Biodiversity - the replacement scare.
Twenty years ago, the Earth Summit in Rio resulted in a Convention on Biological Diversity, now signed by 193 nations, to prevent species loss. But can we tell how many species are becoming extinct?
One statement on the Convention's website claims: "We are indeed experiencing the greatest wave of extinction since the disappearance of the dinosaurs."
While that may (or may not) be true, the next sentence is spuriously precise: "Every hour three species disappear. Every day up to 150 species are lost."
Even putting aside the apparent mathematical error in that claim (on the face of it, if three species are disappearing every hour, 72 would be lost every day) there is an obvious problem in generating any such number. No-one knows how many species exist. And if we don't know a species exists, we won't miss it when it's gone....
It is possible to count the number of species known to be extinct. The International Union for Conservation of Nature (IUCN) does just that. It has listed 801 animal and plant species (mostly animal) known to have gone extinct since 1500.
But if it's really true that up to 150 species are being lost every day, shouldn't we expect to be able to name more than 801 extinct species in 512 years?
...That's why scientists prefer to use a mathematical model to estimate species loss.
Recently, however, that model has been attacked in the pages of Nature. Professor Stephen Hubbell from the University of California, Los Angeles, says that an error in the model means that it has - for years - over-estimated the rate of species loss....
The level of uncertainty faced by researchers in this field means it is perhaps not surprising that no-one can be sure of the scale of species loss. It also means that when a representative of the Convention of Biological Diversity claimed "every hour three species disappear" he must have known it was too precise.
But the fact that the precise extinction rate is unknowable does not prove that the problem is imagined.
Braulio Dias, executive secretary of the Convention on Biological Diversity, says: "We know that the drivers behind species loss are mostly increasing - land conversion and degradation, pollution, climate change. And of course the human population is still growing and consumption is growing - and most of that consumption is not sustainable."
Professor Hubbell, too, thinks species loss is a serious issue, even though he believes it has been exaggerated.
According to IUCN data, for example, only one animal has been definitely identified as having gone extinct since 2000. It was a mollusc.
Exaggerated numbers, wild estimates from inadequate models, spurious precision of urgency of problem, non-sustainable consumption and human population growth to blame, why does this all sound so familiar.