Global extinction rates: why do estimates vary so wildly?

Several leading analysts applauded the estimation technique used by Regnier. But others have been more cautious about reading across taxa. They say it is dangerous to assume that other invertebrates are suffering extinctions at a similar rate to land snails. Mark Costello, a marine biologist of the University of Auckland in New Zealand, warned that land snails may be at greater risk than insects, which make up the majority of invertebrates. "Because most insects fly, they have wide dispersal, which mitigates against extinction," he told me.

The same should apply to marine species that can swim the oceans, says Alex Rogers of Oxford University. Only 24 marine extinctions are recorded by the IUCN, including just 15 animal species and none in the past five decades. Some think this reflects a lack of research. But Rogers says: "Marine populations tend to be better connected [so] the extinction threat is likely to be lower."

Whatever the drawbacks of such extrapolations, it is clear that a huge number of species are under threat from lost habitats, climate change, and other human intrusions. And while the low figures for recorded extinctions look like underestimates of the full tally, that does not make the high estimates right.

Can we really be losing thousands of species for every loss that is documented? Some ecologists believe the high estimates are inflated by basic misapprehensions about what drives species to extinction. So where do these big estimates come from?

Mostly, they go back to the 1980s, when forest biologists proposed that extinctions were driven by the "species-area relationship." This relationship holds that the number of species in a given habitat is determined by the area of that habitat. The biologists argued, therefore, that the massive loss and fragmentation of pristine tropical rainforests - which are thought to be home to around half of all land species - will inevitably lead to a pro-rata loss of forest species, with dozens, if not hundreds, of species being silently lost every day. The presumed relationship also underpins assessments that as much as a third of all species are at risk of extinction in the coming decades as a result of habitat loss, including from climate change.

But, as rainforest ecologist Nigel Stork, then at the University of Melbourne, pointed out in a groundbreaking paper in 2009, if the formula worked as predicted, up to half the planet's species would have disappeared in the past 40 years. And they haven't. "There are almost no empirical data to support estimates of current extinctions of 100, or even one, species a day," he concluded.

He is not alone. In 2011, ecologist Stephen Hubbell of UC Los Angeles concluded, from a study of forest plots around the world run by the Smithsonian Institution, that as forests were lost, "more species always remained than were expected from the species-area relationship." Nature is proving more adaptable than previously supposed, he said. It seems that most species don't simply die out if their usual habitats disappear. Instead they hunker down in their diminished refuges, or move to new habitats.

Claude Martin, former director of the environment group WWF International - an organization that in his time often promoted many of the high scenarios of future extinctions - now agrees that the "pessimistic projections" are not playing out. In his new book, On The Edge, he points out that El Salvador has lost 90 percent of its forests but only three of its 508 forest bird species. Meanwhile, the island of Puerto Rico has lost 99 percent of its forests but just seven native bird species, or 12 percent.

Some ecologists believe that this is a temporary stay of execution, and that thousands of species are living on borrowed time as their habitat disappears. But with more than half the world's former tropical forests removed, most of the species that once populated them live on. If nothing else, that gives time for ecological restoration to stave off the losses, Stork suggests.

But we are still swimming in a sea of unknowns. For one thing, there is no agreement on the number of species on the planet. Researchers have described an estimated 1.9 million species (estimated, because of the risk of double-counting). But, allowing for those so far unrecorded, researchers have put the real figure at anywhere from two million to 100 million.

Last year Julian Caley of the Australian Institute of Marine Sciences in Townsville, Queensland, complained that "after more than six decades, estimates of global species richness have failed to converge, remain highly uncertain, and in many cases are logically inconsistent."

That may be a little pessimistic. Some semblance of order is at least emerging in the area of recorded species. In March, the World Register of Marine Species, a global research network, pruned the number of known marine species from 418,000 to 228,000 by eliminating double-counting. Embarrassingly, they discovered that until recently one species of sea snail, the rough periwinkle, had been masquerading under no fewer than 113 different scientific names.