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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.
"The main implication is that we’re really rolling the
dice," said John Alroy, a paleobiologist at Macquarie University in
Sydney, Australia. "We don’t know which groups will suffer the most, which groups
will rebound the most quickly, or which ones will end up with higher or lower
long-term equilibrium diversity levels."
What seems certain is that the fate
of 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 called
brachiopods, which are similar to clams and oysters. They dominated the
Paleozoic era from 540 million to 250 million years ago, and branched out into
new species during two huge adaptive spurts of growth in diversity - each time
followed by a big crash.
The brachiopods then reached a low, but steady, equilibrium
over the past 250 million years in which there wasn’t a surge or a crash in
species’ numbers, and still live on today as a rare group of marine animals.
Counting creatures
better
In the past, researchers have typically counted species in
the fossil record by randomly drawing a set number of samples from each time
period - a method that can leave out less common species. In fact two studies
using the PaleoDB used this approach.
Instead, Alroy used a new approach called shareholder
sampling, in which he tracked how frequently certain groups appeared in the
fossil record, and then counted enough samples until he hit a target number
representative of the proportion for each group.
"In some sense the older methods are a little like the
American voting system - the first-past-the-post-winner method basically makes
minority views invisible," said Charles Marshall, a paleontologist at the University of California, Berkeley, who did
not take part in the study. "However, with proportional systems,
minority views still get seats in parliament."
Marshall added that
the study was the "most thorough quantitative analysis to date
using global
marine data." But he added that researchers will probably debate
whether the PaleoDB data represents a complete-enough picture of the fossil
record.
Nothing lasts forever
The idea that rules of diversity change should not come as a
surprise for most researchers, according to Marshall.
"To me,
the really interesting possibility is that some groups might not yet be close
enough to their caps to have those caps be manifest yet," Marshall told
LiveScience. Or "evolutionary innovation" might happen so quickly
that new groups emerged to increase overall diversity, even if each sub-group
reached a cap on diversity.
If anything, the record of past
extinctions has shown the difficulty of predicting which groups win out in
the 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 in
diversity can apparently happen at any time, according to Alroy. He added that
the background extinction of individual species has also remained consistent - the
average species lasts just a few million years
Of course, the ongoing extinction crisis of modern times
goes far beyond the background extinction rate. Alroy noted that it could not
only wipe out entire branches of evolutionary history, but may also change
the ecosystems shaped by each species.
That means today’s species matter for environments around
the world, and so humans can’t simply expect replacements from the diverse
species of the future.
"If we lose all the reef builders, we may not get back
the physical reefs for millions of years no matter how fast we get back all the
species diversity in a simple sense," Alroy said.
- Original Story: Mass Extinction Threat: Earth on Verge of Huge Reset Button?
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