Lost Giants

Did mammoths vanish before, during and after humans arrived?

Before humans arrived, the Americas were home to woolly mammoths, saber-toothed cats, giant ground sloths and other behemoths, an array of megafauna more impressive than even Africa boasts today. Researchers have advanced several theories to explain what did them in and when the event occurred. A series of discoveries announced last fall, at fi rst glance apparently contradictory, add fresh details to the mystery of this mass extinction. One prominent theory pegs humans as the cause of the demise, often pointing to the Clovis people, who left the earliest clear signs of humans entering the New World roughly 13,500 years ago. The timing coincides with the disappearance of megafauna, suggesting the Clovis hunted the animals to extinction or infected them with deadly disease. Another hypothesis supposes that climate was the culprit: it had swung from cold to warm twice, including a 1,300-year-long chill known as the Younger Dryas; such abrupt shifts might have overwhelmed the creatures’ abilities to adapt.

To pin down when the megafauna vanished, paleoecologist Jacquelyn Gill of the University of Wisconsin–Madison and her colleagues analyzed fossil dung, pollen and charcoal from ancient lake sediments in Indiana. The dung of large herbivores harbors a fungus known as Sporomiella, and its amounts in the dung gives an estimate of how many mammoths and other megafauna were alive at different points in history. Pollen indicates vegetation levels, and charcoal signals how many fires burned; the extent of flora and wildfi res is related to the presence of herbivores, the researchers say in the November 20 Science. Without megaherbivores to keep them in check, broad-leaved tree species such as black ash, elm and ironwood claimed the landscape; soon after, buildups of woody debris sparked a dramatic increase in wildfi res. Putting these data together, Gill and her team conclude that the giant animals disappeared 14,800 to 13,700 years ago—to 1,300 years before Clovis.

A different study, however, suggests that this mass extinction happened during Clovis. Zooarchaeologist J. Tyler Faith of George Washington University and archaeologist Todd Surovell of the University of Wyoming carbon-dated prehistoric North American mammal bones from 31 different genera (groups of species). They found that all of them seemed to meet their end simultaneously between 13,800 to 11,400 years ago, findings they detailed online November 23 in the Proceedings of the National Academy of Science USA.

But if ancient DNA recovered from permafrost is any sign, megafauna survived in the New World millennia after humanity arrived. As the permafrost in central Alaska cracked during springtime thaws, water that held DNA from life in the region leaked in, only to freeze again during the winter. As such, these genes can serve as markers of “ghost ranges”—remnant populations not preserved as fossil bones. Looking at mitochondrial DNA, evolutionary biologist Eske Willerslev of the University of Copenhagen and his colleagues suggest mammoths lasted until at least 10,500 years ago (as did horses, which actually originated in the Americas only to vanish there until the Europeans reintroduced them). The Proceedings of the National Academy of Science USA published those findings online December 14.

Although the three papers appear to conflict with one another, they could be snapshots from the beginning, middle and end of a mass extinction. “If they seem to disagree, it is for the same reason as in that fable about the three blind men trying to describe an elephant—or mammoth?—by touching different parts of it,” says ecologist Christopher Johnson of James Cook University in Australia, who did not take part in any of the studies.

Johnson suggests the fungus research is superb evidence for when the decline began,
but it is not as good at confirming exactly when the extinction was completed, especially over larger areas where sparse populations might have persisted. The DNA finds, on the other hand, can detect late survivors, he says, “maybe very close to the actual time that the last individuals were alive, at least in Alaska.” The bones analyzed from the period roughly in between show that the extinction process afflicted many species simultaneously. Those fossils came from the contiguous U.S., which back then was separated from Alaska by the massive Laurentide and Cordilleran ice sheets and so, Faith notes, could explain why the pattern of extinction differed up there.

So what caused the decline? The jury’s still out, says Willerslev’s collaborator Ross MacPhee of the American Museum of Natural History in New York City. Johnson notes that archaeologists are turning up evidence of humans in the New World before Clovis, and he suggests they overhunted the megafauna. The beautifully crafted fluted spear points linked with the Clovis might reflect strategies that developed once the giants became rare and harder to hunt, Johnson adds.

