Earth has had five mass extinction events in the last 540 million years, in which more than three-quarters of animal species went extinct. We're on the verge of the sixth mass extinction, but it's not too late to stop it.
Paleobiologists at UC Berkeley have examined today's extinction trends in relation to those at the time of the five mass extinctions. As lead researcher Anthony D. Barnosky explains, we are reaching the tipping point for mass extinction number six:
"If you look only at the critically endangered mammals — those where the risk of extinction is at least 50 percent within three of their generations — and assume that their time will run out, and they will be extinct in 1,000 years, that puts us clearly outside any range of normal, and tells us that we are moving into the mass extinction realm. If currently threatened species — those officially classed as critically endangered, endangered and vulnerable — actually went extinct, and that rate of extinction continued, the sixth mass extinction could arrive within as little as 3 to 22 centuries."
Obviously, mass extinctions don't happen overnight, and it might be difficult to get too worked up about something that isn't due until somewhere between the years 2300 and 4200. But as Barnosky points out, this would be the first mass extinction where a single species would be one of the root causes. I'll give you one guess as to which species that is:
"So far, only 1 to 2 percent of all species have gone extinct in the groups we can look at clearly, so by those numbers, it looks like we are not far down the road to extinction. We still have a lot of Earth's biota to save. It's very important to devote resources and legislation toward species conservation if we don't want to be the species whose activity caused a mass extinction."
There are a lot of threats we need to deal with if we're going to prevent this mass extinction, including repairing fragmented habitats, stopping the spread of invasive species and disease, and getting global warming under control.
Of course, the fossil record is woefully incomplete, so it's extremely difficult to estimate accurately extinction rates from millions of years ago, let alone compare them in a meaningful way to the rates we observe today. Fellow researcher Charles Marshall explains how they got around these problems:
"If we find a mass extinction, we have great difficulty determining whether it was a bad weekend or it occurred over a decade or 10,000 years. But without the fossil record, we really have no scale to measure the significance of the impact we are having. This paper, instead of calculating a single death rate, estimates the range of plausible rates for the mass extinctions from the fossil record and then compares these rates to where we are now."
Even setting the highest possible bar for what constitutes a mass extinction, today's extinction rates are troubling. At least eighty mammal species have gone extinct in the last 500 years out of a total of about 5,570. But the fossil record suggests an average extinction of less than two species every million years. Yes, it's possible that the numbers are way off and there's nothing to worry about. But, as Barnosky explains, the chances of that are exceedingly unlikely, and time is running out for us to start taking this treat seriously:
"It looks like modern extinction rates resemble mass extinction rates, even after setting a high bar for defining 'mass extinction. Obviously there are caveats. What we know is based on observations from just a very few twigs plucked from the enormous number of branches that make up the tree of life. Our findings highlight how essential it is to save critically endangered, endangered and vulnerable species. With them, Earth's biodiversity remains in pretty good shape compared to the long-term biodiversity baseline. If most of them die, even if their disappearance is stretched out over the next 1,000 years, the sixth mass extinction will have arrived."