Australasian Science: Australia's authority on science since 1938

Final Megafauna Extinctions Not Climate-Related

By Stephen Luntz

A study of Australia’s climate and vegetation over 135,000 years has cast doubt on the possibility that the last megafauna extinctions could have been climate-related, while confirming a 20-year-old prediction about the after-effects of the final wave of extinctions.

Prof Patrick De Deckker of the Australian National University took samples of sediment cores off the coast of Kangaroo Island. Two were examined for sea-surface temperatures indicated by foraminifera shells deposited, while one was also the subject of a study of the plant vegetation washed down through the Murray River and its tributaries.

De Deckker found a 3°C change in temperature around the last extinctions, far smaller than at other periods and insufficient to explain the disappearance of the megafauna that previously dominated the continent.

“The finger points to humans, but we don’t have the evidence to say it was necessarily us,” says De Decker. “But we are saying it’s definitely not climate change.”

The vegetation deposited on the sea floor near the mouth of the Murray records a 3000-year period of huge fires, which De Deckker says occurred after the megafauna disappeared and would have been a consequence of their demise. “Prof Tim Flannery suggested that the abrupt extinction of the herbivorous megafauna meant shrubs grew unchecked, increasing the amount of flammable material. This explanation has caused considerable and ongoing controversy, but is now supported by our evidence,” De Deckker says.

In research published in Nature Geoscience, De Deckker reported signs of a change in the plant types growing in the Murray-Darling Basin during this period. Alkanes collected from sediments record the proportion of plants using the C3 and C4 photosynthesis pathways. The C4 pathway is now dominant in northern Australia, while the majority of Victorian plants use C3, with a gradual shift in between.

Prior to the megafauna extinction 70% of the plants in the Murray-Darling Basin used the C4 pathway, and had done so for 10,000 years. However, this dropped to as low as 30% after the great browsers were removed, suggesting that the fires caused a change in the dominant vegetation.

Recently Prof Stephen Wroe of the University of NSW argued that most megafauna species disappeared from the fossil record well before the earliest possible date for human arrival (AS, July/August 2013, pp.12–13). De Deckker acknowledges that his own work may only relate to a small number of megafauna species, but says these were the ones whose giant consumption of plant material had the most influence on the ecology of south-eastern Australia.