Researchers determined species die off about 1,000 times more frequently than they did 60 million years before people came along.
The new estimate suggests the pre-human extinction rate was 10 times lower than researchers previously believed, meaning today's rate is 10 times worse, Brown University reported.
"This reinforces the urgency to conserve what is left and to try to reduce our impacts," said de Vos, who began the work while at the University of Zurich. "It was very, very different before humans entered the scene."
The paper calculates a "normal background rate" of extinctions of 0.1 extinctions per million species per year. This study revises the one extinction per million species per year calculation made in 1990 to a rate of 100 extinctions per million species per year. The calculation uses orders of magnitude rather than precise numbers because that is the highest achievable accuracy.
The new calculation goes beyond the typical information obtained from fossils and looked directly at the evolutionary family trees (phylogenies) of numerous plant and animal species. They looked at how groups of species have changed and diversified over time.
"The diversification rate is the speciation rate minus the extinction rate," said co-author Lucas Joppa, a scientist at Microsoft Research. "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."
A third approach suggested the climb of species diversity should steeply incline in the current era because newer species have not yet gone extinct.
"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 the rise of the number of species with historical trends the researchers were able to create a predictive model of the historical extinction rate. The team tested their models with simulated data of the known extinction rate, and these models turned out accurate results.
We've known for 20 years that current rates of species extinctions are exceptionally high," said author Stuart Pimm, a Duke University professor. "This new study comes up with a better estimate of the normal background rate -- how fast species would go extinct were it not for human actions. It's lower than we thought, meaning that the current extinction crisis is much worse by comparison.
The findings were published recently online in the journal Conservation Biology.
