The analysis, by a team led by Anthony Barnosky of the University of California at Berkeley, concluded that species extinctions over the past few thousand years are higher than the typical rate seen in the fossil record.
If currently threatened species become extinct within the next 100 years and the pace continues unchecked, within 240 to 540 years extinctions will rise to a level not seen since the last five major mass extinctions in Earth's history, Barnosky found.
The major drivers in the five mass extinctions were prolonged volcanic eruptions, changing climate, or even comet or asteroid collisions with Earth. The main driver today is widely seen as the impact of human activities on climate, landscape, and oceans.
He's and Hubbell's work tries to get at an important aspect influencing the pace of extinction.
At the heart of the discussion is the the "species-area relationship," a long-established relation between the amount of area scientists look at and the number of new species they discover as they expand the patch of land or ocean they explore.
As the search area expands, Hubbell explains, scientists add a new species when they spot even one specimen of a previously unknown, undescribed, or reclassified organism.
This has led to the development of a mathematical relationship between the amount of new territory explored and the number of new species one could expect to find.
But nothing similar existed for extinctions. So researchers figured that one approach might be, in essence, to throw this "discovery" calculation into reverse to see how many species one would lose as habitat shrank.
That's where the conceptual problem arises, He and Hubbell say. Extinction is a different beast from the discovery approach to ecology.