Climate change robbed mega fauna of protein rich plants to favor grasses

The climate killed many of the large mammals after the latest Ice Age. But what more specifically was it with the climate that led to this mass extinction? The answer to this is hidden in a large number of sediment samples from around the Arctic and in the gut content from permafrozen woolly rhinos, mammoth and other extinct ice age mammals.

It is a bit of a shift in paradigm, Eske Willerslev and co-workers publish in this week’s edition of the journal Nature. The common image of a light-brown grass-steppe dominating the Northern Hemisphere during the Ice Age does not hold any longer. The landscape was far more diverse and stable than today, and big animals like woolly rhino and mammoth fed on grasses and particularly on protein-rich flowering plants.

But at the Last Glacial Maximum 25,000 to 15,000 years ago, at a time when the climate was at its coldest and driest, a major loss of plant diversity took place. The animals barely survived.

After the Ice Age ended about 10,000 years ago it became warmer again. After the large reduction of plant diversity during the Last Glacial Maximum another kind of vegetation now appeared. One of the key food sources of the large mammals, the protein-rich plants, did not fully recover to their former abundance. This likely proved fatal for species like woolly rhino, mammoth, and horse in Asia and North America. Even though it became warmer again after the end of the Ice Age the old landscapes did not return.

Willerslev is director of Centre of Excellence in GeoGenetics and the National CryoBank and Sequencing Facility, situated at the National History Museum and the Biological Institute, University of Copenhagen.

“We knew from our previous work that climate was driving fluctuations of the megafauna populations, but not how,” he said. “Now we know that the loss of protein-rich forbs was likely a key player in the loss of the ice age megafauna.” Forbs are the flowering plants like sunflowers, clover and milkweed.

Christian Brochmann, a botanist at the Natural History Museum at the University of Oslo in Norway, notes that the permafrost contains a vast, frozen DNA archive left as footprints from past ecosystems.

“. . . we can decipher this archive by exploring the collections of plants and animals stored in natural history museums,” he said. “Using DNA from  museum collections as reference, we could identify the different plant species that co-occurred with extinct ice age mammals.”

“For the first time, ecologists have been able to piece together the characteristics of more complete plant communities occurring in the Arctic during the last 50,000 years,” said Mari Moora, who with Martin Zobel is a vegetation ecologist from the University of Tartu, Estonia. “The new information shows clearly that the vegetation of the Late Pleistocene was rich in forbs but lost considerable diversity at the peak of the ice age.”

The research involved 30 teams from 12 countries involving many disciplines.

The article in Nature elaborates on the Willerslev group’s results from 2011 where the researchers pointed at climate as the culprit for the mass extinction of some of the large mammals.

But in 2011 the researchers lacked a smoking gun. Now they have it. Some 242 permafrost sediment samples and eight fossil samples from large mammals from around the Arctic have been dated and analyzed for DNA, according to the academic article. The data shows that the likely main reason for the mass extinction of the large mammals after the latest Ice Age is changes in the vegetation, it said.

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