Has Earth's Sixth Mass Extinction Already Arrived?

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With the steep decline in populations of many animal species, from frogs and fish to tigers, some scientists have warned that Earth is on the brink of a mass extinction like those that occurred only five times before during the past 540 million years.

Each of these 'Big Five' saw three-quarters or more of all animal species go extinct.

In a study to be published in the March 3 issue of the journal Nature, University of California, Berkeley, paleobiologists assess where mammals and other species stand today in terms of possible extinction, compared with the past 540 million years, and they find cause for hope as well as alarm.

"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 principal author Anthony D. Barnosky, UC Berkeley professor of integrative biology, a curator in the Museum of Paleontology and a research paleontologist in the Museum of Vertebrate Zoology.

"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.

Nevertheless, Barnosky added, it's not too late to save these critically endangered mammals and other such species and stop short of the tipping point. That would require 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."

Coauthor Charles Marshall, UC Berkeley professor of integrative biology and director of the campus's Museum of Paleontology, emphasized 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."

"The modern global mass extinction is a largely unaddressed hazard of climate change and human activities," said H. Richard Lane, program director in the National Science Foundation's Division of Earth Sciences, which funded the research. "Its continued progression, as this paper shows, could result in unforeseen -- and irreversible -- negative consequences to the environment and to humanity."

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. These are "like comparing apples and oranges," Barnosky said. For one thing, the fossil record goes back 3.5 billion years, while the historical record goes back only a few thousand years. In addition, the fossil record has many holes, making it is impossible to count every species that evolved and subsequently disappeared, which probably amounts to 99 percent of all species that have ever existed. A different set of data problems complicates counting modern extinctions.

Dating of the fossil record also is not very precise, Marshall said.

"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," he said. "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, 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."

Barnosky's team 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 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 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.

"Obviously there are caveats," Barnosky said. "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."

He 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 added. "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."

Coauthors with Barnosky and Marshall are UC Berkeley integrative biology graduate students Nicholas Matzke, Susumu Tomiya, Guinevere Wogan, Brian Swartz, Emily L. Lindsey, Kaitlin C. Maguire, Ben Mersey and Elizabeth A. Ferrer; post-doctoral fellow Tiago B. Quental, now at the University of Sao Paulo, Brazil; and recent Ph.D. Jenny McGuire, now a post-doctoral fellow with the National Evolutionary Synthesis Center at Duke University in Durham, North Carolina.

Plenitude Of New Worlds Challenges Skills Of Planetary Modelers

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SEATTLE, WA--About two years ago, astrophysicist Douglas Lin recalls, speaker after speaker at an astronomy meeting in Hawaii deplored the lack of convincing evidence for new planets outside of our solar system. Lin then arose to state that making planets is incredibly easy. Researchers weren't finding them, he said, for a simple reason: Most young planets migrate into their parent stars, which consume them.

The civil audience pelted Lin with figurative tomatoes. But a lot has changed in two years, and no one's hurling fruit now.

Indeed, Lin and other theorists are straining to keep up with the whirlwind of planetary discoveries that has blown through astronomy since late 1995. They have stretched their models of how such systems evolve to accommodate a startling variety of planets, from ones with eccentric looping orbits to "hot Jupiters" that practically skim the outer atmospheres of their stars. Even the history of our own solar system--heretofore a peaceful scenario--is undergoing new scrutiny.

"Planets appear ubiquitous, and planetary systems are extremely diverse," says Lin, professor of astronomy and astrophysics at the University of California, Santa Cruz. "But to form a system that looks like ours, or one that can support the existence of life, may be a rare event." Lin will give a status report on the newly energized field of planetary-system modeling on Thursday, February 13, during "Old Worlds and New Worlds," a special two-day seminar at the AAAS meeting in Seattle.

Lin launched his recent modeling efforts in October 1995 when Swiss astronomers announced the first of the new batch of planets, called 51 Pegasi B. A team at UCSC's Lick Observatory, led by UCSC alumnus Geoffrey Marcy, rapidly confirmed the planet. From the outset its very existence seemed impossible. The Jupiter-sized object raced around its star once every four days, at a distance just one-twentieth that of Earth from our Sun. How could the planet, presumably a giant ball of gas, withstand this blast-furnace orbit? More puzzling still, how did it get there at all?

Within a day of the announcement Lin, Peter Bodenheimer of UCSC, and Derek Richardson of the University of Toronto had their solution. The planet coalesced in a colder region of its star's nebula, perhaps 100 times further away than it is today. Then, in a million-
year gravitational tug-of-war among the star, the planet, and gas and dust in the rest of the disk, 51 Pegasi B spiraled slowly but relentlessly toward the star. Finally, in the model's biggest surprise, inward and outward forces on the planet's orbit canceled each other out just before the star would have devoured the planet. The team published its paper in Nature on April 18, 1996.

Lin's idea that infant planets can migrate either toward or away from their stars dates to the 1970s, but he doesn't hesitate to call those initial concepts "wild speculations." If his new model is correct, the truth is even weirder. "I never thought the migration could stop, especially so close to the star," he says. "That was a real shocker."

Two other curious systems (70 Virginis and HD 114762) have large planets that swoop close to their stars and then out again, almost like huge comets. In a paper to appear in the Astrophysical Journal, Lin and Shigeru Ida of the Tokyo Institute of Technology suggest that each star may have possessed a massive disk of gas and dust, spawning several large planets. Within a few million years, the pernicious effects of gravity could have perturbed the planets sufficiently to make their orbits cross. Then, inevitable collisions created a single enormous object with a bizarre orbital path.

If this notion sounds vaguely familiar, it should: Immanuel Velikovsky proposed that similar events in our own solar system could explain certain oddities in the rotations and positions of planets. Velikovsky's 1950 book Worlds in Collision went way off the deep end, but Lin acknowledges that the basic premise has merit--if not here, than elsewhere.

"Dynamics among planets and within a planetary disk is a very rich game," he says. For instance, our outer solar system is "marginally stable." If Saturn, Uranus, and Neptune each had the same mass as Jupiter, their orbits might degenerate within a billion years--less than the Sun's lifetime--and wreak havoc throughout the entire system. "We are safe, but we are just safe," Lin says. "It takes only subtle differences in initial conditions to cause very diverse evolutionary paths."

Edging further along this limb, Lin suggests that life most likely would arise in systems with single massive planets tucked close to their stars, like 51 Pegasi B. He envisions such planets as the last in a succession of gas giants that migrate to their fiery dooms, sweeping up any rocky terrestrial planets in their paths. Then, according to his models, the gas and dust that remains in the wake of the final planet spreads out, seeding a second generation of earthlike bodies. With no gaseous titans further out to disrupt them, these planets could settle into stable orbits for billions of years. It's not out of the question, Lin says, that such a chain of events marked our solar system's childhood.

This conjecture flatly counters a tenet in astronomy that earthlike planets form in the toasty conditions close to their stars while gas giants dominate the frigid outer reaches, and never the twain shall meet.