Tuesday, July 13, 2010


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A new analysis of the extinction of woolly mammoths and other large mammals more than 10,000 years ago suggests that they may have fallen victim to the same type of "trophic cascade" of ecosystem disruption that scientists say is being caused today by the global decline of predators such as wolves, cougars, and sharks.

In each case the cascading events were originally begun by human disruption of ecosystems, a new study concludes, but around 15,000 years ago the problem was not the loss of a key predator, but the addition of one – human hunters with spears.

In a study published in the journal BioScience, researchers propose that this mass extinction was caused by newly-arrived humans tipping the balance of power and competing with major predators such as saber-toothed cats. An equilibrium that had survived for thousands of years was disrupted, possibly explaining the loss of two-thirds of North America's large mammals during this period.

"For decades, scientists have been debating the causes of this mass extinction, and the two theories with the most support are hunting pressures from the arrival of humans, and climate change," said William Ripple, a professor of forest ecosystems and society at Oregon State University, and an expert on the ecosystem alterations that scientists are increasingly finding when predators are added or removed.

"We believe humans indeed may have been a factor, but not as most current theory suggests, simply by hunting animals to extinction," Ripple said. "Rather, we think humans provided competition for other predators that still did the bulk of the killing. But we were the triggering mechanism that disrupted the ecosystem."

In the late Pleistocene, researchers say, major predators dominated North America in an uneasy stability with a wide range of mammals: mammoths, mastodons, ground sloths, camels, horses, and several species of bison. The new study cites previous evidence from carnivore tooth wear and fracture, growth rates of prey, and other factors that suggest that there were no serious shortages of food caused by environmental change 10,000 to 15,000 years ago.

Quite contrary to that, the large herbivores seemed to be growing quickly and just as quickly had their numbers reduced by a range of significant carnivorous predators, not the least of which was lions, dire wolves, and two species of saber-toothed cats. Food was plentiful for herbivores, the system was balanced, but it was dominated by predators.

"When human hunters arrived on the scene, they provided new competition with these carnivores for the same prey," said Blaire Van Valkenburgh, an expert at UCLA on the paleobiology of carnivores, and a co-author with Ripple on this study.

"The humans were also omnivores, and could live on plant foods if necessary," Van Valkenburgh said. "We think this may have triggered a sequential collapse not only in the large herbivores but ultimately their predators as well. Importantly, humans had some other defenses against predation, such as fire, weapons and living in groups, so they were able to survive."

But the driving force in eliminating the large mammals, according to the new theory, was not humans – they just got the process started. After that, predators increasingly desperate for food may have driven their prey to extinction over long periods of time – and then eventually died out themselves.

In recent studies in Yellowstone National Park and elsewhere, scientists from OSU and other institutions have explored these "trophic cascades," often caused by the loss or introduction of a single major predator in an ecosystem. With the elimination of wolves from Yellowstone, for instance, the numbers of elk exploded. This caused widespread overgrazing; damage to stream ecosystems; the slow demise of aspen forests; and ultimate effects on everything from trees to beaver, fish, birds, and other life forms. When wolves were re-introduced to Yellowstone, studies are showing that those processes have begun to reverse themselves.

"We think the evidence shows that major ecosystem disruptions, resulting in these domino effects, can be caused either by subtracting or adding a major predator," Ripple said. "In the case of the woolly mammoths and saber-toothed tiger, the problems may have begun by adding a predator, in this case humans."

The new analysis draws on many other existing studies in making its case.

For instance, other research describes this process with a model in modern times in Alaska. There, the allowance of relatively limited human hunting on moose caused wolves to switch some of their predation to sheep, ultimately resulting in a precipitous decline in populations not only of moose but also wolves and sheep.

The loss of species in North America during the late Pleistocene was remarkable – about 80 percent of 51 large herbivore species went extinct, along with more than 60 percent of important large carnivores. Previous research has documented the growth rates of North American mammoths by studying their tusks, revealing no evidence of reduced growth caused by inadequate food – thus offering no support for climate-induced habitat decline.

It seems that diverse and abundant carnivores kept herbivore numbers below levels where food becomes limiting. By contrast, the large population of predators such as dire wolves and saber tooth cats caused them to compete intensely for food, as evidenced by heavy tooth wear.

"Heavily worn and fractured teeth are a result of bone consumption, something most carnivores avoid unless prey is difficult to acquire," says Van Valkenburgh.

