July 7, 2005

Brush fires caused Australian extinctions – study

WASHINGTON (Reuters) - A study of ancient eggshells and
teeth supports the controversial theory that early humans
caused the extinction of many of Australia's huge animals by
setting brush fires, researchers said on Thursday.

The study also showed why it sometimes does not pay to be a
picky eater -- the giant birds that were more choosy about
their diets perished, while the indiscriminate emu survived.

"We speculate that human firing of landscapes rapidly
converted a drought-adapted mosaic of trees, shrubs, and
nutritious grasslands to the modern fire-adapted desert scrub,"
the researchers wrote in Friday's issue of the journal Science.

"Animals that could adapt survived; those that could not,
became extinct."

Australia is known for its unusual fauna, from kangaroos to
the Tasmanian devil. But it once supported an even broader
range of large creatures, including an ostrich-sized bird
called Genyornis, a relative of the wombat as big as a
hippopotamus, and a snake 25 feet long and three feet (one
meter) in diameter.

They became extinct, along with every marsupial bigger than
220 pounds (100 kg), suspiciously close to the arrival of the
first human beings 50,000 to 60,000 years ago.

Gifford Miller of the University of Colorado and colleagues
have been trying to collect evidence to support their theory
that the first human arrivals set large fires that changed
Australia's interior and forced many creatures into extinction.

For years they have been studying the remains of the big
animals, looking in particular at the various types of carbon,
called isotopes, that built up in eggshells and teeth.

"What your mother told you is true: You are what you eat,"
said Marilyn Fogel of the Carnegie Institution in Washington,
who worked on the study.

"Different types of plants metabolize different forms of
carbon in distinctive ways from the carbon dioxide they take up
during photosynthesis," Fogel added in a statement.


They saw a sudden shift in carbon content of the shells and
teeth at about the same time humans would have arrived,
suggesting the animals switched to different plant foods.

"The animals that relied mostly on the more palatable plant
forms died out, such as Genyornis, while the animals that
adapted to the less nutritious plants survived, including the
emu," Fogel concluded.

Not everyone agrees that people forced the change in diet.

Large animals went extinct across the world between 50,000
and 10,000 years ago, just as humans spread. But other
researchers say climate change may have been responsible.

A team at Australia's Queensland University of Technology
found evidence that smaller species disappeared around the same
time that the big animals did, and also found evidence of
cooler, drier conditions that changed plant species make-up.

Fogel and colleagues say they found evidence of dramatic
climate changes, but no changes in the diet and environment
until the sharp transition around the time of the arrival of
humans. "Humans are the major suspect," said Fogel.

"Bands of people set large-scale fires for a variety of
reasons, including hunting, clearing, and signaling other

Christopher Johnson of James Cook University in Queensland
said the case has not yet fully been made for brush fires as
the source of the changes.

"But regardless of where the story goes from here, we now
have a stronger case than ever that the arrival of humans had a
larger impact than the last glacial cycle on ecological change
in Australia," Johnson wrote in a commentary.