An article on the BBC News website outlines methods for determining numbers used in measures of biodiversity loss, such as the claim by the Convention on Biological Diversity in 2007 that we are “experiencing the greatest wave of extinction since the disappearance of the dinosaurs”. One problem, the BBC explains, is that
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.
However, a model used to estimate species loss has been criticised by Professor Stephen Hubbell from the University of California, Los Angeles. The article explains that the problem arises from running a model that assesses the number of species present as you expand a sample area, in reverse. Fundamentally, the problem is
if you increase a habitat by, say, five hectares, and your calculations show that you expect there to be five new species in those five hectares, it is wrong to assume that reversing the model, and shrinking your habitat, eliminates five species.
There are other factors affecting this issue, such as determining the background level of extinctions if human beings were not the cause.
Professor Hubbell agrees with the Convention on Biological Diversity that species loss is a serious issue, even though he believes it has been exaggerated.