Extinctions are about 1,000 times more frequent now than in the 60 million years before people came along. The explanation from lead author Jurriaan de Vos, a Brown University postdoctoral researcher, senior author Stuart Pimm, a Duke University professor, and their team appears online in the journal Conservation Biology
In absolute, albeit rough, terms the paper calculates a "normal background rate" of extinction of 0.1 extinctions per million species per year. That revises the figure of 1 extinction per million species per year that Pimm estimated in prior work in the 1990s. By contrast, the current extinction rate is more on the order of 100 extinctions per million species per year
"The diversification rate is the speciation rate minus the extinction rate," said co-author Lucas Joppa, a scientist at Microsoft Research in Redmond, Wash. "The total number of species on earth has not been declining in recent geological history. It is either constant or increasing. Therefore, the average rate at which groups grew in their numbers of species must have been similar to or higher than the rate at which other groups lost species through extinction"
For a third approach, de Vos noted that the exponential climb of species diversity should take a steeper upward turn in the current era because the newest species haven't gone extinct yet
"It's rather like your bank account on the day you get paid," he said. "It gets a burst of funds — akin to new species — that will quickly become extinct as you pay your bills"
By comparing that rise of the number of species from the as-yet unchecked speciation rate with the historical trend (it was "log-linear") evident in the phylogenies, he could therefore create a predictive model of what the counteracting historical extinction rate must have been