New York: A widespread and permanent loss of forests in Madagascar that occurred 1,000 years ago was due not to climate change or any natural disaster, but to human settlers who set fire to the forests to make way for grazing cattle, new research has found.
The results suggest that humans used "slash and burn techniques" around this time to create pastureland for cattle, said one of the researchers David McGee, assistant professor at Massachusetts Institute of Technology (MIT).
The researchers came to this conclusion after determining the composition of two stalagmites from a cave in northwestern Madagascar.
"The transition from ephemeral forager to dedicated agro-pastoralist occurred, probably across Madagascar, around 1,000 years ago," Laurie Godfrey, professor at University of Massachusetts at Amherst, noted.
Stalagmites form from water that percolates from the surface, through the soil, and into a cave.
These finely layered pillars can be preserved for thousands of years, and their composition serves as a historical record of the environment above ground.
From their analysis, the team found that around 1,000 years ago, both stalagmites' calcium carbonate composition shifted suddenly and completely, from carbon isotope ratios typical of trees and shrubs, to those more consistent with grassland, within just 100 years.
The researchers found that this landscape transformation was not triggered by climate change.
Around the same period, they found that oxygen isotope levels remained unchanged in both stalagmites, indicating that rainfall rates -- and climate in general -- 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," McGee said.
"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 noted.
"Both the speed at which this shift occurred and the fact that there's no real climate signal suggest human involvement," McGee pointed out.
The findings appeared in the journal Quaternary Science Reviews.