"I'm optimistic that, because we haven't lost those species yet, if we redouble our conservation efforts we can stem the tide of extinctions and have those species around in the future," he added.
The study's 30 million-year timeline allowed the researchers to compare species diversity over a period of dramatic change in the landscape. The Rocky Mountains and Sierra Nevada ranges formed in the West, while there were dramatic swings in climate that may have been larger than and as fast as the Earth is seeing today, said co-author and UC Berkeley research associate Marc A. Carrasco. Yet these changes did not have a great effect on mammal diversity, compared to what happened when the last glacial period ended, the ice retreated in North America, and humans crossed from Asia into America.
"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 analysis by Barnosky, Carrasco and Russell W. Graham, professor of geosciences at Penn State in University Park, Pa., appears online this week in the open-access journal PLoS One.
Their analysis combined two databases compiled over the past 15 years ago by Graham and one database created by a UC Berkeley team led by Barnosky and Carrasco in the past few years. Graham's databases are FAUNMAP I, which lists all mammal fossils and their geographic ranges in the United States between 40,000 and 500 years ago, and FAUNMAP II, a compilation of mammalian fossils dating from 40,000 to 5 million years ago. The UC Berkeley database is MIOMAP, which includes all fossil occurrences in the U.S. between 5 and 30 million years ago, which covers the Miocene and part of the Oligocene periods. The databases include all terrestrial mammals f
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University of California - Berkeley