Fossil and genetic evidence tell us that animals have been going extinct since long before the existence of modern humans – but how might extinction rates differ if humans suddenly ceased to exist? A recently updated calculation suggests the rate of die-offs would be lower. Much, much lower.
According to a team led by Stuart Pimm of Duke University in Durham, North Carolina, human activities are driving species to extinction at roughly 1000 times the background rate, i.e. the rate at which species would die out in the absence of humans. That's at the upper end of a previous estimate of Pimm's, published in Science in 1995, that not only underestimated the rate at which species are currently disappearing, but also overestimated the background rate over the last 10–20 million years.
That's scary for any number of reasons, but the most frightening of all may be how little we understand about the balance of speciation and extinction on the global scale. Is the inflated extinction rate pushing us toward some catastrophic tipping point? At what point , if any, does the rapid loss of species cause large-scale ecological systems to break down? As New Scientist's Peter Aldhous puts it:
The big unknown is what the high current extinction rate means for the health of entire ecosystems. Some researchers have suggested "sustainable" targets for species' loss, but there's still no scientific way to predict at what point cumulative extinctions cause an ecosystem to collapse. "People who say that are pulling numbers out of the air," says Pimm.
The upshot? Earth may be on the cusp of its sixth mass extinction. Of course, if we've learned anything from the last five major die-outs, it's that our planet (and its various inhabitants) has a way of recovering. Whether humanity recovers with it remains to be seen.