Trophic cascades initiated by humans are broadly demonstrated, the researchers report. In North America, it may have started with the arrival of the first humans, but continues today with the extirpation of wolves, cougars and other predators around the world. The hunting of whales in the last century may have led to predatory killer whales turning their attention to other prey such as seals and sea otters - and the declines in sea otter populations has led to an explosion of sea urchins and collapse of kelp forest ecosystems.

"In the terrestrial realm, it is important that we have a better understanding of how Pleistocene ecosystems were structured as we proceed in maintaining and restoring today's ecosystems," the researchers wrote in their conclusion. "In the aquatic realm, the Earth's oceans are the last frontier for megafaunal species declines and extinctions."

"The tragic cascade of species declines due to human harvesting of marine megafauna happening now may be a repeat of the cascade that occurred with the onset of human harvesting of terrestrial megafauna more than 10,000 years ago. This is a sobering thought, but it is not too late to alter our course this time around in the interest of sustaining Earth's ecosystems."

(Photo: Mauricio Anton, courtesy of Oregon State University)

Oregon State University


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David Reed is a big hit at parties. That is, once people get past the "yuck" factor of his job. Reed, the associate curator of mammals at the Florida Museum of Natural History (FLMNH), studies lice. Apparently, those same insects that drive parents and youngsters crazy during a school infestation are helping to uncover some deep scientific mysteries.

"It's really fascinating how interested people are in lice," says Reed. "Lice are gross and disgusting, and I think that's what first piques people's interest about them. But when you tell people that you can learn a lot about our own evolutionary history by studying lice, they get interested for another reason, too."

With funding from the National Science Foundation (NSF), Reed and his team study the evolutionary history of humans and the parasites that have accompanied people from around the time Homo sapiens left Africa nearly 100,000 years ago.

For decades, scientists have been studying human migration patterns based on fossil evidence and molecular data like DNA sequences. Studying lice gives researchers one more tool. By collecting lice from around the world and studying their DNA, Reed has been able to aid in reconstructing the past.

"We find these "aha" moments where we see that, for some reason the parasites are telling us something different about their shared evolutionary history than what we've learned already from their hosts," says Reed.

Scientists know that there are two different types of lice that occur in the New World. One of the questions Reed studies is: When did these two types of lice first come to the New World?

"Did one come with the Europeans and one with the First Peoples? That's something we'd like to know more about," says Reed. "If we can find out from the lice when people got to the New World, we can use that information to trace their routes, the timing of when they got to the New World, to see if it confirms or changes what we know about human migration patterns."

For example, it is still unclear exactly where the journey began in Asia and when people began their long migration to North and South America. By looking at the lice of today from those New World continents and comparing them to lice from Mongolia, or Siberia, Reed can try to determine the origin.

It is also helpful to study lice on other primates, such as chimps and gorillas. "Chimps and humans shared a common ancestor about five to seven million years ago," says Reed. "We also see that the lice that occur on chimpanzees and humans also shared a common ancestor, about five to seven million years ago. So we know that these parasites have been co-evolving with their hosts for at least 25 million years."

Recently, Reed received additional NSF funding via the American Recovery and Reinvestment Act of 2009 (ARRA). The new funding allowed him to hire technicians and a graduate student, as well as to purchase much needed infrastructure, such as a digital imaging system to make high-resolution images of museum specimens available to anyone over the Internet. "This funding also allows us to train students to better communicate their research findings to a general audience, an area in which scientists usually receive little or no training," adds Reed.

Practical, as well as historical, knowledge can also be gained from Reed's research. "One thing we can learn from lice is better ways to treat head lice. It's a very, very common problem because these head lice are resistant to the shampoos that we use. The more that we can learn and teach the public about the evolution of resistance to insecticidal shampoos or antibiotics, or whatever, I think the better we can treat different diseases that affect humans," he explains.

For millennia, parents of small children have had to deal with the treatment of lice. "We do find with head lice, there's such a stigma associated with it, that there's loss of work for the parent, and for the child going to school. There are billions of dollars lost in the economy every year as a result of head lice infestations," says Reed.

"It is all about educating the community in advance of an outbreak," says Deborah Altschuler, president of the National Pediculosis Association (pediculosis meaning "infestation with lice"). Altschuler deals with parents who are horrified and frustrated when their kids come home with head lice.

"We at NPA encourage parents to comb, and for a variety of important health reasons. Combing enables families to accurately determine whether there's an infestation or not. It is the best prevention we know," said Altschuler.

