gathering, settlers eventually introduced cattle farming into their lifestyles. In the future researchers hope to sample more caves across Madagascar to get a better picture of how the island was transformed by early humans. 

Their study was recently published in the journal Quaternary Science Reviews.

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