This modern-day kangaroo apparently grazed its way
to the 21st century, while its distant relative marsupials browsed themselves
to extinction.
Gene
Feldman, NASA.
A similar debate over extinctions has raged in Australia, where every mammal, reptile or bird weighing more than 100 kilograms went extinct in the past 100,000 years. New data indicate that the extinctions occurred just after humans walked across a land bridge from Southeast Asia about 50,000 years ago.
For
years, climate was blamed for the losses, says Gifford Miller, who studies
earth history at the University of Colorado's Institute of Arctic and
Alpine Research. New and better dating seem to finger human activity --
direct or indirect.
Miller studied (see "Pleistocene Extinction... " in the bibliography) a flightless bird, Genyornis newtoni. Tests on more than 700 eggshells dated the youngest to about 50,000 years old, and a separate study showed a similar pattern among seven extinct marsupials.
Until
recently, the extinctions were thought to have occurred during maximum
glaciation, about 30,000 years ago, which made climate a prime suspect.
But the new dates indicate that the deaths occurred when climate was "similar
to the present day, was not changing particularly fast," Miller says.
"We conclude that this can't be the case, and have eliminated the most
plausible climate mechanism."
Burned up
If the extinctions occurred after people arrived, does
that place hunters in the crosshairs of suspicion? Perhaps not, since
no Australian kill sites have been found from that period. A more likely
explanation is ecosystem change, Miller says. "Virtually all the extinct
marsupials are browsers. Grazers make it through, but browsers and the
large carnivores dependent on them die." Why would animals be hunted based
on a preference for leaves over grasses?
Emus
survived the die-off in Australia that wiped out species that browsed
leaves for a living. USDA.
He points to an illustrative contrast between the leaf-eating but extinct Genyornis and the emu, a flightless bird that still survives on a diet grass, shrubs, fruits and insects.
So what happened to the trees and shrubs? Quite likely they were burned. Aborigines today burn the rangeland to flush out game and recycle nutrients, Miller says, and consecutive fires destroy the ecosystems that would have fed Genyornis.
So is being big helpful or harmful in evolutionary terms?
Bibliography | Credits | Feedback | Search