The Missing Factor in Paul Ehrlich's Calculation of a "Sixth Great Mass Extinction"
Wesley Smith has already noted Paul Ehrlich's recent admonition regarding an imminent "sixth great mass extinction event," induced through the activity of modern humans. In an interview with the Telegraph, Ehrlich summarized the message of a journal article that he co-authored in Science Advances:
Species are disappearing up to about 100 times faster than the normal rate between mass extinctions, known as the background rate.
Our calculations very likely underestimate the severity of the extinction crisis. There are examples of species all over the world that are essentially the walking dead.
According to the Abstract:
The oft-repeated claim that Earth's biota is entering a sixth "mass extinction" depends on clearly demonstrating that current extinction rates are far above the "background" rates prevailing between the five previous mass extinctions. Earlier estimates of extinction rates have been criticized for using assumptions that might overestimate the severity of the extinction crisis. We assess, using extremely conservative assumptions, whether human activities are causing a mass extinction. First, we use a recent estimate of a background rate of 2 mammal extinctions per 10,000 species per 100 years (that is, 2 E/MSY), which is twice as high as widely used previous estimates. We then compare this rate with the current rate of mammal and vertebrate extinctions. The latter is conservatively low because listing a species as extinct requires meeting stringent criteria. Even under our assumptions, which would tend to minimize evidence of an incipient mass extinction, the average rate of vertebrate species loss over the last century is up to 100 times higher than the background rate. Under the 2 E/MSY background rate, the number of species that have gone extinct in the last century would have taken, depending on the vertebrate taxon, between 800 and 10,000 years to disappear. These estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way. Averting a dramatic decay of biodiversity and the subsequent loss of ecosystem services is still possible through intensified conservation efforts, but that window of opportunity is rapidly closing.
While the criteria may sound quantitative, and the increase in extinction rate qualitatively higher, there is a missing factor in this calculation.
First, the extinction rate in the past is determined from fossils. Since almost by definition, fossil animals are nearly all extinct today, the extinction rate is close to 100 percent. But the key thing is that not all species are represented in the fossil record.
Second, the present extinction rate is determined from surveys of living organisms. This is going to bias the much smaller populations observed today, as well as bias the surveys that "haven't seen a delta smelt in twenty years."
The net result of these two considerations is that fossil extinction rates are smoothed over tens of millennia, whereas today's extinction rates are spiky and represent maybe tens of decades at most. If we smoothed our present extinction rates over a few millennia, it would be amazing how much smaller the "rate" would be. In mathematical terms, dx/dt is scale dependent.
In addition, fossils represent such a small fraction of the living population that it is very difficult to infer an extinction rate from them. When it is attempted, one is undoubtedly looking at species that have widespread populations, whereas the exact opposite is true of most "extinct" species measured by living surveys today. The net result is that Ehrlich is undoubtedly comparing fossil extinction rates at the genus or family level with extinction rates of living species at the sub-species level.
So no, I wouldn't worry about Ehrlich's sixth mass extinction, at least not until we are all fossils and can do an apples-to-apples comparison.
Image: Delta smelt (Hypomesus transpacificus), by Peterson, B. Moose / U.S. Fish and Wildlife Service () [Public domain], via Wikimedia Commons.