SFGATE: The analysis by Barnosky, research associate Marc Carrasco and Penn State’s Russell Graham was published this week in the scientific journal PLoS ONE. It compares the extinctions of mammals in North America after humans arrived 13,000 years ago to the five mass extinctions on Earth over the past 450 million years. The least severe of those extinctions wiped out the dinosaurs 68 million years ago and killed off 75 percent of the species on the planet.
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.” 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 epochs. 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.” MORE