Study offers warning about next potential mass extinction

The Earth has undergone mass extinctions, during which more than 75% of existing species disappear, exactly five times in the past 540 million years. If things continue as they are we may be at the beginning of the sixth, a group of biologists and paleontologists suggest in a cautionary paper in Wednesday's edition of the journal Nature.

But they also signal that the threat is at such an early stage, with so many questions yet to be answered, that there's hope for avoiding this outcome.

The projection comes from researchers at the University of California, Berkeley, the University of Sao Paulo in Brazil and the National Evolutionary Synthesis Center in Durham, N.C. They looked at the animals currently listed as "critically endangered," "endangered" and "threatened" by the International Union for Conservation of Nature.

The IUCN lists 18,351 species on its "Red List of Threatened Species," considered the global standard for the conservation status of animal and plant species. All are at risk based on current and projected habitat loss or destruction due to human encroachment and climate change. Of those, 1,940 are listed as critically endangered, meaning the species' numbers have decreased, or will decrease, by 80% within three generations.

SCIENCE FAIR BLOG: Eastern cougar officially declared extinct

Then they raise the big "ifs."

If those animals were all to go extinct and if the rate of extinction were to continue unabated, amphibian, bird and mammal extinctions would reach "Big Five" magnitude within as little as three centuries to as far out as 2,200 years, they calculate. All of the mass extinctions of the past also transpired over many centuries or longer.Biologists today estimate that within the last 500 years, at least 80 mammal species have gone extinct out of a beginning total of 5,570 known species.That may be only 1.4%, but "just because the magnitude is low … doesn't mean to say that they aren't significant," says co-author Charles Marshall, UC Berkeley professor of integrative biology, in a press release.

"Walk outside, look around and imagine three-fourths of all the different kinds of life you see gone," says Anthony Barnosky, a paleobiologist at Berkeley and lead author on the paper. "Ask yourself if you'd be happy living in that world."

The researchers say they based their calculations on fairly conservative estimates of known extinction events from the fossil record, and information about extinct and threatened species today. They acknowledge that these are estimates and highlight the need for more research.

Previous extinction events Ordovician, 443 million years ago.

86% of all species lost.

Some possible causes: Waxing and waning ice ages; rise of the Appalachians affecting atmospheric and ocean chemistry.



Devonian, 359 million years ago.

75% of all species lost.

Some possible causes: Global cooling, followed by global warming, tied to new plants evolving and decrease in carbon dioxide; possibly meteor strikes as well.



Permian, 251 million years ago. 96% of all species lost.

Some possible causes: Siberian volcanoes; increased carbon dioxide levels; global warming; an asteroid or meteor strike.



Triassic, 200 million years ago.

80% of all species lost.

Possible cause: Volcanic activity caused by the breakup of the mega-continent Pangaea led to a rise in carbon dioxide levels and increased global temperatures.



Cretaceous, 65 million years ago.

76% of species lost, including the dinosaurs.

Possible causes: Asteroid strike in the Yucatan led to global cataclysm and rapid global cooling.



Source: Nature

Questions still to be answered are whether currently threatened, endangered and critically endangered species will go extinct, whether the rates used in their calculations will increase, decrease or remain steady and how reliably extinction rates in well-studied fossils can be extrapolated to other species in other places.

So they made what they consider cautious extrapolations from the data available.

The paper has "pushed the field forward significantly," says David Jablonski, a paleontologist at the University of Chicago who was not an author on the paper. "It's exciting to have a group of people trying to work it through as rigorously as they can."

And Stuart Pimm, a conservation biologist at Duke University in Durham, N.C., believes the researchers have "done a good job of measuring the size and rates of extinctions, the number of species that are threatened and, I think, drawn some very solid conclusions."

The term, "the sixth extinction," has been thrown around a lot in conservation circles over the past several years, but Pimm says he hadn't used it because he felt it was "poetic" but not based enough on facts. With the publication of this paper "I will use it now," he says.

The good news is that the Earth so far has only lost "a few percentage of species, nothing like the 75% we lost in the past, so we still have a lot more out there to save," says Barnosky.

The bad news is that current extinction rates are accelerating.

Pimm published a paper in the journal in Science in 1995 which found that species are currently going extinct at rates 100 to 1,000 times faster than the normal pace of evolution would give rise to new species and eliminate others. The last 15 years of research have only strengthened those findings, he says.

"But it's not hopeless. Earth's biodiversity is really in pretty good shape, if we can just slow the train of extinction," says Barnosky.

The cause of this latest wave of extinctions is humans and our burgeoning population, set to hit nine billion by 2050, he says.

Human endeavors are causing changing atmospheric conditions, habitat fragmentation, pollution, overfishing and over hunting, the introduction of invasive species and pathogens, the paper notes.

"One thing the fossil record shows us unequivocally is that ecosystems can be pushed to their breaking points," says Jablonski. What tends to be left after these mass extinction events is "a world of rats and weeds and cockroaches," species that are good at living in the margins, says Jablonski.

The solution is not to give up, but to become more efficient, says Barnosky. "We have to realize that there's a fixed amount of land out there for other species habitats' in order to keep biodiversity at the level we're accustomed to having it."