Even if scientists cannot definitively finger the killer, research into the megafauna disappearance “is directly relevant today because we are in the middle of a mass extinction and one for which we know the cause—us,” Gill says. “Large animals are among the most threatened today,” she points out, and no one wants Africa to follow the ancient experience of the Americas.

Source of Information :  Scientific American Magazine February 2010

Hunting a Croc Killer

Mass deaths of South Africa’s Nile crocodiles puzzle biologists BY NAOMI LUBICK

Carcasses of adult crocodiles do not usually signal the return of winter in South Africa, but mass death seems to be becoming the harbinger of the season. Rangers at the Kruger National Park have found Nile crocodiles floating in the Olifants River or bloated and decaying along its banks. Investigators are rushing to figure out the cause and worry that the deaths might be signaling the presence of toxins or pathogens that could threaten not only the croc population but also the livelihoods of the people living near the river.

The Olifants River runs several hundred kilometers through three South African provinces and into Mozambique. It supplies water to industrial agriculture operations that send food to Europe and to the local rural communities, which also depend on those waters for fishing and farming.

The first sign of croc trouble in the river came in the winter of 2008, when rangers collected 170 dead individuals, sometimes at a rate of 20 bodies a week. A survey at the end of this May showed nearly 400 crocs living in the park’s gorge, down from at least 1,000 in 2008. So far, as of August 7, rangers and scientists have found 23 carcasses.

After slicing open some of the crocodile corpses last year, researchers determined some kind of pansteatitis—an inflammation of adipose tissue—was killing the animals. Specifically, their tails were swollen with the hardened, enlarged fat deposits, which had stiffened and immobilized the crocodiles and left them unable to hunt. Samples of the fat showed the deposits had oxidized to bright yellow.

The disease may not be limited to crocs. Scientists found the same kinds of fat deposits in fish in the Olifants River. And in the river’s gorge just upstream from Massingir Dam in Mozambique, which also has seen croc declines, birds were absent, raising the possibility that they, too, have succumbed to the same agent.

But the cause behind the strange fattening remains a mystery. In June a team led by Henk Bouwman of North-West University, Potchefstroom Campus, in South Africa reported test results from crocodile tissues at two European chemistry meetings. “Everything is there,” Bouwman says, referring to the detection of DDT, PCBs, dioxins and brominated flame retardants, “but nothing is screaming, ‘it’s me, it’s me, it’s me.’ ”

One possibility could be related to dinoflagellates and cyanobacteria found upstream in the catchment, which might be releasing toxins similar to those that cause red tides in marine environments, says Peter Ashton, a water resources specialist at the Council for Scientific and Industrial Research in South Africa and the University of Pretoria.

“It never is a quick, easy solution” in which it takes one test to find a culprit, explains Danny Govender, a disease ecologist for South African National Parks. She notes that samples taken from live crocs in 2007 showed that the fat of some crocodiles was beginning to harden. Along with Bouwman, she hypothesizes that all these toxins, found below harmful levels individually, could be acting together in a deadly brew.

Govender cites changes to the river’s ecosystem that stem from infrastructure outside the park, including hundreds of coal-mining operations upstream, where crocodiles have disappeared almost completely, and a dam downstream of the gorge. For the first time in the two decades since it was built, the dam’s reservoir was full last year, slowing down the Olifants’s flow through the crocodiles’ gorge. Govender wonders if the slowed water enabled toxins to build up along the crocodiles’ stretch of the river. Indeed, hydrogen sulfide, ammonia and other compounds from river sediments probably caused massive fish deaths in July, scientists have concluded, and crocs eating these contaminated fish could have been affected.

Even if researchers find the culprits, the impacts could reach further than suspected. “We really underestimated [the number of dead] crocodiles from last year’s count,” Govender adds, noting that their bodies could have been eaten by other crocs or sunk to the bottom of the river. “I suspect we’re losing a lot of breeding females,” whose carcasses are smaller and more easily scavenged. If that is the case, she says, the gorge’s crocodile population may not ever recover, even if scientists can pinpoint the cause of the die-off.

As for the people who depend on the Olifants River, “I don’t know what to tell them,” Bouwman says. His colleague Henrik Kylin of Swedish University of Agricultural Sciences in Uppsala wants to go into Mozambique to test fish there, and possibly people, to see if the croc killer kills more than just crocs.

Source of Information : Scientific American October 2009