What killed the megafauna?

Human activities and population growth have wrought much destruction to life on Earth. But when it comes to megafauna extinctions, evidence suggests we may be off the hook – rather, the major culprit could be climate change.

In Australia, Cosmos recently reported a fossil discovery that revealed giant birds, reptiles and marsupials died out 40,000 years ago due to extreme climatic conditions and environmental degradation.

Across the Pacific, a new study published in the journal Nature Communications has linked North American megafauna extinctions during the Late Quaternary to extreme temperature changes – not with overhunting by humans, as suggested by some.

More than 10,000 years ago, many giant critters roamed the continent, including mammoths (Mammuthus) and enormous beavers (Castoroides), horses, ground sloths (Megalonyx) and a one-tonne armadillo look-alike, the Glyptodon.

What drove their extinction is a “contentious topic”, according to Mathew Stewart from Germany’s Max Planck Institute and team, led by senior author Huw Groucutt.

Some blame human population growth and the arrival of highly skilled “big-game” hunters around 14,000 years ago, for which the giant animals were no match. Others argue that archaeological evidence doesn’t support this notion, pointing to climatic and ecological disruptions.

Indeed, around that time there were two major temperature shifts – abrupt warming that started around 14,700 years ago and a return of near-glacial conditions around 12,900 years ago, during a period known as the Younger Dryas.

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A simplified radiocarbon-dated event (REC) model showing that decreases in megafauna numbers coincided with the return to near-glacial conditions at the start of the Younger-Dryas. Credit: Hans Sell & Christopher W Carlton

To shed more light on the debate, Stewart and colleagues applied a new statistical approach derived from their work on past extreme events, which they say addresses limitations of previous methods to estimate human population growth through time.

With this new method – Radiocarbon-dated Event Count (REC) modelling – they tested the overhunting and climate change hypotheses of the megafauna extinctions.

“On the one hand, we reasoned that if expanding human populations drove megafauna extinctions, that the human and megafauna population proxies would be negatively correlated,” explains Stewart.

“On the other hand, if climate change was responsible, we expected a correlation – either negative or positive – between our temperature and megafauna population proxies.”

Analyses included all megafauna together and five taxa with decent numbers separately, focussing on the whole continent and the American Great Lakes and Southwest regions individually.

Results showed megafauna populations had no associations with human numbers but were consistently correlated with temperature. As North America warmed up their numbers increased, and their subsequent declines and extinctions coincided with the cold snap.

However, the team says the story is likely to be much more complex and needs to be considered along with ecological changes associated with climatic variation – and humans could also have contributed indirectly through other means such as habitat fragmentation. They call for researchers to develop more reliable methods to clarify what really happened.

What’s the relevance for us now?

“We are undoubtedly in the midst of an anthropogenically driven biodiversity crisis – sometimes referred to as the sixth mass extinction,” says Stewart. “Increasingly, scholars are looking to the past to make sense of and inform the current crisis. 

“And the causes of the extinction of the Earth’s largest animals during the Late Quaternary have featured heavily in these discussions. Therefore it is important that we have a good understanding of the processes involved in these extinctions.”

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