If the course of human history is any model, then the wheels are already turning on Earth's sixth mass extinction, thanks to habitat destruction, pollution and now global warming, a scientific analysis of millions of years of data revealed Friday.
The study of the fossil and archaeological record over the past 30 million years by UC Berkeley and Penn State University researchers shows that between 15 and 42 percent of the mammals in North America disappeared after humans arrived.
That means North American mammals are well on the way - perhaps as much as half way - to a level of extinction comparable to other epic die-offs, like the one that wiped out the dinosaurs.
As you might expect, it's all our fault:
Anthony Barnosky, a UC Berkeley professor of integrative biology and co-author of the study, said the most dramatic human-caused impacts on the ecosystem have occurred in the last century.
"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," Barnosky said. "So it looks like we are going into another one of these extinction events."...
Although humans clearly did not have anything to do with the previous extinctions, many scientists are afraid that global warming and other environmental problems caused by the ever-increasing human population could have similarly catastrophic consequences.
"Here we are again, astronomically increasing the number of humans on the face of the globe, plus unusual climate change," Barnosky said. "That seems to be a recipe for extinction that we saw in the past, and we are seeing again."
Humans reached North America about 13,000 years ago and more than 50 species disappeared over the next 2,000 years, including mammoths, saber-toothed cats, giant ground sloths and other large animals, according to the study.
The arrival of humans coincided with the end of the last ice age, but the study pointed out that 38 other ice ages had occurred in North America over the past 2 million years and there were no comparable die-offs during the others.
"The only difference is that 13,000 years ago, humans appear on the scene," 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 work, which was supported by the National Science Foundation, looked at the number, distribution and range of every mammal from shrews to mammoths in the area of the continental United States between 500 years ago and 30 million years ago.
Previous research has shown that most mammal extinctions in North America, Australia, Europe and Northern Asia have occurred within a few thousand years after the arrival of humans. This study puts that data into historical perspective, providing the percentage of animals that went extinct during certain time periods compared with other