Global extinction rates: why do estimates vary so wildly?

Costello says double-counting elsewhere could reduce the real number of known species from the current figure of 1.9 million overall to 1.5 million. That still leaves open the question of how many unknown species are out there waiting to be described. But here too some researchers are starting to draw down the numbers.

Back in the 1980s, after analyzing beetle biodiversity in a small patch of forest in Panama, Terry Erwin of the Smithsonian Institution calculated that the world might be home to 30 million insect species alone - a far higher figure than previously estimated. His numbers became the received wisdom. But new analyses of beetle taxonomy have raised questions about them.

In June, Stork used a collection of some 9,000 beetle species held at London's Natural History Museum to conduct a reassessment. He analyzed patterns in how collections from particular places grow, with larger specimens found first, and concluded that the likely total number of beetle species in the world might be 1.5 million. From this, he judged that a likely figure for the total number of species of arthropods, including insects, was between 2.6 and 7.8 million.

Some researchers now question the widely held view that most species remain to be described - and so could potentially become extinct even before we know about them. Costello thinks that perhaps only a third of species are yet to be described, and that "most will be named before they go extinct."

Does all this argument about numbers matter? Yes, it does, says Stork. "Success in planning for conservation ... can only be achieved if we know what species there are, how many need protection and where. Otherwise, we have no baseline against which to measure our successes." Or indeed to measure our failures.

None of this means humans are off the hook, or that extinctions cease to be a serious concern. Extinction rates remain high. And, even if some threats such as hunting may be diminished, others such as climate change have barely begun. Moreover, if there are fewer species, that only makes each one more valuable.

But Stork raises another issue. He warns that, by concentrating on global biodiversity, we may be missing a bigger and more immediate threat - the loss of local biodiversity. That may have a more immediate and profound effect on the survival of nature and the services it provides, he says.

Ecosystems are profoundly local, based on individual interactions of individual organisms. It may be debatable how much it matters to nature how many species there are on the planet as a whole. But it is clear that local biodiversity matters a very great deal.