A new study has laid down the possibility that Earth may already be on the brink of a sixth mass extinction, like those that occurred only five times before during the past 540 million years.
"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," said Anthony D. Barnosky at the University of California, Berkeley.
"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," he said.
However, he added, there was still something we could do to save these critically endangered mammals and other such species and stop short of the tipping point.
That could be done by dealing with a perfect storm of threats, including habitat fragmentation, invasive species, disease and global warming,
"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," Barnosky said.
"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."
Charles Marshall, UC Berkeley professor of integrative biology, said that the small number of recorded extinctions to date does not mean we are not in a crisis.
"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," he said.
"Even though the magnitude is fairly low, present rates are higher than during most past mass extinctions."
According to H. Richard Lane, program director in the National Science Foundation''s Division of Earth Sciences, the mass extinction is a largely unaddressed hazard of climate change and human activities.
"Its continued progression, as this paper shows, could result in unforeseen – and irreversible – negative consequences to the environment and to humanity,” he said.
The study originated in a graduate seminar Barnosky organized in 2009 to bring biologists and paleontologists together in an attempt to compare the extinction rate seen in the fossil record with today''s extinction record.
However, the fossil record goes back 3.5 billion years, while the historical record goes back only a few thousand year and dating of the fossil record also is not very precise, say the researchers.
To get around this limitation, Marshall said, "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."
Biologists estimate that within the past 500 years, at least 80 mammal species have gone extinct out of a starting total of 5,570 species.
"It looks like modern extinction rates resemble mass extinction rates, even after setting a high bar for defining ''mass extinction,''" Barnosky said.
After looking at the list of threatened species maintained by the International Union for Conservation of Nature (IUCN), the team concluded that if all mammals now listed as "critically endangered," "endangered" and "threatened" go extinct, whether that takes several hundred years or 1,000 years, Earth will be in a true mass extinction.
Barnosky admitted that there are limitations to the claim of the study but urges similar studies of groups other than mammals in order to confirm the findings, as well as action to combat the loss of animal and plant species.
"Our findings highlight how essential it is to save critically endangered, endangered and vulnerable species," Barnosky concluded.
"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.”
The study will be published in the March 3 issue of the journal Nature.