Dung fungus reveal that humans, not climate change, killed Australia’s giant beasts

By Ed Yong | March 22, 2012 2:00 pm

In Australia, an ancient murder mystery is coming to a riveting conclusion, thanks to an unusual clue – not a fingerprint, or a bloody weapon, but fungal spores preserved in fossilised dung. The spores belong to Sporormiella, a fungus that only grows in mammal and bird droppings. Large plant-eaters provide it with vast banquets. In turn, the fungus reveals how many big vegetarians were living in the neighbourhood.

Scientists have used these spores to study the deaths of the giant animals that once grazed North America – “megafauna” such as mammoths, woolly rhinos, and more. Now, it’s Australia’s turn. By using Sporormiella records, along with other buried clues, Susan Rule from the Australian National University has found strong evidence that acquits a changing climate in the death of Australia’s giants. Instead, her study points the finger squarely at humans.

Australia was once home to giant versions of its current animals, as well as strange forms with no modern counterpart. These included Megalibgwilia, an echidna the size of a sheep; Procoptodon, a huge kangaroo more than twice the size of today’s reds; Thylacoleo, a lion-like hunter with a bone-crushing bite; Diprotodon, a hippo-sized wombat; and a giant monitor lizard, even bigger than the Komodo dragon.

By 40,000 years ago, all of these animals were extinct. This was shortly after humans first arrived on the continent, and many scientists think these facts aren’t coincidental. Humans either killed the beasts themselves, or made the land inhospitable by regularly burning it – a practice that still happens today.

But there’s an alternative theory: Australia’s climate had been drying out for a long time, forcing the large animals to the edge of extinction. Humans merely nudged them off. Or perhaps, humans and a changing climate were equally culpable.  None of these competing explanations has gained a clear upper hand. It’s not clear precisely when the megafauna died out, and places that hoard their remains are few and far between.

To settle the debate, Rule and her team travelled to Lynch’s Crater, a huge lake in north-east Australia. The team drilled into the crater and extracted two cylinders of rock, or “cores”. Each one represents fossils, soils and chemicals that have been laid down over the course of 130,000 years. They are time capsules, where millennia of Australia’s history have been crushed into solid rock.

The Sporormiella spores within the cores suggested that giant animals were common in Australia until around 41,000 years ago. At that point, the spores almost completely vanished. This gives us a very precise estimate for when these animals finally died out.

Around 100 years after this fungal vanishing act, the land also experienced more fire, as evidenced by the growing amount of charcoal in the cores. Two centuries after that, the local plants also changed.  Pollen grains, preserved in the cores, showed that rainforest plants, like flowers and conifers, started to decline. They were replaced by grasses and sclerophylls – the trademark plants of the Australian bush, such as eucalyptus.

This accurate timeline shows that Australia’s climate and plant life were stable for at least five millennia before the great beasts died out, and only started changing later. Rule’s study also shows that the megafauna were relatively unaffected by earlier climatic upheavals. Australia experienced two great periods of climate change around 120,000 and 75,000 years ago, when it became cooler and drier. Neither was accompanied by a fall in Sporormiella. The climate shifted, but the megafauna took it in their stride. Their numbers (and thus the fungi in their dung) only declined after humans arrived.

Stephen Wroe, who has championed the climate hypothesis before, says that the new study is a “very useful contribution”, but Rule has assumed that Sporormiella reflects the presence of Australia’s megafauna. “Even in North America, Sporormiella is associated with many species, not just big plant eaters,” he says.

Jacquelyn Gill , who used the dung fungus to study the loss of North America’s megafauna, has no such objections. She says, “Understanding the causes of megafaunal extinctions in North America have been complicated because humans arrived while climate was changing rapidly. This study neatly avoids that problem.”

Gifford Miller, who studies ancient climates, notes that there’s some uncertainty about the precise dates in Rule’s timelines, but that their timings relative to each other are “very strong”. He adds, “[The study] suggests that the arrival of humans indeed is the leading explanation for the demise of Australia’s megafauna.”

