Changing Climate, Not Humans, Killed Australia’s Massive Mammals

If you think Australia is full of weird creatures now, you should have seen it at the end of the last Ice Age. There were wombats the size of Volkswagons, koala cousins that resembled the mythical Drop Bear and enormous, venomous lizards larger than today’s Komodo dragons. But why did these fantastic beasts disappear? After a decade of debating this question, a new study is helping to revive a hypothesis that had previously been pushed aside.

What happened in Australia is just one part of a global story in the decline of the world’s massive mammals. From that island continent through Asia, Europe, Africa and the Americas, the close of the Ice Age 12,000 years ago saw the worldwide downfall of many large, charismatic creatures from the giant ground sloth to the beloved woolly mammoth. In every case, both humans and a warming climate have been implicated as major suspects, fueling a debate over how the extinction played out and what—or who—was responsible.

As far as Australia goes, humans have been promoted as prime culprits. Not only would early-arriving aboriginals have hunted megafauna, the argument goes, but they would have changed the landscape by using fire to clear large swaths of grassland. Some experts point to Australia’s megafauna crash after human arrival, around 50,000 years ago, as a sure sign of such a human-induced blitzkrieg.

For example, a region called the Sahul—which included Australia, Tasmania and New Guinea during the Ice Age—lost 88 species of animal that weighed over 220 pounds. These included oversized kangaroos that strutted rather than hopped, real-life ninja turtles with tail clubs and flightless birds twice the size of today’s emus. 

The problem is, there’s no hard evidence that humans were primarily to blame for the disaster that befell these giants. Judith Field, an archaeologist at the University of New South Wales who focuses on megafauna and indigenous communities in Australia and New Guinea, says the hunting hypothesis has hung on because of its appealing simplicity. “It’s a good sound bite” and “a seductive argument to blame humans for the extinctions” given how simple of a morality fable it is, she says. But when it comes to hard evidence, Field says, the role of humans has not been substantiated.

So what really happened? The picture is far from complete, but a paper by Vanderbilt University paleontologist Larisa DeSantis, Field and colleagues published today in the journal Paleobiology argues that the creeping onset of a warmer, drier climate could have dramatically changed Australia’s wildlife before humans even set foot on the continent. And while this event was natural, it is a frightening portent of what may happen to our modern wildlife if we do nothing to stop the scourge of today's human-caused climate change. 

 The researchers focused on a spot in southeastern Australia known as Cuddie Springs, which turned out to be an ideal place to interrogate the fate of the continent’s megafauna. Initial scientific forays focused on searching for fossil pollen to reconstruct ancient environments, Field says. But in the process, researchers also found fossils and archaeological artifacts that indicated megafauna and humans lived alongside each other there for 10,000 years or more.

“The combination of the fossil bone, the pollen record and the archaeology make this a really unique opportunity to investigate the relationship between the three,” Field says.

Even better, DeSantis says, Cuddie Springs boasts older beds of fossils deposited long before human arrival. This provided an opportunity to document changes over a longer span of time, “and assess dietary responses to long-term shifts in climate,” she says. To that end, the paleontologists focused on fossils laid out in two horizons—one 570,000-350,000 years old and the other between 40,000 and 30,000 years old. Drawing on chemical clues about diet and microscopic damage to marsupial teeth found in those layers, the researchers were able to document who was around and what they were eating at each layer.

If you were able to take a time machine between the two time periods, you’d be forgiven for thinking that you had moved through space as well as time. “Cuddie Springs, around 400,000 years ago, was wetter,” DeSantis says, and there was enough greenery for the various herbivores to become somewhat specialized in their diets. Kangaroos, wombats and giant herbivores called diprotodontids browsed on a variety of shrubby plants, including saltbush. By 40,000 years ago, a warmer, drying climate had transformed the landscape and the diets of the mammals on it.

By late in the Ice Age, the plant-eating marsupials were all eating more or less the same thing, and the sorts of plants that were better at holding water for these mammals were much rarer. Saltbush, for example, became less palatable because, DeSantis says, “if you haven’t been able to find water for days, the last thing you are going to eat is salty food that requires you to drink more water.” The desert became drier, resources became scarce, and competition for the same food ramped up.

Altogether, DeSantis says, this suggests “climate change stressed megafauna and contributed to their eventual extinction.”

Knowing how climate change impacted Australia’s mammals thousands of years ago isn’t just ancient history. NASA recently reported that we’ve just gone through the hottest year on record in an ongoing string of exceptionally warm years. The only difference is that now, our species is driving climate change. “Australia is projected to experience more extreme droughts and intense precipitation events,” DeSantis says, including a projected temperature increase of around 1-3 degrees Celsius by 2050, thanks to Homo sapiens and our forest-razing, fossil-fuel-burning, factory-farm-dependent lifestyles.

Looking to the past may help us get ready for what’s coming. “Data from Cuddie Springs suggest that there is likely a tipping point beyond which many animals will go extinct,” DeSantis says. We’re on track to play out such a catastrophe again—and today’s changing climate can’t be halted or reversed, the least our species can do is prepare for it. “I always learned in school that the importance of studying history is to make sure that history doesn’t repeat itself,” DeSantis says.

Looking at the ghosts of climate change past gives us a preview of what’s coming—and what we might lose if we do not act.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.