Washington: Thanks to global warming and ecological degradation, the sixth mass extinction is already on the way, equal to the "big five" that occurred over the past 450 million years, the last of which killed off dinosaurs, warn scientists.
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 catalogues of mammal diversity in the US between 30 million years ago and 500 years ago, University of California-Berkeley (UC-B) 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 about 40,000 years ago, they didn't reach North America until about 13,000 years ago, and most mammal extinctions occurred in the subsequent 1,000-2,000 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-B 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, a UC-B release said.
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. So it looks like we are going into another one of these extinction events."
"The only difference is that 13,000 years ago humans appeared on the scene," UC-Berkeley research associate Marc A. Carrasco said. "The bottom line is, mammals in general were able to deal with these changes in the past. Only when humans arrive, do the numbers fall off a cliff."
The analysis appeared online this week in PLoS One.
First Published: Saturday, December 19, 2009, 17:23