Before ancient megafauna went extinct, mastodons kept broad-leaved vegetation, such as black ash trees, in check.
Credit: Barry Roal Carlsen, University of Wisconsin-Madison.
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.