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Mass Extinction Threat: Earth on Verge of Huge Reset Button? 
Mass extinctions have served as huge reset buttons that dramatically changed the diversity of species found in oceans all over the world, according to a comprehensive study of fossil records. The findings suggest humans will live in a very different future if they drive animals to extinction, because the 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 mass extinction. 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 Anthropocene Epoch, marked by major changes to global temperatures and ocean chemistry, increased sediment erosion, and changes in biology that range from altered flowering times to shifts in migration patterns of birds and mammals and potential 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 of future success for any one group of animals, given that mass extinctions more or less wiped the slate clean, according to studies such as the latest one.

Then and now

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

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

http://news.yahoo.com/s/livescience/20100902/sc_livescience/massextinctionthreatearthonvergeofhugeresetbutton  (http://news.yahoo.com/s/livescience/20100902/sc_livescience/massextinctionthreatearthonvergeofhugeresetbutton)