If the planet is headed for another mass extinction like the previous five, each of which wiped out more than 75 percent of all species on the planet, then North American mammals are one-fifth to one-half the way there, according to a University of California, Berkeley, and Pennsylvania State University analysis.
Many scientists warn that the perfect storm of global warming and environmental degradation both the result of human activity is leading to a sixth mass extinction equal to the "Big Five" that have occurred over the past 450 million years, the last of which killed off the dinosaurs 68 million years ago.
Yet estimates of how dire the current loss of species is have been hampered by the inability to compare species diversity today with the past.
By combining data from three catalogs of mammal diversity in the United States between 30 million years ago and 500 years ago, UC Berkeley and Penn State researchers show that the bulk of mammal extinctions occurred within a few thousand years after the arrival of humans, with losses dropping after that. Although modern humans emerged from Africa into Europe and Asia by about 40,000 years ago, they didn't reach North American until about 13,000 years ago, and most mammal extinctions occurred in the subsequent 1-2,000 years.
"The optimistic part of the study is that we haven't come all that far on extinction in the past 10,000 years," said co-author Anthony Barnosky, UC Berkeley professor of integrative biology. "We have this pulse when humans had their first effect about 13,000 years ago, but diversity has remained pretty steady for about 10,000 years."
He expects to see a similar pattern in Europe after the invasion of Homo sapiens some 40,000 years ago.
In the last 100 or so years, however, "we are seeing a lot of geographic range reductions that are of a greater magnitude than we would expect, and we are seeing loss of subspecies and even a few species.
|Contact: Robert Sanders|
University of California - Berkeley