Such events have happened only five times before in the last 540 million years, and each saw three-quarters or more of all animal species become extinct.
“Just because the magnitude is low compared to the biggest mass extinctions we’ve seen in a half a billion years doesn’t mean to say that they aren’t significant,” says professor of integrative biology Charles Marshall. “Even though the magnitude is fairly low, present rates are higher than during most past mass extinctions.”
Comparing the extinction rate seen in the fossil record with today’s extinction record is unreliable, as the fossil record goes back 3.5 billion years, while the historical record dates back only a few thousand. In addition, the fossil record has many holes and isn’t very precise.
“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,” says Marshall. “But without the fossil record, we really have no scale to measure the significance of the impact we are having.”
To get around this limitation, the team estimated the range of plausible rates for the mass extinctions from the fossil record and then compared those rates to where we are now.
They chose mammals as a starting point because they’re well studied today and are well represented in the fossil record going back some 65 million years. Biologists estimate that within the past 500 years, at least 80 mammal species have become extinct out of a starting total of 5,570 species.
The team’s estimate for the average extinction rate for mammals is less than two extinctions every million years, far lower than the current extinction rate for mammals.
“It looks like modern extinction rates resemble mass extinction rates, even after setting a high bar for defining ‘mass extinction’,” says professor Anthony D. Barnosky.
“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.”
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