Updated | The extinction of megafauna across the globe at the end of the last Ice Age appears to have been driven, in part, by moisture from thawing glaciers and permafrost. As moisture in the air increased, grasslands turned to bog and ecosystems collapsed, eventually causing species like the woolly mammoth, giant sloth and sabre-toothed cat to die out.
Between 15,000 and 11,000 years ago, large animals—known as megafauna—across the globe disappeared. Extinctions were most pronounced across Eurasia and the Americas, in what is now Europe, Siberia and North and South America.
What caused this mass trend is unclear. These species were in contact with early humans and Neanderthals and fossil evidence shows they were killed for their meat. Some scientists believe that as humans became more successful, they hunted megafauna to extinction. However, most agree that this alone could not have led to such widespread species extinction. Rather, they believe a combination of climate change and hunting was responsible.
In a study published in Nature Ecology & Evolution, a team of scientists led by Alan Cooper of the University of Adelaide, analyzed the teeth and bones of herbivorous megafauna to understand the impact of environmental change.
They took megafauna samples from across the globe, allowing the team to understand environmental changes on a continental scale. Findings showed thawing permafrost and melting glaciers led to a dramatic increase in landscape moisture. This, they found, caused grasslands to turn into bogs and peatlands and megafauna populations were forced to fragment. Effects were particularly prominent in Eurasia and the Americas, but less so in Africa.
"We didn't expect to find such clear signals of moisture increases occurring so widely across all of Europe, Siberia and the Americas," Cooper says in a statement. "The timing varied between regions, but matches the collapse of glaciers and permafrost and occurs just before most species go extinct.” Because herbivore megafauna were critical to the food chain, any decline in their populations would have a ricochet effect on the rest of the ecosystem and any species within it.
Scientists also say moisture levels can help explain why megafauna extinctions in Africa were far less pronounced. Environmental changes to moisture here were far less pronounced: “The stable presence of grasslands surrounding the central forested belt of Africa during this period helps to explain why proportionally fewer African megafauna became extinct during the late Pleistocene,” the scientists write.
Cooper adds: "The idea of moisture-driven extinctions is really exciting because it can also explain why Africa is so different, with a much lower rate of megafaunal extinctions and many species surviving to this day. Africa's position across the equator means that grassland zones have always surrounded the central monsoon region. The stable grasslands are what has allowed large herbivores to persist—rather than any special wariness of hunters learned from humans evolving there."
Adrian M Lister, from the Vertebrates and Anthropology division of London’s Natural History Museum, says the research adds to our understanding of megafauna extinctions. In an email interview with Newsweek, he says: “The idea that the grazing megafauna depended on a dry grassland habitat (the 'mammoth steppe'), that died away as the climate warmed and moisture increased, has been around for a long time (associated especially with Andrei Sher and Dale Guthrie, who termed it 'mammoth steppe'). Mainly it was based on the evidence of fossil pollen and insects, so it's very nice to have completely independent evidence from another source—stable isotopes—in this study.”
However, Gifford H Miller, Professor of Geological Sciences at the University of Colorado, disagrees. He says that while the study uses an “impressive data set,” there is no comparative study from previous ice ages to show widespread extinctions as a result of an abrupt shift in climate.
“Climate change associated with glacial/interglacial mode switches no doubt caused major ecosystem reorganization as the balance between temperature and precipitation underwent major shifts,” he tells Newsweek in an email interview. “Herbivores dependent on specific ecosystems—and no doubt partially responsible for the perpetuation of those ecosystems, as the authors point out—would have experienced stress. But there is no evidence that the most recent glacial/interglacial transition was in some fundamental way different than previous transitions during which megafauna remained intact.
“In the Americas the only real difference was that modern humans were also present, and in Eurasia, it was the first transition with Homo sapiens on the landscape. Without evidence indicating the effective moisture changes of the most recent glacial/interglacial transition were unprecedented, I think the case for effective moisture change being the major driver of megafaunal extinction is weak.”
This story has been updated to include Adrian Lister's and Gifford H. Miller's comments.