【The New York Times】Extinction and its Discontents

By ANDREW C. REVKIN

Fairly far along in my career as an environmental journalist, I had to start writing pieces that amounted to obituaries for species. First came Miss Waldron’s red colobus (which popped back on the radar, at least for now). Then the Yangtze River dolphin, or baiji (pictured below), then the Chinese paddlefish.

But most of the time, extinction is happening invisibly, a presumptive consequence of habitat loss and other factors. Now a statistical fight is brewing over the quality of calculations long made to estimate the rate at which species are departing.

The authors of a new paper in Nature, “Species–area relationships always overestimate extinction rates from habitat loss,” claim to have mathematical proof showing results of such calculations, using the “species to area curve,” are uniformly too high.

They stress that this does not negate the reality of the wave of species loss under way in an increasingly human-dominated planet:

Although we conclude that extinctions caused by habitat loss require greater loss of habitat than previously thought, our results must not lead to complacency about extinction due to habitat loss, which is a real and growing threat.

But their paper strongly challenges longstanding conventions used to estimate vanishings that still mainly occur invisibly. Gaps in understanding of the processes leading to extinction were already raising big questions more than a decade ago, as I reported in this news story in 2000: “Extinction Turns Out to Be a Slow, Slow Process.” I’ve appended a highly relevant excerpt at the bottom of this post.

Stuart L. Pimm, a Duke University ecologist long involved in estimates of extinction rates, has strongly disputed the conclusions. The basics of the debate between Pimm and the authors, Fangliang He and Stephen P. Hubbell, are laid out by Virginia Gewin in an article in Nature’s news section.

But Pimm also sent me a strong criticism of the paper by e-mail, calling it a “sham.” I shared his critique with Hubbell, who sent a reply. I’ve appended the exchange below. The discussion will play out in the peer-reviewed literature in the end, but this is a useful rough sketch of the issues. First is Pimm:

The paper is a sham: it does not report extinction rates or the numbers of species that are threatened. Despite its posturing, it deals with a different issue.

The paper is riddled with false statements. For instance, the paper states: “Estimates of extinction rates based on (the species-area) method are almost always much higher than those actually observed.”

It is unequivocally false. One reference used to support this (Pimm and Askins) uses a species-area relationship to predict 4.5 bird extinctions following deforestation in Eastern North America and then notices that four species went extinct and one is threatened. There are dozens of other studies of many taxa around the world that find equally compelling agreements between predicted and observed extinctions. A small selection of them follows.

So what does the paper model — and why does it poorly address the issue of extinctions? Imagine destruction that wipes out 95% of the habitat in an area metaphorically “overnight”. How many species have disappeared “the following morning”? The paper tells you. It is not many, just those wholly restricted to the 95% (and absent from the 5% where they would survive).

The important question is: How many of additional species living lonely lives in their isolated patches (the 5%) would become extinct eventually because their population sizes are too small to be viable? A different species-area curve applies — the one for islands, which are isolated. It is a much larger number of extinctions, of course, and the one used in the studies mentioned above that find such compelling agreement between predicted against observed extinctions.