After Mass Extinction: Just Animals at Extremes

As human civilization drives more animals to extinction, extreme sizes will likely become the new norm.

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."

Watch "Racing Extinction" on Discovery Channel, Dec. 2, at 9 PM ET/PT.

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."

Under mass extinction, pays to be small (or big).

The world's sixth mass extinction began about 40 years ago, with humans contributing to the loss of biodiversity, according to a new study in the journal Science Advances. The authors call for fast action to conserve remaining threatened species, populations and habitat, but warn that the window of opportunity is rapidly closing. "We have maybe a decade to turn the boat in the right direction," co-author Anthony Barnosky of the University of California at Berkeley told Discovery News. "If we don't start cranking the rudder hard now, we won't make the turn in time." He and lead author Gerardo Ceballos explained that a mass extinction is a relatively sudden, global decrease in the diversity of life forms. Barnosky added that it's "when three out of four familiar species disappear from the face of the earth in a short time period." Already extinct is the Pinta Island tortoise, famously represented by a single male individual, "Lonesome George," for many years. He died on June 24, 2012, without siring any offspring. With him went an entire subspecies of tortoise.

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This colorful little toad from Costa Rica was declared extinct in the past few decades. A single male was spotted in 1989, but no one has seen this species since. Even many surviving amphibian species have dropped to dangerous population lows in recent years, with habitat loss, climate change, viruses and other factors theorized to contribute to the losses.

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A camera trapping study conducted from 2000–2004 hoped to capture a Formosan clouded leopard in a photo, but came up empty. No one has seen these big cats for decades, so it is presumed that the species -- native to the island of Taiwan -- is extinct. Ceballos told Discovery News that past mass extinctions have been driven by sudden climate change, meteorite hits, cataclysmic volcanic eruptions and other major natural disasters. Now, with human activity adding to the existing "backdrop" of natural disasters, yet another mass extinction is underway, he said. For the study, he and his colleagues compared a highly conservative estimate of current extinctions with this "backdrop" rate estimate. They even doubled the latter to bring the two rates as close to each other as possible. Despite this adjustment, they found that species are disappearing up to about 100 times faster than the normal rate. Human activity that continues to alter or destroy natural habitats includes the following: land clearing for farming, logging and settlement; introduction of invasive species; carbon emissions that drive climate change and ocean acidification; toxins that alter and poison ecosystems.

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Japanese sea lions are thought to have become extinct in the 1970s. This species, and all others that have gone extinct, once played vital roles in their ecosystems. "To me, the most important loss is that we'd be wiping out in one fell swoop what took millions of years for evolution to create," Barnosky said. "It's like going into the Louvre with a razor blade and slashing up at least three out of every four great works of art on display there. Others would argue that the tremendous monetary losses would be as important. (There would be a) loss of fisheries, forests, and other ways nature serves us, which scientists call 'ecosystem services.'"

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The Javan tiger is a tiger subspecies that once inhabited the Indonesian island of Java until it was declared extinct in the mid 1970s. Ceballos points out that "75 percent of all medicine initially derived from plants and animals," so extinctions deprive us of potential cures for countless diseases and illnesses.

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The last known dusky seaside sparrow, a bird once found in Florida, died in June of 1987. The subspecies was declared extinct in December 1990. Discoveries of new species happen each year, but unfortunately many of these newly found species are already highly endangered at the time of their discovery. There is no question that humans led to the demise of the dusky seaside sparrow. In an attempt to reduce the mosquito population where the birds lived, workers flooded the bird's habitat, devastating the sparrow's nesting grounds. Later, a highway was constructed at the site. Pollution and pesticides were also introduced into the area. Only six or seven birds survived the onslaught, and they have all since died, presumably without producing any offspring.

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Once common in parts of Spain and France, the Pyrenean ibex was declared extinct in January 2000. There have been attempts to clone them using the DNA from one of the last surviving females, but so far those efforts have proven to be mostly unsuccessful. People basically hunted this animal to death, but habitat loss and other factors played a role in their demise too.

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The Northern white rhino and the following two species on this list represent animals that are not extinct yet, but are endangered. Hunting, primarily for the rhino's ivory tusk, is driving the demise of this particular species. The situation is so dire that the rhino shown here, along with three others, is under 24-hour armed guard in Kenya.

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According to Ceballos, orangutan numbers in the wild "are so very low now." The Sumatran orangutan was once distributed over the entire island of Sumatra and further south into Java. Now, the species in the wild is restricted to just two sections in the north of the island. Of the nine known groups of these primates, only two are thought to have prospects for long-term viability. A factor in this and other losses is human population growth. "Each one of us has an ecological footprint that contributes to stomping out other species," Barnosky said. "Even conservative estimates of population growth indicate there will be 2–3 more billion people on the planet by 2050."

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There are two types of African elephants still in existence: the African bush elephant and the African forest elephant. Both are listed as "vulnerable" by the IUCN. "Poaching at current rates would cause the extinction of all wild elephants within 20 years," Barnosky said. As co-author Paul Ehrlich of Stanford University said, "There are examples of species all over the world that are essentially the walking dead."

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