July 7, 2005

Researchers Theorize on Animal Extinction

WASHINGTON -- The extinction of most of Australia's large animals occurred around 45,000 years ago, shortly after the arrival of humans. A study suggests that human burning of the landscape forced dietary changes that killed off many of the animals.

Researchers studying ancient eggshells from two types of large birds, as well as the teeth of wombats, found a change in the types of carbon the animals had ingested, indicating a change in diet.

Before the extinction, grasses, trees and shrubs were commonly eaten - but then grasses disappeared from the animals' diet, the researchers reported in Friday's issue of the journal Science.

"Humans are the major suspect," Marilyn Fogel of the Carnegie Institution said in a statement.

People set large-scale fires for a variety of reasons, including hunting, clearing and signaling other groups, she said.

Gifford Miller of the University of Colorado commented that "the opportunistic feeders adapted and the picky eaters went extinct."

The scientists studied changes in the eggshells of the now-extinct bird Genyornis and the surviving emu. They then corroborated their findings by analyzing the carbon in ancient teeth from wombats, which are also vegetarians.

The types of carbon in the emu eggs and wombat teeth changed, indicating a change in diet to more trees and shrubs. The Genyornis eggs didn't change and the bird died out.

Researchers discounted the possibility of overhunting or disease introduced by humans as likely contributors to the dieoff because those would not have involved changes in the animals' basic diet. And they noted that there was no major climate change under way at the time.

Christopher N. Johnson of James Cook University in Australia called the report "strong evidence that human arrival caused changes in vegetation and animal dieoff."

But he suggested that overhunting could have played a role.

In Africa, Johnson noted, the appetites of large animals suppress the growth of scrublands. A sudden elimination of such animals in Australia might have allowed the growth of different plants, forcing a dietary change in the surviving animals.

Johnson noted that the disappearance of mammoths, ground sloths and other large mammals followed the arrival of humans in the Americas, but it is difficult to determine the role of people in that case because climate was also changing rapidly at the time.

The research was funded by the U.S. National Science Foundation and the Australian Research Council.


On the Net:

Science: http://www.sciencemag.org/