As the megafauna died, the plants they ate began to thrive. That provided more fuel for severe fires that were lit either by lightning strikes or by the newly arrived humans. The fire left the charcoal that Rule would eventually find. It also changed the vegetation even further, killing off the sensitive rainforest plants and allowing the tolerant sclerophylls to prosper in their ashes. A similar chain of events probably happened in North America, according to Gill’s research.

Understanding what killed the megafauna isn’t just an academic exercise in historical finger-pointing. What happened then can tell us about what is happening right now. Australia’s plants also changed during the two earlier periods of climate change, but they reacted just as strongly (if not more so) to the loss of the megafauna. The demise of these iconic animals had just as much an impact on their environment than big shifts in climate across an entire continent.

Today, other giants, such as elephants and rhinos, are also at risk of extinction. “Large animals are some of the most threatened species on the planet today, and unlike past extinctions, the causes of the modern ones aren’t controversial,” says Gill. “The study is important because it suggests that those extinctions may have a legacy of landscape change.”

Reference: Rule, Brook, Haberle, Turney, Kershaw & Johnson. 2012. The Aftermath of Megafaunal Extinction: Ecosystem Transformation in Pleistocene Australia. Science http://dx.doi.org/10.1126/science.1214261

Image: Diprotodon by Peter Murray

More on the megafauna:

Comments (8)

  1. Anthony David

    This is another excellent piece in the puzzle that is the Austrlaina megafauna extinction debate.
    Minor quibbles. The cores of the lake sediment would have been mud, not rock. In fact, the supporting material refers to peat in the cores used for radiocarbon age dating. The doi article link does not work, though it does match that used in the original Science article. I assume it will be created soon.

  2. I thought the debate was already quite settled – of course we’ll never know for “certain”, but the evidence gathered so far is pretty convincing if you ask me. Maybe I’m being mean-minded, but I’d say that some people continue to oppose the idea that humans were a main reason behind the extinction of megafaunal species all over the world because they still cling to the romantic idea that our ancestors lived in harmony with nature, as if they were Sindarin Elves / Na’vi / Children of the Forest (though presumably less elvish, less blue and taller respectively)

  3. Following Walter’s comment, I’ll admit to a bias of my own, not in interpreting the science (which I’m happy to accept whichever way it falls), but in what I think makes the most satisfying story: what I would like to be true. It’s certainly true that Aboriginal cultures among others place a lot of emphasis on the need to conserve nature (with strict limits on hunting and so forth), but this imperative must have come from somewhere, and I like to imagine that once upon a 40,000-year-old campfire a conversation went something like: “So where have all the diprotodon gone?” “Oh, yeah. Whoops. Maybe we should have been more careful.”

    The reason I find this narrative satisfying is because I firmly believe that the single most noble thing you can say about any culture is that it has learned from its mistakes. For that reason, I can’t imagine a viewpoint that respects Aboriginal culture more. I mean, look at us: we’re having trouble with that.

  4. meh

    “Dung fungus” – great band name!

