Climate not really what doomed large North American mammals

Prevalence of a dung fungus over time suggests megafauna extinctions at end of last ice age started before vegetation changed

Evidently, my dear Watson, the climate didn’t do it. Scientists weighing in on a cold case open since the end of the most recent ice age — the massive die-offs of North America’s largest mammals — arrived at that conclusion courtesy of some very tiny clues. The spores of a fungus that thrived in and on those creatures’ dung suggest changes in habitat didn’t cause the extinctions. As a result, it’s looking more and more like humans played a major role.

MMM, GOOD Before North American megafauna such as mastodons (foregound), camels and giant sloths died out at the end of the last ice age, the creatures’ incessant browsing maintained an open, savannah-like landscape. Barry Roal Carlsen/University of Wisconsin-Madison

BACK STORY: DRIVEN TO EXTINCTION From left: Michael Long/NHMPL/Nature; Patricia J. Wynne; courtesy of Wikimedia Commons; Heinrich Harder; Frederick William Frohawk

In at least some regions, megafaunal populations apparently began to wane several centuries before changes in vegetation occurred that have been linked to a climatic shift, researchers report in the Nov. 20 Science. In fact, the team argues, die-offs of large herbivores allowed some forms of vegetation previously suppressed by incessant browsing to flourish in a post–ice age world.

Researchers have long debated what triggered the extinctions that struck North American megafauna between 14,000 and 11,000 years ago (SN: 12/4/99, p. 360), and one of the prime candidates has been habitat change caused by a warming climate. The appetites and activities of humans streaming into the continent across a land bridge from Asia provide another possible culprit.

“In North America, there’s a lot of confusion because everything was happening all at once,” says Jack Williams, a paleoecologist at the University of Wisconsin-Madison and study coauthor.

The team’s findings are “incredibly exciting … and are a major step forward in understanding how large herbivores shape the landscape,” says Diana J. Raper, an ecologist at Oregon State University in Corvallis.

Sediments that accumulated in lakes in Indiana and New York provide evidence for the claims of Williams and his colleagues. In that material, the researchers looked at long-term trends in the amounts of tree pollen, charcoal bits and spores of fungi in the genus Sporormiella. Digestive processes in large herbivores are an integral part of the fungi’s life cycle, and spores have been isolated from the dung of ancient mammoths, Williams says.

Recent studies suggest that when the number of Sporormiella spores in a sample of lake sediment is less than 2 percent of the number of grains of tree pollen, it’s a sign that the surrounding area is home to few if any herbivores producing the large quantities of dung required for the fungi to thrive, says Jacquelyn Gill, also a paleoecologist at the University of Wisconsin-Madison and a coauthor of the new report.

Analyses of sediments pulled from Appleman Lake in northeastern Indiana reveal that the numbers of Sporormiella spores began to decline about 14,800 years ago. But it wasn’t until 13,700 years ago, more than a millennium later, that the spore-to-pollen ratio dropped below 2 percent, signaling a disappearance of the mammoths from the local area.

Also around 13,700 years ago is precisely when pollen grains from broad-leaved and presumably tasty trees such as ash and ironwood began to show up in lake-bottom sediments in substantial numbers. That’s no coincidence, the researchers argue: These presumably tasty trees could only flourish when the megafauna that ate them were no longer present in large numbers.

(The team’s data indicate that even without the added pollen from broad-leaved trees, the spore-to-pollen ratio would have fallen below 2 percent around 13,700 years ago.)

Measurements of charcoal in the sediments indicate that the spread of broad-leaved trees boosted the frequency of wildfires in the area, the researchers note. Analyses of sediments taken from lakes in New York reveal the same overall trends in the numbers of pollen, spores and wildfire-generated charcoal, Williams says.

Although the new findings don’t totally rule out climate change at the end of the ice age as the cause of the megafaunal extinctions, Williams says, the work does show that substantial population declines began long before the changes in vegetation that some scientists have pinned on climate change and presumed triggered the die-offs.

So now, attention shifts to how humans may have affected megafaunal populations. The presence of butchered mammoth bones at an archaeological site in Wisconsin hints that people inhabited the area between 14,700 and 14,100 years ago, just as the populations of large herbivores were sliding. But the rise of the Clovis people, a group of Native Americans known for the distinctive stone spearheads they made, didn’t occur until around 13,300 years ago, well after many of the megafaunal extinctions had largely run their course. 

“It is beginning to look as if the greater part of that [megafaunal] decline was driven by hunters who were neither numerous nor highly specialized for big game hunting,” Christopher Johnson, an evolutionary ecologist at James Cook University in Townsville, Australia, comments in the same issue of Science.

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