|
http://news.excite.com/news/r/010608/10/odd-extinction-dc Human Hunters Spelled Doom for Ice Age Behemoths
Updated: Fri, Jun 08 10:31 AM EDT By Will Dunham
WASHINGTON (Reuters) - The arrival of human hunters triggered a mass extinction of large animals in North America, dooming such Ice Age behemoths as the woolly mammoth and mastodon, as well as one in Australia tens of thousands of years earlier, scientists said on Thursday.
Two studies appearing in the journal Science discount climate change as the cause of the calamities on the two continents, instead pinning the blame squarely on prehistoric people who were fanning out around the globe.
John Alroy, evolutionary biologist at University of California at Santa Barbara, ran a complicated computer model of North American ecology late in the Pleistocene epoch simulating the population dynamics of 41 large plant-eating mammals and humans, who were arriving on the scene for the first time.
Regardless of how he adjusted the variables, Alroy found that human hunting inevitably caused mass extinctions -- particularly devastating the populations of the largest animals such as mammoths and mastodons, whose slow growth rates and long gestation periods made it hard for them to rebound.
The first large populations of people arrived in North America about 13,400 years ago. Alroy said the extinction appeared to unfold within about 1,200 years after that.
University of Melbourne geochronologist Richard Roberts used advanced techniques to determine when scores of exotic marsupials and huge flightless birds disappeared in Australia. A continent-wide mass extinction took place around 46,400 years ago -- shortly after the ancestors of today's aborigines first landed on the shores of Australia, Roberts said. Among the marsupials (mammals that shelter their babies in pouches) lost were: kangaroos that towered nine-feet tall, had sharp claws and may have eaten meat; Diprotodon, which looked like a wombat but was bigger than a cow; and Thylacoleo, a fearsome predator known as the marsupial lion. Also becoming extinct were the heaviest bird ever known, Genyornis, which weighed 220 pounds, stood nine-feet tall and could not fly, and Megalania, a flesh-eating lizard measuring 26 feet in length.
A SPIRITED DEBATE
There has been a spirited debate among scientists about what caused these losses. Some argue that climate change was to blame. Others promote the "blitzkrieg" or "overkill" theory proposed by University of Arizona scientist Paul Martin in 1967 that humans armed with primitive weapons decimated populations of animals that never previously had encountered people.
In a blow to the climate-change theory, the new research demonstrated that in different parts of the world and at times separated by tens of thousands of years, the lone common thread in the extinctions was the arrival of humans on the scene.
Alroy said his computer simulation showed that it was "overwhelmingly likely" that "overkill" was the cause. The model correctly predicted the survival or extinction of 32 of 41 species studied. About 30 of the species became extinct.
"It shows, to my mind, conclusively that it was possible for a relatively small population of humans to cause a major mass extinction over a relatively short period of time with very modest amounts of hunting," Alroy said in an interview.
Scientists believe humans crossed an Ice Age land bridge from Asia into North America, arriving when the continent was half covered by an ice sheet a mile (1.6 km) thick. Suddenly, three-quarters of America's large mammals were gone.
Among the casualties were: two kinds of mammoths and the mastodon, relatives of the modern elephant; woolly bison; enormous ground sloths; tapirs; half a dozen species of horses; a camel; llamas; giant antelopes; oxen; the giant armadillo; and glyptodonts, an enormous mammals with tank-like armor.
Alroy said these losses also spelled the end for key predators such as the saber-toothed cats and the Dire wolf.
HUMAN ARRIVAL WAS "LIKE A PLAGUE"
Humans "swept through like a plague as far as large animals were concerned," Martin said in an interview.
"Clearly when humans come for the first time, you tend to get these catastrophic losses," added Ross MacPhee, a biologist with the American Museum of Natural History in New York. But MacPhee faulted Alroy for failing to examine whether diseases that may have accompanied humans' arrival played a role.
In Australia, Roberts used techniques that had advantages over traditional carbon dating from 28 sites linked with the remains of large animals to determine that the mass extinction took place 46,400 years ago, plus or minus 3,000 years. Humans arrived there between 60,000 and 50,000 years ago.
Roberts said environmental changes caused by people -- such as burning the landscape to facilitate hunting -- may have played as big a role as actual culling of animals. Click HERE to return to the Alerts page.
Images on this page created by the author Copyright. In accordance with Title 17 U.S.C. Section 107, any copyrighted work posted within this site is distributed under fair use without profit or payment for non-profit research, editorial and educational purposes only. [Ref. http://www4.law.cornell.edu/uscode/17/107.html ] |Email|PAWPRINTS|Home|Wildlife Backgrounds|THE PREDATOR|
|
