How many species are going extinct? It's an impossible question, not least because we don't know how many species there are. We do, of course, know pretty accurately the numbers of species of birds, mammals and reptiles. The big uncertainties apply to insects, beetles and plants.
The low estimates are two million species and of which we are losing 0.01% per year. That implies a loss of 200 species pa. Other estimates are as much as ten times greater implying a loss rate of 20,000 species per year.
But the real frightener is that even these numbers may be a serious underestimate.
The main driver of species extinction is widely believed to be habitat loss but it has proved difficult to prove this quantitatively. However, work by Stefan Dullinger and Franz Essl (of the University of Vienna) and co-workers appears to have resolved this puzzle. In research published in 2013 (PNAS, April 30, 2013. vol. 110, no. 18) they showed that species loss in many species does not occur when the habitat is destroyed or degraded but decades later.
An analysis of threatened species from seven taxonomic groups and 22 European countries showed that the number of such species was best explained by what had happened in 1900. The most useful variable was the intensity of land use - or how much of the production of the land we humans take for ourselves. This accounted for 35% of threatened species - twice as many as were predicted by land use data for the year 2000.
It's not hard to see why.
Intensification of land use such as deforestation or even hedgerow removal does not usually kill every member of a species but it does reduce their numbers and creates smaller, more or less isolated, populations. These populations are vulnerable to accidents, in-breeding, human disturbance and further habitat loss. Over a period of decades rare events become rather likely causing the small populations to become extinct. Even where physically possible isolation prevents recolonisation of the area and the species moves towards extinction.
What this means, practically, is that we have seriously under-estimated the impact of the human activity on the natural world because much of it has not yet happened. And, insofar as our predictions of future impacts are based on our experience those impacts are also under-estimated.
A separate study by Oliver Wearn and co-workers at Imperial College (Science, 337, p 228) estimates
that deforestation to date will ultimately eliminate 5% of species of
Amazonian birds, mammals and amphibians. However, if deforestation
continues at its present rate Amazonian species loss will reach 40% by
2050 and may ultimately reach 65%.
And this, please note, is before we assess the impacts of climate change!
Pessimists are surely right to consider ours the time of the Sixth Great (species) Extinction - comparable to the asteroid strike that killed the dinosaurs.
The difference is that the asteriod did not know what it was doing. We do.