IELTSwithJurabek

PASSAGE 2

Read the text and answer questions 14-26

MAMMOTH KILL

What led to the disappearance of the giant mammals? Kate Wong examines the theories.

Although it is hard to imagine in this age of urban sprawl and automobiles, North America once belonged to huge, elephant-like mammoths, camels, bear-sized beavers and other giant beasts, collectively known as megafauna. Some 11,000 years ago, however, these large-bodied mammals - about 70 species in all - disappeared. Their demise coincided roughly with the arrival of humans in this region and dramatic climate change, factors that have inspired several theories about the die-off. Yet despite decades of scientific investigation, the exact cause remains a mystery. Now new findings offer support to one of these controversial hypotheses: that human hunting drove these huge megafauna species to extinction.

This belief resulted in the overkill model which emerged in the 1960s, when it was put forth by Paul S. Martin of the University of Arizona. Since then, critics have charged that no archaeological remains exist to support the idea that the first Americans hunted to the extent necessary to cause these extinctions. But at the annual meeting of the Society of Vertebrate Paleontology in Mexico City in October 1999, specialist John Alroy of the University of California at Santa Barbara argued that in fact, hunting-driven extinction is not only plausible, it was unavoidable. He has determined, using a computer simulation, that even a very modest amount of hunting would have wiped out these animals.

Assuming an initial human population of 100 people that grew no more than two per cent annually, Alroy determined that, if each band of, say, 50 people killed 15 to 20 large animals a year, humans could have eliminated the animal populations within 1,000 years. Large mammals in particular would have been vulnerable to the pressure because they have longer gestation periods than smaller mammals and their young require extended care.

However, not everyone agrees with Alroy's assessment. For one thing, the results depend on population size estimates for the extinct animals - estimates that are not necessarily reliable. But a more specific criticism comes from mammal expert Ross D. E. MacPhee of the American Museum of Natural History in New York City, who points out that the relevant archaeological record contains barely a dozen examples of stone points embedded in mammoth bones, and none are known from other megafaunal remains - hardly what one might expect if hunting drove these animals to extinction. Furthermore, some of these species had a vast range, covering the whole continent. The Jefferson's Ground Sloth, for example, lived as far north as the Yukon and as far south as Mexico, which would have made hunting them in numbers sufficient to cause their extinction rather unlikely, he says.

MacPhee agrees that humans most likely brought about these extinctions, as well as others around the world that coincided with human arrival, but not directly. Rather than through hunting, he suggests that people may have introduced a deadly disease, perhaps through their dogs or accompanying vermin, which then spread wildly among the native species because of their low resistance to the new introductions. Repeated outbreaks of a deadly disease could thus quickly drive them to the point of no return. So far, MacPhee does not have empirical evidence for this theory, and it will not be easy to come by: such disease would kill far too quickly to leave its signature on the bones themselves. But he hopes that analyses of tissue and DNA from the most recent animal remains will eventually reveal the microbes responsible.

The third explanation for what brought on this North American extinction does not involve human beings. Instead, its proponents blame the loss on the climate. The Pleistocene epoch in question witnessed considerable climate instability, explains Russell W. Graham of the Denver Museum of Nature and Science. As a result, their regular habitats disappeared, and species that had once formed communities split apart.

For some animals, this brought opportunity. For much of the megafauna, however, the increasingly uniform terrain left them with shrinking geographical ranges - a death sentence for large animals, which need correspondingly large ranges. Although these creatures managed to maintain viable populations through most of the Pleistocene period, the final major climate fluctuation pushed them over the edge, Graham says.

For his part, Alroy is still convinced that human hunters were the destroyers of the giant animals. The overkill model explains everything the disease and climate scenarios explain, he asserts, and in addition makes accurate predictions about which species would eventually become extinct.

PASSAGE 3

Read the text and answer questions 27-40

Keeping the fun in funfairs

A. Fun is becoming a tricky issue for ride designers. In order to increase excitement, they have been ramping up the accelerations to create the most dizzying forces possible. But getting it right is far from easy. Err on the side of caution and people will not bother with a second ride. Go too far, however, and they may not be able to. Experts realize we are now at the limit of how much acceleration a human body can take, and designers are finding it hard to think up ways of keeping the public coming back for more. The problem is that true innovation has been lacking for a while, and fairground rides have become more about survival than actual enjoyment. So if our thrill-seeking bodies can really take no more, what is going to keep dragging us back to amusement parks? Creating something new and exciting, yet safe, is going to take some careful thought.

27B. When the Disney Corporation asked German designer Walter Stengel to design a giant loop ride for them in the 1970s, he went to NASA, the aeronautics and space foundation, to discover the effects of sustained acceleration on pilots. NASA's research suggests that the maximum level we can endure is 9g, with g being the standard unit of acceleration due to gravity. