Why the big animals died: new study shakes up extinction science

The following article featuring research by Professor Corey Bradshaw and Professor Alan Cooper appeared in The Lead on 24 July, 2015:

By Sarah Keenihan / 24th of July, 2015


IF YOU are going to challenge the status quo, best make sure you’ve got solid numbers to back you up.

A study published today in the journal Science does just that, applying a tailored analytical approach to offer a new explanation as to why iconic mammals such as mammoth and bison became extinct.

Lead by Professor Alan Cooper, a researcher at the University of Adelaide in South Australia, the paper presents evidence that sudden warm climactic periods were the primary driver of large animal extinctions in Eurasia and North America around 10-50,000 years ago.

The findings counter the common idea that Ice Age low temperatures were to blame.

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Professor Corey Bradshaw getting down and dirty. Image courtesy of Haley Renee.

“The idea that cold killed off these large animals is completely wrong,” said co-author Professor Corey Bradshaw, who developed the new analytical approaches that give the study its scientific rigor.

“Yes, cold periods did force animals to retract into smaller and more isolated populations – but most species survived these times,” he explained.

“It was the warming events known as interstadials that were critical.”

Interstadials are rapid and abrupt periods of relative warmth – increases from 4°C to 16°C – that separate much colder icy periods. They occurred towards the end of the last Ice Age due to a combination of factors, including features of Earth’s rotation pathway, ice-melt feedbacks, changing ocean currents and carbon dioxide fluctuations.

But our ancestors are not completely blameless – humans also played a part.

“What we think happened is that some of these warming events disrupted meta-populations of animals – that is, groups of animals that were able to breed with each other – and then humans took advantage of the situation with their hunting practices,” said Professor Bradshaw.

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Mammoth vertebrae in ice, Yukon Territory, Canada. Photo courtesy of Kieren Mitchell.

The result was waves of climate-linked extinction across many thousands of years, including of mammoth, bison, woolly rhinoceros and giant ground sloth.

“This is analogous to what’s happening now, with massively degraded natural habitats, animals living in stressful situations and global temperatures increasing.”

“It’s like a one-two punch that can lead to extinction,” Professor Bradshaw said.

“Right now, every major predator in the world is under threat, including sharks.”

“Current climate trends terrify paleontologists because we’ve seen it all before.”

The idea that studying past climactic and extinction events can inform modern science is well supported.

“It’s the only indication we’ve got for what’s going to happen in the future!” said Dr Paul Willis, Director at the Royal Institute of Australia and a paleontologist by training.

“Current climate trends terrify paleontologists because we’ve seen it all before,” he said. “Knowing about past extinctions, we wonder whether we’re heading towards something similar in the future.”

“This research neatly cuts through what has been a very polarised debate; were the megafaunal extinctions caused by humans or climate?”

“Now we can say that it appears to be a bit of both.”

This new study is highly unique, combining several lines of evidence.

As well as radiocarbon dating, DNA recovered from fossils was used to create accurate extinction timelines, and to track animal population dynamics and geographic distributions.

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Professor Alan Cooper abseiling in the 100 foot pitch in Natural Trap Cave in Wyoming to excavate megafaunal bones. Photo courtesy of Laura Weyrich.

To add an extra layer of robustness, Professor Bradshaw applied a statistical measure known as the ‘Signor-Lipps effect’ to account for bias associated with the fossil dating methods.

The extinction patterns were analysed on a background of two sources of climate information – the Greenland ice core record, and marine sediments off the coast of Venezuela.

Professor Bradshaw is confident the results reflect a real phenomenon.

“The data have been reviewed many times, and re-analysed over and over again,” he explained. “All sorts of assumptions relating to each piece of data have been checked, and the result still persists. So it’s watertight.”

The paper is published in Science, the journal of the American Association for the Advancement of Science.

“Publication in Science is a guarantee that the material has been reviewed properly,” said New Zealand ecologist Dr Stephane Boyer, who was not involved in the study.

“Such journals look very closely at what they publish, and are subjected to a high level of scrutiny.”

With these results now in the public domain, Professor Bradshaw does not hold back when it comes to extrapolating their importance.

“This study shows that the species of huge animals that were widely distributed across the northern continents of the world were wiped out in a geological heartbeat,” he said.

“It’s powerful. This study is foreshadowing what’s happening today.”

The paper ‘Abrupt warming events drove Late Pleistocene Holarctic megafaunal turnover’ was published by Professors Cooper and Bradshaw with lead co-author Chris Turney, Professor of Earth Sciences and Climate Change at University of New South Wales, along with other Australian and American colleagues.

Professor Bradshaw offers additional reflections on this paper at his blog ConservationBytes.

Key contacts

Professor Alan Cooper 
Director, Australian Centre for Ancient DNA
61 08 8313 5950
Professor Corey Bradshaw 
Sir Hubert Wilkins Chair of Climate Change 
Director, The Environment Institute’s Climate and Ecology Centre
08 8313 5842
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