  5. John Dodds

    Can we get some added information on the dating? We are blithly jumping from 100,000 years ago (lots of megafauna) to 40,000 years ago (some or last of megafauna & some(?) people).
    My understanding is that from 110,0000 years ago to 12000 years ago we were in an Earthly ice age (little or no ice in Australia except in the mountains!) when it was several degrees colder. The only warm periods are the last 10,000 years and for about 5000 years about 130,000 years ago. except for periodic very short term spikes in temperature which do not even show up in the Antarctic ice cores due to the longer periods between samplings.
    Just how convincing is the sampling of the dung? Was it done every 300-500 years like the Antarctic ice cores OR was it done every 10 years like the GISP northern Hemisphere ice cores?
    ALSO was it the humans or the Fires or both(?) that wiped out the big animals or was it a reduction in their available food source caused by the reduced temperature/ice age/fires? I find it hard to believe that a tiny little(?) human population could wipe out all those big animals on the entire continent/island (similar to USA~3K x 1.5K miles) without some help from a reduction in food plants caused by the ongoing ice age from 115,000 years ago to 12,000 years ago?
    Next question Were there humans in Tasmania and when? Does the separation of Tasmania by the water/ocean from the rest of Australia contribute to differences in the timing of the dying out of the mega fauna (note that the mega fauna in Tasmania IS different from the megafauna on the rest of Australia., because I suspect that it probably took man (the aborigines) some time to expand from Africa, to Northern Australia/Queensland) and on to Tasmania 1500 miles to the south where it was already colder. Is there a lake core sample in Tasmania that can confirm the same timing “that man was the cause, and climate change was not? Aside from the bias in the reporting! (commented on in the BLOG on The Conversation website maintained by Australian scientists)

  6. Suz

    Denial is becoming the most unappealing feature in the human race. We want so much to be righteous, but, unfortunately, we are only human. Thinking that our ‘less than technologically advanced’ counterparts were any different than us with their self-centered perceptions is completely unrealistic. It is a shame that for the vast majority of us, complete destruction preludes enlightenment. An even greater shame is that with so much history written down, we still have to learn things the hard way.

  7. amphiox

    It’s certainly true that Aboriginal cultures among others place a lot of emphasis on the need to conserve nature (with strict limits on hunting and so forth), but this imperative must have come from somewhere, and I like to imagine that once upon a 40,000-year-old campfire a conversation went something like: “So where have all the diprotodon gone?” “Oh, yeah. Whoops. Maybe we should have been more careful.”

    I should point out that even an aboriginal culture which already had a strong emphasis on conserving nature would not be immune to causing such an ecological catastrophe. Proper ecological management requires in depth knowledge of the specifics of the ecology, and every ecology is unique. For a culture that has newly arrived on a hithertoo unknown and virgin land, whose ecology is in some ways very different and vulnerable in different ways from what they had previously known, there is great danger for causing much damage simply from lack of knowledge. All the traditional knowledge they bring with them would have applied to the old ecology they came from, but may not apply to the new one. There is even danger that activities that actually help in the old ecology would prove disastrous in the new. And as it takes many generations to acquire such knowledge, it may have come too late to reverse the inadvertent damage done in the early phase of arrival.

    When human migrations contribute to mass extinctions, it is often attributed to naive animals not having prior experience with human pressure and failing to adapt fast enough. But one must not forget that the arriving humans are also naive themselves to the new ecology they find themselves in, and thus may not successfully adapt fast enough to living sustainably in the new ecology, even if that had been their intent.

  8. Goob

    Agree with Meh- Dung Fungus would be a great name for a band!!

    Don’t agree so much with the conclusions of the new research though. Numerous studies have shown how numerous climatic (e.g., humid v arid), hydrological (shallow water v deep water) and geomorphological (size/shape of lake or swamp) factors influence fungal abundance, concentration and preservation. These issues have not been considered at all in the new work.

    Also, they argue for a mass extinction at 41 ka. If that’s correct, then how come the fungus increases to near all-time highs again at 30 ka??? The authors explain it as some extant kangaroos coming into the area…. but why can’t they be megaherbivores??? Or vice versa- why can’t the earlier spikes just be regular kangaroos??? How can you tell the difference between fungus on a steaming pile of giant wombat turd, versus that from a growing over the dung of a kangaroo, or a koala, or a rat for that matter (note that the dung fungus is also found lots of small mammals, as well as birds and reptiles!).

    Finally, that there is no climate change at or after 40 thousand years ago is a furphy. There is a massive dataset that shows that there were huge droughts at that time; the summer monsoon retracted significantly; and El Nino became super intense.

    I’m not supporter of either the ‘humans did it’ or ‘climate change’ extinction ideas, I’m just saying that these scientists need to be honest about the existing datasets and what the data actually say.

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