Early Australians to Blame for Mass Extinctions, Study Finds
for National Geographic News
|July 7, 2005|
Humans first reached Australia around 60,000 years ago and changed the landscape forever, scientists have concluded.
Roughly 60 species of the continent's large mammals and some bird species became extinct around 45,000 to 50,000 years, as a result of a change in the ecosystem brought on by massive fires set by the early settlers.
The exact purpose of the fires is unclear; the settlers may have been clearing land, signaling other tribes, or hunting. What is clear is that the fires changed the landscape from a mosaic of forests and grasses to the fire-adapted shrubs and spinifix (a grass) found today.
Climate change is often thought to have caused extinctions in other parts of the world. The researchers were able to eliminate this possibility by studying the carbon isotopes of the eggshells of emus and the teeth of wombats going back 140,000 years. There were many large climate shifts during that period that did not induce a change in the ecosystem. In addition, evidence from dust in marine sediments off the coast of Australia suggests that the climate was relatively stable 45,000 to 50,000 years ago.
However, the evidence showed a clear shift in the diet of many animals 45,000 years ago.
"Humans are the major suspect," said Marilyn Fogel, a researcher at the Carnegie Institution of Washington. "We don't think that over-hunting or new diseases are to blame for the extinctions, because our research sees the ecological transition at the base of the food chain. The best fit for the sudden change in diet is human-induced change in the vegetation available."
Fogel is a co-author of the study is published in the July 8 issue of the journal Science.
Emu Eggs and Wombat Teeth
The eggshells of the Australian emu Dromaius novaehollandiae and the extinct giant flightless bird Genyornis newtoni were recovered from three widely separated geographic sites. By analyzing the carbon isotope ratios, the researchers were able to determine the animals' diets.
There are three isotopes, or varieties, of carbon found in nature. The C4 isotope is found mainly in tropical and arid-adapted grasses. C3 is more commonly found, including in desert scrub and grasses.
Before 50,000 years ago, the emu ate a wide range of foods. The isotopic evidence suggests they lived on the abundant nutritious grasses available during wet years, and on shrubs and trees in drier years.
After 45,000 years ago, however, there was a major disruption in the range of food sources, and the plants available were largely inedible and low in nutrition. The carbon isotopes in wombat teeth reflect a similar change in diet.
"We saw a quick shift in dominance from C4 drought-resistant trees, shrubs, and grasses to C3 desert plants," said Fogel.
The timing of the abrupt change in diet, and the extinction of up to 50 giant marsupial species, coincides with the arrival of humans.
Many of the extinct marsupial megafauna were large, herbivorous browsers, some weighing several tons. The emu, with its wide range of food sources, was able to adapt and survived the changes to the ecosystem. Animals like the Genyornis, which had a more specialized diet, did not.
In a commentary accompanying the report in Science, Christopher Johnson, a biologist at James Cook University in Australia, raises the possibility that over-hunting by humans may have caused the extinction of the large browsers, triggering a shift in the landscape from abundant grasses to desert scrub.
But the authors maintain that neither over-hunting nor human-introduced diseases could have caused such dramatic changes at the base of the food chain.
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