The disappearance of the huge herbivores that once roamed North America triggered a massive change in the environment with new trees and more fires.
Reporting in Science, researchers say lake sediments show that the decline of mammoths, mastodons, ground sloths and giant beavers allowed broadleaved trees to flourish as the beasties weren’t around to eat them. The rise of these trees then meant that more fuel for fires accumulated.
“Our work thus shows close connections among the late-glacial histories of fire, vegetation, and mammalian herbivores and suggests that the loss of a broad guild of consumers contributed to substantial restructuring of plant communities and an enhanced fire regime,” write Jacquelyn Gill, of the University of Wisconsin, Madison, and her colleagues.
To make these links Gill’s team looked at sediment from Appleman Lake in Indiana and other sediments from New York sites. They traced fungus spores that live on dung as a proxy for megafauna. As the number of spores in the sediments decreases about 13.8 thousand years ago, new types of pollen appear, showing the increasing dominance of the broadleaf trees. At the same time a big increase in charcoal is seen, showing the increasing number of fires.
The big question though is what does this tell us about why the mammoth died out…
As the paper says, the cores can’t answer this question directly, but they can rule out a couple of possibilities. Firstly the slow decline of fungus spores seems to rule out a meteor or comet impact 12.9 thousand years ago as the killer. They also rule out climate change induced changes in vegetation killing of the big beasts, as the decline in dung-fungus predates the change of pollens.
The research may also let humans of the Clovis civilisation off the hook. The Clovis culture – with their stone spears and specialised big animal hunting – only appeared around 13,000 years ago.
Although the arrival of the Clovis people does coincide with the last dates of megafaunal fossils, Gill’s paper puts the start of the megafauna decline much earlier.
“If people were responsible for that decline, they must have been pre-Clovis settlers,” writes Christopher Johnson in an editorial accompanying the new research. “The existence of such people has been controversial, but archaeological evidence is slowly coming to light and is consistent with their arrival around the beginning of the megafaunal decline. It is beginning to look as if the greater part of that decline was driven by hunters who were neither numerous nor highly specialized for big-game hunting.”
Image: Barry Roal Carlsen / UWM