Tuesday, December 26, 2006

 

Humans Responsible for Australian Extinctions

The mystery of what killed Australia's giant animals - the so-called 'megafauna' - during the Last Ice Age is one of the longest-running and most emotive debates in palaeontology. Scientists have now published clear evidence from south-eastern Australia to show that climate change was not the driving force behind the extinctions, which took place between 50 and 40 thousand years ago.

This refocuses attention on humans as the main cause. The latest study, published in the January 2007 issue of the respected international journal Geology, is unique in providing - for the first time - a long-term perspective on the responses of the megafauna in the Naracoorte Caves (info) region of south-eastern Australia to cyclical swings in Ice Age climates.

"Climate change was certainly not the main culprit in the extinctions. Our data show that the megafauna was resilient to climatic fluctuations over the past half-million years", said team leader and palaeontologist Dr Gavin Prideaux from the Western Australian Museum and Flinders University.

Australia lost 90% of its large fauna, including rhino-sized marsupials, 3-metre tall kangaroos and giant goannas within 20 thousand years of human arrival. Opinions are divided between the relative importance of climatic changes and the activities of humans themselves via habitat disturbance or over-hunting. Unfortunately, the debate has been hamstrung by a lack of basic data on how communities responded to climate changes before humans arrived.

The new fossil evidence from Naracoorte reveals surprising stability in the mammal composition through successive wet and dry phases. "Although populations fluctuated locally in concert with cyclical climatic changes, with larger species favoured in wetter times, most if not all of them survived even the driest times - then humans arrived", said Dr Prideaux.

The Naracoorte Caves World Heritage Area in south-eastern South Australia contains the richest assemblage of Pleistocene (1.8 million to 10 thousand years ago) animals anywhere in Australia. What makes the record more remarkable is that it can be directly compared to a 500 thousand-year record of local rainfall preserved in the stalagmites in these caves.

The fossils were dated by two independent methods (optically stimulated luminescence and uranium-series dating) at the Universities of Wollongong and Melbourne, by geochronologists Professor Richard 'Bert' Roberts, Dr Kira Westaway and Dr John Hellstrom. The multi-disciplinary team also included Dr Dirk Megirian from the Museum of Central Australia in Alice Springs, who studied the sediments for additional clues of the prevailing climate conditions.

"These analyses have allowed us to pinpoint the ages of the fossils and the major shifts in climate. Our evidence shows that the Naracoorte giants perished under climatic conditions similar to those under which they previously thrived, which strongly implicates humans in their extinction" said Professor Roberts.

The research project was supported by the South Australian Department for Environment and Heritage, GreenCorp, the Friends of the Naracoorte Caves, the Cave Exploration Group of South Australia, the Commonwealth Natural Heritage Trust Extension Bushcare Program, and the Australian Research Council.

Original PR available via Media Releases (Dec. 22 entry) [Paleontology]

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Based on The Geological Society of America's* Geology paper:

"Mammalian responses to Pleistocene climate change in southeastern Australia"

by Gavin J. Prideaux, , Richard G. Roberts, Dirk Megirian, Kira E. Westaway, John C. Hellstrom, Jon M. Olley

Abstract
NYA at: http://dx.doi.org/10.1130/G23070A.1

Resolving faunal responses to Pleistocene climate change is vital for differentiating human impacts from other drivers of ecological change. While 90% of Australia's large mammals were extinct by ca. 45 ka, their responses to glacial-interglacial cycling have remained unknown, due to a lack of rigorous biostratigraphic studies and the rarity of terrestrial climatic records that can be related directly to faunal records. We present an analysis of faunal data from the Naracoorte Caves in southeastern Australia, which are unique not only because of the species richness and time-depth of the assemblages that they contain, but also because this faunal record is directly comparable with a 500 k.y. speleothem-based record of local effective moisture. Our data reveal that, despite significant population fluctuations driven by glacial-interglacial cycling, the species composition of the mammal fauna was essentially stable for 500 k.y. before the late Pleistocene extinctions. Larger species declined during a drier interval between 270 and 220 ka, likely reflecting range contractions away from Naracoorte, but they then recovered locally, persisting well into the late Pleistocene. Because the speleothem record and prior faunal response imply that local conditions should have been favorable for megafauna until at least 30 ka, climate change is unlikely to have been the principal cause of the extinctions.

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*About the GSA:

"...Established in 1888, The Geological Society of America provides access to elements that are essential to the professional growth of earth scientists at all levels of expertise and from all sectors: academic, government, business, and industry.

The Geological Society's growing membership unites thousands of earth scientists from every corner of the globe in a common purpose to study the mysteries of our planet and share scientific findings..."

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