Australasian Science: Australia's authority on science since 1938

Australia's Megafauna Extinctions: Cause and Effect

By various experts

Australian research has found new evidence that human hunters were primarily responsible for the disappearance of Australia’s giant vertebrates about 40,000 years ago, and concluded that the extinctions caused changes to the Australian landscape.

“We are excited by this research not just because it helps us to understand why Australia's megafauna went extinct. More importantly, it moves the question on by focusing on the ecological impacts of that extinction. Big animals have big impacts on plants. It follows that removing big animals should produce significant changes in vegetation.

"Twenty years ago, Tim Flannery argued that one of the results of megafaunal extinction was increased fire in Australian landscapes, as fine plant matter that would have been eaten by giant kangaroos, Diprotodons etc. accumulated and provided fuel for wildfires.

"Our results suggest that he may have been right, at least for some places. They also suggest that as well as releasing fire, taking out big herbivores had direct effects on the structure and composition of vegetation, making it more dense and uniform. Getting a better understanding of how environments across Australia changed as a result of megafaunal extinction is a big and interesting challenge, and will help us to understand the dynamics of contemporary Australian ecosystems.”

Chris Johnson is lead author of the research paper, and a Professor of Wildlife Conservation & ARC Australian Professorial Fellow in the University of Tasmania’s School of Zoology.

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“This is an important contribution to our understanding of what happened 40,000 years ago or so when 90% of our large terrestrial species disappeared, depriving us of the sight of giant short-faced kangaroos, marsupial ‘lions’, giant horned tortoises and herds of Diprotodons meandering through the outback.

“The authors present a fascinating dataset from a sediment-filled crater in north-eastern Australia and inject new data into a field of enquiry that aches under the strain of opinion pieces and the tired reworking of published data. From the significant drop-off in the abundance of a herbivore dung fungus, they make the reasonable inference that this corresponds to a substantial drop in local herbivore biomass around 40,000 years ago. They argue that this reflects the local loss of the larger Pleistocene species hitherto inhabiting the area, and that the changes in vegetation and fire that followed were a direct result of the extinctions.

“The timing of the inferred extinction coincides with early human presence in the region, but not with significant climatic change. This supports a mounting number of studies that have argued that climate change was not primarily responsible for the Late Pleistocene extinctions in other parts of the continent.”

Dr Gavin Prideaux is a lecturer in vertebrate palaeontology in the School of Biological Sciences at Flinders University, South Australia

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“The data are superb and decisive: all you have to do is look at the diagrams showing the raw data and the conclusions become obvious. The key new data are the spore counts, and in combination with the charcoal and rainforest pollen data they tell the whole story. There is simply no reasonable way to argue with the authors' conclusions.

“The age model is excellent and the sampling is very intensive over a very long time interval. But the statistical analyses have all sorts of problems: in some cases the authors used unorthodox time series methods that are dubious, and in others they used relatively simplistic methods whose assumptions are violated by the data. But again, all you have to do is look at the diagrams. The statistics really don't matter.

“The debate really should be over now. Hunting did it: end of story. Personally, though, I never understood what there was to debate because nothing else made sense.

“Most Quaternary palaeoecologists immediately point to climate change whenever there is an interesting pattern to explain, but huge climate changes happened over and over again throughout the Pleistocene and there was no mass extinction. And the new data rule out any role for climate change at all.

“The only other viable hypotheses was the landscape burning idea. But that never really seemed plausible because all sorts of organisms also should have gone extinct if only fire was important. The fact that only very large mammals (and birds) went extinct only ever made any sense on the theory that hunting was the mechanism.

“In reality, the debate should have started and ended with Paul Martin's landmark analysis from 1967. But it has dragged on for nearly a half-century now because the idea that Stone Age hunters could cause such utter havoc across three entire continents over very short time spans strikes many people as incredible. Like it or not, though, it's the truth, and it's time for us to all confront it.”

Dr John Alroy is a Future Fellow in Macquarie University’s Department of Biological Sciences.

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The paper by Rule et al reports the result of an analysis of spores, palynological indicators of schlerophyllous (dry country) vs rainforest vegetation and lake levels and charcoal (fire history). Rule et al argue that on the basis of this study that megafauna became extinct at 41ka (by hunting) in NE Queensland, and that this precipitated major vegetation changes as a consequence.

The issues that I see with this paper are:

1. The (unproven) assumption that Sporormiella is a proxy for megaherbivores (read megafauna) in Australia.

2. The (unproven) assumption Sporormiella therefore tracks (mega-) faunal populations through time.

3. That climate was stable through this period (the opposite is generally accepted – see Antarctic Core records below).

4. Fire histories as an accurate record of human activities/presence has been soundly refuted by Mooney et al 2010 (and not cited in this paper).

5. Humans hunted megafauna to extinction (no evidence to support this assertion and refuted by current evidence, again not cited in this paper).

6. The (unproven) assumption that megafauna were present in high enough densities that would have catalysed such a vegetation shift on their sudden disappearance – when there is no evidence that they were there at all, except for this undemonstrated proxy. Extant herbivorous birds such as Cassowary may well have been there, or even the Emu.

The facts of the matter are:

1. Most megafauna were extinct nearly 100,000 years before human arrival and there is no evidence for any particular time period to be significant in terms of faunal extinctions (see Field et al. 2008, 2011; Field and Wroe 2012).

2. Only two sites on the Sahul landmass (Pleistocene Australian–New Guinea) record co-occurrence of humans, only one of these on the Australian continent – ie little evidence for a human megafauna co-existence.

3. There is no evidence from any of the Australian archaeological sites that humans were exploiting megafauna, let alone actively hunting megafauna. This is not to say they would not have taken megafauna on encounter, there is just no evidence.

4. The only evidence we have from Queensland for megafauna indicates that they were gone before humans arrived (ie >45ka).

5. Earliest evidence for human occupation in NE Queensland is c. 40ka and population sizes are likely to be small , as indicated by the small Pleistocene assemblages recovered from archaeological sites of this age. The oldest archeological site we know of close to Lynch’s Crater has dates to c. 30ka.

6. Current evidence can find no clear link between a human presence and biomass burning. Their conclusion is: “There is no distinct change in fire regime corresponding to the arrival of humans in Australia at 50 ± 10 ka and no correlation between archaeological evidence of increased human activity during the past 40 ka and the history of biomass burning. However, changes in biomass burning in the last 200 years may have been exacerbated or influenced by humans.” (Mooney et al QI, 2011)

7.Climatic instability through MIS3 is well documented and broadly accepted: e.g. Cohen et al . Geology (2011) conclude “the paleohydrological record indicates a progressive shift to more arid conditions on the Australian continent, with marked drying after 45 ka.”

The results of this paper are interesting, the interpretations drawn from it are unsubstantiated and can be explained by other mechanisms despite the assertions of the authors – ie onset of ENSO, D–O cycles, and the invariable instability of climate right through the MIS 3 (c.60–30ka.), biomass burning etc. To assert that these changes came after faunal extinctions is not supported by the data – the assumed proxies are not demonstrated.

Dr Judith Field is Senior Research Fellow in the School of Biological, Earth and Environmental Sciences at The University of NSW.

Source: AusSMC