By Sid Perkins, Science News
BRISTOL, ENGLAND—Studies that have mostly blamed the arrival of humans for die-offs among Australia’s large mammals 50,000 years ago missed the role played by a changing climate, new research suggests.
Most assessments of Australian extinctions have used evidence gathered at sites that typically include fossils from only one narrow interval of time, Gilbert Price, a vertebrate paleontologist at the University of Queensland in St. Lucia, Australia, reported September 23. But he and his colleagues have analyzed fossils of creatures both large and small from Darling Downs, a site in eastern Australia with a fossil record that extends from about 120,000 to about 55,000 years ago. In all, the team has tallied about 70 species that lived nearby at some point during that interval.
The data don’t support a previously proposed human-only cause for Australian megafaunal extinctions, Price noted. From strata deposited about 120,000 years ago, the researchers recovered the remains of 15 species of large mammals. About 90,000 years ago, only eight species of large mammals lived there. By 55,000 years ago, still several millennia before humans arrived in the area, only four large mammal species remained.
That long-term drop in diversity also appeared among small creatures, and the types of species that disappeared suggest climate change played a role, Price said. Sediments deposited from 120,000 to 90,000 years ago contain the fossils of rodents, frogs and land snails as well as large mammals, suggesting that the surrounding area was a patchwork of woodlands, vine-choked thickets and open grasslands. By 55,000 years ago, however, many of the wet-loving and forest-adapted species had largely disappeared, signaling a transition to drier, more open conditions.
The new findings don’t pin the blame for Australia’s final spate of mammal extinctions on either climate change or human presence, Price cautioned. The long-term trend in species diversity at Darling Downs does hint, however, that climate change caused some species to die out. And the changes may have reduced the populations of other species enough that human arrival easily tipped them over the edge to extinction.