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December 18th, 2000

Changes in Australian ecosystems tied to arrival of exotic animals

A study of old and new emu eggshells collected from central Australia indicates a dramatic loss of grasslands beginning roughly 300 years ago was due to the arrival of Europeans and the introduction of exotic grazing animals, according to a new study.

The eggshells of the large, flightless bird are unique in that they can be dated by traditional radiocarbon methods and by a process known as amino acid racemization, said University of Colorado at Boulder Professor Gifford Miller. Racemization involves studying mirror-like changes in amino acids in the eggshells over time, which has proven to be an accurate time clock for dating events going back thousands of years.

"Because these eggshells can be dated by both radiocarbon methods and amino acids, they offer a unique opportunity to look back at ecosystem changes over the last 70,000 years," said Miller, a fellow of INSTAAR.

The study indicates that grazing mammals introduced in the last few centuries by Europeans have contributed to "a major change in ecosystems" in central Australia, said Miller. "We see a major reduction of plant biomass beginning in the 1800s, which looks like a result of overgrazing by pastoral animals and the introduction of rabbits."

The results of the study were presented at the fall 2000 meeting of the American Geophysical Union held in San Francisco Dec. 15 to Dec. 20. Other authors on the paper include Beverly Johnson, a former CU graduate student of Miller's now at the University of Washington, John Magee and Michael Gagan of Australian National University and Marilyn Fogel of the Carnegie Institution in Washington, D. C.

The eggshells were collected by Miller and Johnson, who conducted lengthy north-south and east-west transects in south-central Australia over the past two years. Miller believes the introduction of rabbits about 150 years ago was a major contributor to the demise of grasslands in the region.

The emu eggshells were biochemically analyzed for two different carbon isotopes that reflect the birdsÍ diet, said Miller, also a professor in CU-BoulderÍs geological sciences department. High proportions of the "heavy carbon," or carbon 13, in the eggshells indicate the emus were eating primarily grasses, while lower proportions of carbon 13 indicate their primary diet was bushes and trees.

To evaluate changes in vegetation in south-central Australia since European colonization, the team derived proportions of grasses compared to bushes and trees in the diet of emus that laid their eggs between 300 and 14,000 years ago. The team then compared eggshells collected in the past 25 years with those going back to the last glacial maximum 15,000 years to 30,000 years ago and even as far back as 65,000 years ago, he said.

"Not even at the last glacial maximum has the proportion of emu diet of grasses been so small," he said. "We suggest unprecedented changes in ecosystems occurred in south-central Australia within the past 300 years, resulting in a dramatic reduction in the proportion of grasses in the emu diet."

Eggs laid by emus during the last glacial maximum reflect a lack of grasses in their diet, he said. A 1997 study by Miller and colleagues indicated systematic burning by the earliest human colonizers altered the vegetation patterns enough to diminish the effectiveness of summer monsoons that periodically drenched northern Australia, triggering increased aridity over much of the continentÍs interior and decreasing grasses.

In January 1999, Miller and colleagues published a paper in Science magazine indicating the earliest humans who peopled Australia some 55,000 years ago inadvertently disrupted the continentÍs food chain by burning vast areas of native vegetation, resulting in the extinction of most large animal species. The study showed an ostrich-sized bird known as Genyornis--as well as 85 percent of the Australian "megafauna" weighing more than 100 pounds -- disappeared about 50,000 years ago.

"Australia has a fragile ecosystem where the soils are significantly depleted in nutrients," said Miller. "I would speculate that even if all the exotic animals were removed, it would take the ecosystems a long time to recover because the soils lack resiliency."