OPINION: What killed Australia’s megafauna?

By Stephen Wroe 23 May 2013
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Climate change, not human activity, drove Australia’s megafauna to extinction, says Dr Stephen Wroe.

THROUGHOUT THE ICE AGE that characterised our planet for much of the last two million years, mainland Australia, Tasmania and New Guinea formed a single landmass — Sahul.

It was a strange and hostile place with a vast arid core that expanded to encompass 70% or more of the continent. And it was dominated by giants.

This “megafauna” included the largest marsupial that ever lived, Diprotodon, the size of a large rhinoceros; huge, short-faced kangaroos that exceeded 200kg in body mass; and massively-built terrestrial birds, around the height of an emu – but twice as heavy.

They were preyed upon by a venomous goanna that may have been as big as a large saltwater crocodile, and a deadly marsupial “lion” with incredibly powerful jaws and bolt-cutting teeth. All up, around 90 of these large to gigantic species and subspecies existed.

Now they are gone; only a few big kangaroos still survive.

Where did the megafauna go?

Explaining these extinctions has locked scientists in heated debate since the 19th century. While arguments have changed, the identity of the proposed “culprits” have not. Was it climate, or was it humans?

Underpinning all arguments for a human-driven extinction were two key assumptions. The first is that the megafauna were present when the first Aborigines arrived; the second is that all previous glacial maxima (times of peak cold) – the last was between 28 and 19 thousand years ago – were much of a muchness. If there was nothing extreme about the last two or three glacial maxima, the only feasible cause for the extinction of megafauna was the arrival and activity of people.

It is now clear that the first of these assumptions is poorly founded. Humans arrived around 50-45 thousand years ago, but it is increasingly evident that many of the megafauna had disappeared before humans arrived. At most 14 and as few as eight species of now extinct megafauna clearly overlapped in time with humans.

It is also now clear that the second assumption was incorrect. In fact many palaeoclimatologists have long been of the opinion that Sahul was subject to a protracted, stepwise deterioration in climate over the last 300-400 thousand years. The long term trend is an increasingly arid and erratic climate.

Across geological time the vast majority of species that have ever lived have gone extinct, and the vast majority of these in the complete absence of humans. Climate or climate-related influences are undoubtedly to blame in almost every instance.

Were humans responsible for wiping out megafauna?

In recent years the evidence for the protracted, stepwise aridification of Sahul has firmed, backed by new and mounting data from Antarctic ice cores and analyses of ancient central Australian lake levels. The 800-thousand-year Antarctic ice core record in particular has provided unprecedented resolution on the Southern Hemisphere story – and it has revealed a distinct change from 450 thousand years ago if not earlier.

From this time on things started to become more extreme. Moreover, the ice core record shows marked drying, beginning at around 50-45 thousand years ago – the time humans arrived. This is consistent with evidence for the decline of once vast inland mega-lakes. Other recent studies have suggested that climatic deterioration may have taken place to varying degrees across the planet – beginning as early as 700 thousand years ago.

Still further cracks have emerged in arguments for a human role. Spikes in fire activity deduced from charcoal analyses had been assumed by some to indicate increased burning by humans, laying a basis for the argument that human-driven environmental change drove the megafaunas’ demise. But more recent work shows that increased burning characterised Sahul long before people arrived.

The loss of a giant flightless bird from south-central Australia at around 50 thousand years ago had been attributed by some to human activity, but it is now clear that its disappearance clearly coincided with escalating climatic variability.

Many questions remain. Humanity’s role in the demise of now-extinct species that were still present when people arrived cannot be entirely discounted, but this remains to be demonstrated. However, it is increasingly clear that the disappearance of megafauna from Sahul took place over tens if not hundreds of millennia under the influence of an inexorable, albeit erratic, climatic ratchet, and that the first Aborigines made footfall at a time when conditions were already rapidly deteriorating.

Dr Stephen Wroe is a palaeontologist and associate professor in Biological, Earth and Environmental Sciences at the University of New South Wales, in Sydney.

This is an edited version of an article first published on The Conversation.