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Human Invasion of Australia Led to Extinction of Mammals: Study

Posted on: Wednesday, 20 July 2005, 15:00 CDT

Human invasion of Australia led to extinction of mammals: study

LOS ANGELES, July 7 (Xinhua)-- A shifting diet of two flightless birds inhabiting Australia tens of thousands of years ago indicates that early humans may have led to the disruption of ancient ecosystem and the extinction of large terrestrial mammals, scientists reported.

Humans altered the continent's interior with fire, changing it from a mosaic of trees, shrubs and grasses to the desert scrub. The mammals disappeared shortly after humans colonized the continent about 50,000 years ago, said Gifford Miller, a professor from the University of Colorado at Boulder.

Using isotopic studies of fossil eggshells from both indigenous emus and the extinct, ostrich-sized Genyornis, a new study by Miller and colleagues published in the July 8 issue of Science shows that the ecosystem's flora changed swiftly and dramatically after humans arrived.

The analyses, which scientists used to pinpoint particular plant groups ingested by the birds, indicated that emus living before 50,000 years ago preferred nutritious grasses characteristic of milder temperatures and warm summer rains, Miller said.

After 45,000 years ago, the eggshell evidence showed emus successfully switched to a diet of mostly shrubs and trees characteristic of drier conditions.

But Genyornis, which also preferred the nutritious grass prior to 50,000 years ago, failed to make the dietary switch and became extinct shortly after humans arrived, according to the researchers.

"The opportunistic feeders adapted and the picky eaters went extinct," said Miller. "The most parsimonious explanation is these birds were responding to an unprecedented change in the vegetation over the continent during that time period."

The researchers analyzed nearly 1,500 fragments of fossilized emu and Genyornis eggshells dating back 140,000 years from three different regions in Australia's interior, including Lake Eyre, Port Augusta and the Darling-Murray Lakes. Each region has a distinct local climate and geography.

They also looked at carbon isotopes in fossil wombat teeth collected from the Port Augusta and Darling-Murray sites, where such teeth often are found in association with fossil-bird eggshells.

While the analyses showed the diet of the vegetarian wombat consisted of a much larger proportion of the grasses favored by emus and Genyornis prior to 50,000 years ago, wombats, like emus, successfully switched to other vegetarian food sources after 45, 000 years ago.

"Neither over-hunting nor human-induced diseases, the two most widely cited alternative agents for a human-caused extinction event in Australia, would result in the dramatic changes at the base of the food web documented by our datasets," wrote the authors in Science.

"The reduction of plant diversity, however it came about, would have led to the extinction of specialized herbivores and indirectly to the extinction of their non-human predators."

The earliest human colonizers in Australia are believed to have arrived by sea from Indonesia about 50,000 years ago, using fire as a tool to hunt, clear paths, signal each other and promote the growth of certain plants, Miller said.

More than 85 percent of Australia's large mammals, birds and reptiles weighing more than 50 kg went extinct shortly after humans arrived, including 19 species of marsupials, a huge lizard and a tortoise.

"This study shows that the environmental footprints of humans can have very large and unexpected consequences, which I think is relevant to what is happening with human activity on Earth today," Miller said.

"A cumulative series of small changes can have unintended large- scale consequences, in this case a complete restructuring of the ecosystems," he added.

Source: Xinhua News Agency - CEIS

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