As human civilization drives more and more animals to extinction, what will be left? Extreme sizes will likely become the new norm, according to research published in the latest issue of the journal Science.
"Survivors tend to be either much smaller or much larger than most of their relatives," lead author Lauren Sallan of the University of Pennsylvania told Discovery News. "Those extremes favor survival through either fast breeding and large populations, or large ranges and ability to weather the storm."
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Sallan and co-author Andrew Galimberti, who is now a graduate student at the University of Maine, focused on marine animal body-size trends after what is called the end-Devonian mass extinction that occurred 359 million years ago. The researchers believe, however, that their findings, when combined with prior research, suggest patterns that could affect all animal life - both marine and terrestrial - after all known mass extinctions.
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Their analysis of 1,120 fish fossils spanning the period from 419 to 323 million years ago determined that, in line with a theory known as Cope's rule, the fish tended to evolve larger body sizes because of the evolutionary advantages of being larger. These include avoiding predation and being better able to catch prey.
On the other hand, they also found support for yet another theory, known as the Lilliput Effect, which holds that after mass extinctions, there is a temporary trend toward small body sizes.
Sallan also said that it will "take 5–20 million years (for surviving animals) to recover in species numbers and begin diversifying."
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The present mass extinction is unprecedented in terms of speed and ultimate triggers, she said, yet "it is likely that recovery will take just as long as every other event: tens of millions of years. Ecosystems will remain fragile during that entire time, with many additional species lost."
In terms of predicted winners and losers of the current die off, the researchers suspect that, in the marine realm, sharks will unfortunately be goners.
"Since we are killing the sharks directly, they likely won't make it this time, however, large ‘living fossil' trash fishes (not desirable to sport anglers or for human consumption), like gar and bowfin, which are freshwater apex predators, will likely survive initially, as they have through all other extinctions, but won't contribute much to future biodiversity."
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On land, mice and other small, fast-breeding mammals are expected to "eventually give rise to new radiations, but not for 5–20 million years," Sallan said.
Humans are in a surprisingly good position, given that our primate and even earlier mammal ancestors were among the longer-term success stories, she said. Sallan quickly reminded, however, "survival is a prolonged process, not a one-off event."
In fact, geophysical scientist David Jablonski of the University of Chicago has coined the term "dead clade walking," to refer to survivors of mass extinctions that gradually die off anyway after the big wipe out. He explains that large-bodied animals ultimately become victims due to their lower population sizes and longer generation times.
Another new study, published in the journal Current Biology, suggests that as any animal dies off, there is a domino effect leading to an increased rate of extinction in other species. The authors from the University of Exeter refer to the phenomenon as "extinction cascades."
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While some predictions can be made concerning life after mass extinctions, there are clearly still many unknowns. In short, we can expect the unexpected.
As Sallan said, "The ultimate winners are unlikely to be closely related to the victims or even resemble them. The deck will be reshuffled in ways that are unexpected in terms of biodiversity, but predictable in terms of trends."