In a study published in the journal Nature, researchers at the University of California, Berkeley, assessed where the mammals and other species stand today in terms of possible extinction, compared with the past 540 million years, and they found the cause for hope as well as alarm.
"If you look only at the critically endangered mammals and assume... 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 lead author Anthony Barnosky, a UC professor of integrative biology.
"If currently threatened species actually went extinct and that rate of extinction continued, the sixth mass extinction could arrive within as little as 3 to 22 centuries."
Nevertheless, it`s not too late to save these critically endangered mammals and other such species and stop short of the tipping point, Prof Barnosky, who is also a curator at the Museum of Paleontology in Berkeley, said.
That would require dealing with a perfect storm of threats, including habitat fragmentation, invasive species, disease and global warming, the paleobiologist warned.
"So far, only one to two per cent 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," he said.
For the research, Prof Barnosky and his team estimated the range of plausible rates for the mass extinctions from the fossil record and then compared those rates to present status.
They chose mammals as a starting point because they are well studied today and are well represented in the fossil record going back some 65 million years.
Biologists estimate that within the past 500 million years, at least 80 mammal species have gone 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`," Barnosky said.
After looking at the IUCN -- International Union for Conservation of Nature -- list of threatened species, the team concluded that if all mammals listed endangered now go extinct, whether that takes several hundred years or 1,000 years, Earth will be in a true mass extinction.