The association runs a national campaign at the beginning of the school year called "Comb First," designed to educate parents, students and teachers before an outbreak occurs. Altschuler stressed that head lice prevention is not just a September crisis, and that regular screening can mitigate unnecessary and unsafe problems down the road.

"Parents are really good about this, if they are given good information. They need help and incentives to screen early," said Altschuler.

Some cities now have facilities where kids with head lice can go for combing. The non-profit "Lice Solutions" in West Palm Beach Florida, www.licesolutions.org is one such resource. They also specialize in educating parents, teachers, and school systems about "this unwelcome houseguest." And the organization occasionally provides Reed with live lice for him to study.

Reed recently showed off some live insects with a new digital camera system at the museum. With the lice magnified on a monitor, he showed how the tiny creatures have developed some very useful evolutionary traits, like the ability to clutch onto a human hair. The insects can grasp onto a human hair and lock on solidly.

"A key to their existence is for them to stay attached to hair. If they don't, they are doomed. And so the reason we care about it really is, we want to know how they get from one individual host to another. From one human or from one child to another," says Reed.

Studying lice provides insight about some other intriguing questions. Like, when did humans start wearing clothes? Reed says the answer can be found in studying "clothing lice."

"Being a naked primate, we don't have a lot of body fur for them to live in, so when humans lost their body hair evolutionarily, that was a lot of habitat lost for lice. What's interesting though is that when we started wearing clothing, lice immediately went to this new habitat and became what we know as clothing lice, or body lice. They lay their eggs in clothing and they are really tied to human clothing for their whole existence. They come to the body of the human to feed, but only once per day," he explains.

Body lice are found on people unable to change clothing, such as refugees, who literally only have the clothes on their backs, and often soldiers during wartime.

Reed says he's heard almost every pun imaginable about his work, from, "Of Lice and Men," to "Pair of Lice Lost," to "You must really do some louse-y research."

But he says scientists really are just scratching the surface of what these parasites can tell us, especially since there are about 5,000 different species of lice, living on animals from birds to humans to fur seals. And he's constantly working to get both school children and adults interested in his research.

"As soon as we show gross, icky pictures of lice and talk about what's really cool about them--their life history, their life cycle, their evolution--people get really interested and they realize that science can be interesting in areas where you didn't necessarily expect to be interested before," notes Reed.

Maybe not such "louse-y" research after all!

(Photo: Sarah Bush, University of Utah)

National Science Foundation


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When high-temperature superconductors were first announced in the late 1980s, it was thought that they would lead to ultra-efficient magnetic trains and other paradigm-shifting technologies.

That didn’t happen. Now, a University of Florida scientist is among a team of physicists to help explain why.

In a paper set to appear Sunday in the online edition of Nature Physics, Peter Hirschfeld, a UF professor of physics, and five other researchers for the first time describe precisely how the atomic-level structural elements of high-temperature ceramic superconductors serve to impede electrical current. Their explanation for how “grain boundaries” separating rows of atoms within superconductors impede current is the first to fit a phenomenon that has helped keep the superconductors from reaching their vaunted potential – and puzzled experimental physicists for more than two decades.

“Nobody understood why it was such a strong effect, or why the current was so limited by these grain boundaries,” Hirschfeld said. “And that is what we have explained in this paper.”

High-temperature superconducting ceramic wires are composed of rows of atoms arranged slightly askew to each other, as though one piece of graph paper had been melded atop another with the horizontal and vertical lines at less-than-perfect alignment. Lumps of electrical charge build up at the angles where the lines meet, acting like dams to interrupt the flow of electricity.

Hirschfeld and his colleagues’ contribution was to conceive and construct a mathematical model that fit these observations “very nicely,” he said. ”We abstracted a very theoretical model of a single boundary” that can be applied to all such boundaries, he said.

Unfortunately the model does not suggest a way to break down the barriers, although Hirschfeld said it will give researchers a better tool to interpret results of past and future experiments. This gives the team hope that their model could, over time, lead to high-temperature superconductors with less restrictive grain boundaries. That would be a step toward helping the superconductors, which have found limited applications in areas such as powerful research magnets, reach their heralded potential.

Siegfried Graser, the first author of the Nature Physics paper and a faculty member at the University of Augsburg in Germany, did most of his research while he was a postdoctoral associate in Hirschfeld’s group at UF. The other authors are at the University of Augsburg and the University of Copenhagen. The research was funded by the U.S. Department of Energy.

University of Florida




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