The Sixth Extinction: An Unnatural History

following calculation. Tropical deforestation is notoriously difficult to
measure, but let’s assume that the forests are being felled at a rate of one
percent annually. Using the species-area relationship, S = cAz, and setting
the value of z at .25, we can calculate that losing one percent of the
original area implies the loss of roughly a quarter of a percent of the
original species. If we assume, very conservatively, that there are two
million species in the tropical rainforests, this means that something like
five thousand species are being lost each year. This comes to roughly
fourteen species a day, or one every hundred minutes.
This exact calculation was performed by E. O. Wilson in the late
nineteen-eighties, not long after one of his trips to the BDFFP. Wilson
published the results in Scientific American, and on the basis of them he
concluded that the contemporary extinction rate was “on the order of
10,000 times greater than the naturally occurring background rate.” This,
he further observed, was “reducing biological diversity to its lowest level”
since the end-Cretaceous extinction, an event, he noted, that while not
the worst mass extinction in history, was “by far the most famous,
because it ended the age of the dinosaurs, conferred hegemony on the
mammals and ultimately, for better or worse, made possible the origin of
our own species.”
Like Erwin’s, Wilson’s calculations were shocking. They were also easy
to grasp, or at least to repeat, and they received a great deal of attention,
not just in the relatively small world of tropical biologists but also in the
mainstream media. “Hardly a day passes without one being told that
tropical deforestation is extinguishing roughly one species every hour, or
maybe even one every minute,” a pair of British ecologists lamented.
Twenty-five years later, it’s now generally agreed that Wilson’s figures—
here again like Erwin’s—don’t match observation, a fact that should be
chastening to science writers perhaps even more than to scientists. What
the reasons are for this continue to be debated.
One possibility is that extinction takes time. Wilson’s calculations
assume that once an area is deforested, species drop out more or less