Global extinction rates: why do estimates vary so wildly?

Most ecologists believe that we are in the midst of the sixth mass extinction. Humanity's impact on nature, they say, is now comparable to the five previous catastrophic events over the past 600 million years, during which up to 95 percent of the planet's species disappeared. We may very well be. But recent studies have cited extinction rates that are extremely fuzzy and vary wildly.

The Millennium Ecosystem Assessment, which involved more than a thousand experts, estimated an extinction rate that was later calculated at up to 8,700 species a year, or 24 a day. More recently, scientists at the U.N. Convention on Biological Diversity concluded that: "Every day, up to 150 species are lost." That could be as much as 10 percent a decade.

But nobody knows whether such estimates are anywhere close to reality. They are based on computer modeling, and documented losses are tiny by comparison. Only about 800 extinctions have been documented in the past 400 years, according to data held by the International Union for the Conservation of Nature (IUCN). Out of some 1.9 million recorded current or recent species on the planet, that represents less than a tenth of one percent.

Nor is there much documented evidence of accelerating loss. In its latest update, released in June, the IUCN reported "no new extinctions," although last year it reported the loss of an earwig on the island of St. Helena and a Malaysian snail. And some species once thought extinct have turned out to be still around, like the Guadalupe fur seal, which "died out" a century ago, but now numbers over 20,000.

Moreover, the majority of documented extinctions have been on small islands, where species with small gene pools have usually succumbed to human hunters. That may be an ecological tragedy for the islands concerned, but most species live in continental areas and, ecologists agree, are unlikely to prove so vulnerable.

But the documented losses may be only the tip of the iceberg. That's because the criteria adopted by the IUCN and others for declaring species extinct are very stringent, requiring targeted research. It's also because we often simply don't know what is happening beyond the world of vertebrate animals that make up perhaps 1 percent of known species.

One way to fill the gap is by extrapolating from the known to the unknown. In June, Gerardo Ceballos at the National Autonomous University of Mexico - in collaboration with luminaries such as Paul Ehrlich of Stanford and Anthony Barnosky of the University of California, Berkeley - got headlines around the world when he used this approach to estimate that current global extinctions were "up to 100 times higher than the background rate."

Ceballos looked at the recorded loss since 1900 of 477 species of vertebrates. That represented a loss since the start of the 20th century of around 1 percent of the 45,000 known vertebrate species. He compared this loss rate with the likely long-term natural "background" extinction rate of vertebrates in nature, which one of his co-authors, Anthony Barnosky of UC Berkeley recently put at two per 10,000 species per 100 years. This background rate would predict around nine extinctions of vertebrates in the past century, when the actual total was between one and two orders of magnitude higher.

Ceballos went on to assume that this accelerated loss of vertebrate species would apply across the whole of nature, leading him to conclude that extinction rates today are "up to a hundred times higher" than background.

A few days earlier, Claire Regnier, of the National Museum of Natural History in Paris, had put the spotlight on invertebrates, which make up the majority of known species but which, she said, currently "languish in the shadows."

Regnier looked at one group of invertebrates with comparatively good records - land snails. And to get around the problem of under-reporting, she threw away the IUCN's rigorous methodology and relied instead on expert assessments of the likelihood of extinction. Thus, she figured that Amastra baldwiniana, a land snail endemic to the Hawaiian island of Maui, was no more because its habitat has declined and it has not been seen for several decades. In this way, she estimated that probably 10 percent of the 200 or so known land snails were now extinct - a loss seven times greater than IUCN records indicate.

Extrapolated to the wider world of invertebrates, and making allowances for the preponderance of endemic land snail species on small islands, she concluded that "we have probably already lost 7 percent of described living species." That could mean, she said, that perhaps 130,000 of recorded invertebrates have gone.