WASHINGTON — It seems to be a law of nature that when people come, animals go. It happened in the past, and it's happening again now.
About 11,000 years ago, more than 130 species, including most large mammals such as the woolly mammoth, saber-tooth cat and a 5-ton ground sloth, suddenly vanished from North America.
Scientists are still debating the reasons, but two leading suspects are excessive hunting by humans who'd newly arrived from the Old World and devastating human-borne diseases.
"People come and animals begin to disappear,'' said Ross MacPhee, the curator of the mammal collection at the American Museum of Natural History in New York.
A third commonly cited cause of the massive extinction is climate change at the end of the Ice Age and its effect on plant and animal habitats.
The combination of climate change and human impact was especially destructive, according to Anthony Barnosky, a paleontologist at the University of California, Santa Barbara.
"Today we stand at a similar crossroads,'' Barnosky reported in an analysis of the extinctions during what scientists call the Late Pleistocene age, about 12,000 to 10,000 years ago. "A similar but scaled-up natural experiment is under way today — the exponential growth of human populations at exactly the same time the Earth is warming at unprecedented rates,'' he wrote in the Proceedings of the National Academy of Sciences.
There are disturbing analogies between what happened in the Pleistocene and today's world:
Looking to the future, Dennis Hansen, a biologist at Stanford University in Stanford, Calif., predicted that human activities are "set to cause further extinctions among large vertebrates.''
In an article last month in the journal Science, Hansen pointed to the Indian Ocean island of Mauritius, where the vast majority of animals have gone extinct, leaving the forests "largely populated by ghosts today."
"Hunters who seek out large species for their meat and charismatic species for their hides, ornaments and supposed medicinal value pose a particularly acute problem,'' according to Joseph Wright, a biologist at the Smithsonian Tropical Research Institute in Panama. "Many persecuted species have been extirpated or persist only at greatly reduced abundances,'' he wrote in the journal Biotropica.
Scientists offer conflicting explanations for the rapid burst of extinctions in North America at the end of the most recent Ice Age. No consensus has developed.
For decades, the dominant theory, proposed by Paul Martin, a geoscientist at the University of Arizona in Tucson who's now retired, was a rampage of hunting by humans who'd just arrived across a land bridge from Asia. Within a few hundred years, Martin theorized, a "blitzkrieg'' of killing wiped out more than half of all the mammals that weighed more than 100 pounds.
As a result, "Americans live in a land of ghosts,'' Martin wrote in his 2005 book, "Twilight of the Mammoths."
MacPhee, however, thinks that it wasn't over-hunting but the diseases people brought with them that ravaged animal populations in North America. As a parallel example, he noted how microbes introduced by European conquerors destroyed Central and South American civilizations 500 years ago.
Other scientists, such as Russell Graham, a museum director at Pennsylvania State University in University Park, think that climate change was "the major driving force that caused the extinctions.'' It destroyed animals' native habitats and reduced their geographic range so much that they could no longer survive.
"We're seeing the same thing now,'' Graham added. "The combination of human activities and climate change is extremely lethal.''
Some scientists refer to the current loss of species as the ''sixth extinction.'' Five previous mass extinctions occurred long before humans arrived on the planet, most recently when a huge asteroid or comet helped wipe out the dinosaurs 65 million years ago.
"I don't think there's any question that extinctions have occurred in the past, that they're occurring now and that they will occur in the future,'' Graham said.
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