[font face=Serif]FOR IMMEDIATE RELEASE June 5, 2012
Contact: Megan Sever msever@earthmagazine.org

[font size=5]EARTH: Ecosystem Collapse in Pleistocene Australia[/font]

[font size=3]Alexandria, VA – In the Late Quaternary, Australia was home to an array of megafauna. The half-ton Palorchestes azael, the rhinoceros-sized Diprotodon, and even the giant koala, Phascolarctos stirtoni, roamed Australia’s interior. However, between 50,000 and 45,000 years ago, they all vanished. Although recent studies indicate human colonization as a potential cause of their extinction, the exact mechanism has never been resolved. Now, geologist Gifford Miller from the University of Colorado at Boulder and his colleagues believe they have uncovered the answer.

By analyzing the biominerals found in fossil bird eggshells and marsupial teeth, Miller and his colleagues have pieced together the ancient diets of three Australian megafauna to discover what happened to these creatures. What early mechanisms led to the Australian megafauna’s demise? Read the story online at http://www.earthmagazine.org/article/ecosystem-collapse-pleistocene-australia .

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[font face=Serif][font size=5]Ecosystem collapse in Pleistocene Australia[/font]

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This 140,000-year record of dietary carbon, collected from three broadly separated regions, provides clear evidence for an abrupt ecological shift — and a permanent reduction in available food sources — about the time of human colonization, Miller says. Prior to 50,000 years ago, the emu eggshell carbon isotope values varied widely, exhibiting a pattern consistent with an opportunistic feeder living in an environment where moisture varied considerably, with some years wet enough for nutritious grasslands to grow abundantly and other, drier years dominated by shrubs and trees. From 50,000 to 45,000 years ago, the mean carbon isotope ratios in both emu shells and wombat teeth decreased, and it has remained low ever since. This isotopic shift documents an increased reliance by emus and wombats on shrubby plants and trees, he says. Being opportunistic eaters, emus adapted their diet to a large-scale change in vegetation that occurred around that time.

In contrast, dietary carbon data from Genyornis eggshells from 140,000 to 50,000 years ago indicate a more specialized feeding strategy for this heavier bird. The carbon isotopes indicate that Genyornis always included some grass sources, unlike the emu, which prefer grass but can tolerate a diet based entirely on shrubs and trees. Miller and his colleagues concluded in two studies, published in Science and Climate of the Past, that Genyornis consumed a more restricted diet that was no longer available after 50,000 years ago.

The team’s findings of a dramatic upheaval at the base of the food chain are consistent with the hypothesis that systematic burning of the landscape by humans permanently converted the previous ecosystem from nutritious tree and shrub savanna (with frequent years of rich grasslands) to the modern desert scrub environment. Animals that could adapt survived; those that could not went extinct.

After collecting this evidence that ecosystem change played an important role in the megafauna’s extinction, Miller and his colleagues decided to see if human burning permanently altered the ecosystem via feedbacks that reduced the delivery of monsoon moisture to the continent’s interior. They used a general atmospheric circulation model to evaluate the sensitivity of monsoon precipitation to the continent’s vegetation and soil characteristics, especially rainfall recycling through evapotranspiration.

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