Mass Extinction Threat: Earth on Verge of Huge Reset Button?

Jeremy Hsu

LiveScience Senior Writer

LiveScience.com jeremy Hsu

livescience Senior Writer

     livescience.com – Thu Sep 2, 2:30 pm ET Mass extinctions have served as huge reset buttons that dramaticallychanged the diversity of species found in oceans all over the world, accordingto a comprehensive study of fossil records. The findings suggest humans willlive in a very different future if they drive animals to extinction, becausethe loss of each species can alter entire ecosystems.

     Some scientists have speculated that effects of humans -from hunting to climate change - are fueling another great massextinction. A few go so far as to say we are entering a new geologic epoch,leaving the 10,000-year-old Holocene Epoch behind and entering the AnthropoceneEpoch, marked by major changes to global temperatures and ocean chemistry,increased sediment erosion, and changes in biology that range from alteredflowering times to shifts in migration patterns of birds and mammals andpotential die-offs of tiny organisms that support the entire marine food chain.

     Scientists had once thought species diversity could help buffer a group of animals from such die-offs, either keeping them from heading toward extinction or helping them to bounce back. But having many diverse species also proved no guarantee offuture success for any one group of animals, given that mass extinctions moreor less wiped the slate clean, according to studies such as the latest one.

Then and now

     Looking back in time, the diversity of large taxonomicgroups (which include lots of species), such as snails or corals, mostly hoveredaround a certain equilibrium point that represented a diversity limit ofspecies' numbers. But that diversity limit also appears to have changedspontaneously throughout Earth's history about every 200 million years.

     How today's extinction crisis - species today goextinct at a rate that may range from 10 to 100 times the so-called backgroundextinction rate - may change the face of the planet and itsspecies goes beyond what humans can predict, the researchers say.

"The main implication is that we're really rolling thedice," said John Alroy, a paleobiologist at Macquarie University inSydney, Australia. "We don't know which groups will suffer the most, which groupswill rebound the most quickly, or which ones will end up with higher or lowerlong-term equilibrium diversity levels."

What seems certain is that the fateof each animal group will differ greatly, Alroy said.

His analysis, detailed in the Sept. 3 issue of the journal Science,is based on almost 100,000 fossil collections in the Paleobiology Database (PaleoDB).

     The findings revealed various examples of diversity shifts,including one that took place in a group of ocean bottom-dwelling bivalves calledbrachiopods, which are similar to clams and oysters. They dominated thePaleozoic era from 540 million to 250 million years ago, and branched out intonew species during two huge adaptive spurts of growth in diversity - each timefollowed by a big crash.

     The brachiopods then reached a low, but steady, equilibriumover the past 250 million years in which there wasn't a surge or a crash inspecies' numbers, and still live on today as a rare group of marine animals.

Counting creaturesbetter

     In the past, researchers have typically counted species inthe fossil record by randomly drawing a set number of samples from each timeperiod - a method that can leave out less common species. In fact two studiesusing the PaleoDB used this approach.

     Instead, Alroy used a new approach called shareholdersampling, in which he tracked how frequently certain groups appeared in thefossil record, and then counted enough samples until he hit a target numberrepresentative of the proportion for each group.

"In some sense the older methods are a little like theAmerican voting system - the first-past-the-post-winner method basically makesminority views invisible," said Charles Marshall, a paleontologist at the University of California, Berkeley, who didnot take part in the study. "However, with proportional systems,minority views still get seats in parliament."

     Marshall added thatthe study was the "most thorough quantitative analysis to dateusing globalmarine data." But he added that researchers will probably debatewhether the PaleoDB data represents a complete-enough picture of the fossilrecord.

Nothing lasts forever

    The idea that rules of diversity change should not come as asurprise for most researchers, according to Marshall.

"To me,the really interesting possibility is that some groups might not yet be closeenough to their caps to have those caps be manifest yet," Marshall toldLiveScience. Or "evolutionary innovation" might happen so quicklythat new groups emerged to increase overall diversity, even if each sub-groupreached a cap on diversity.

    If anything, the record of pastextinctions has shown the difficulty of predicting which groups win out inthe long run. "Surviving is one thing and recovering is another,"said Marshall, who wrote a Perspectives piece about the study in the same issue of Science.

    One of the few consistent patterns is that growth spurts indiversity can apparently happen at any time, according to Alroy. He added thatthe background extinction of individual species has also remained consistent - theaverage species lasts just a few million years

     Of course, the ongoing extinction crisis of modern timesgoes far beyond the background extinction rate. Alroy noted that it could notonly wipe out entire branches of evolutionary history, but may also changethe ecosystems shaped by each species.

    That means today's species matter for environments aroundthe world, and so humans can't simply expect replacements from the diversespecies of the future.

"If we lose all the reef builders, we may not get backthe physical reefs for millions of years no matter how fast we get back all thespecies diversity in a simple sense," Alroy said.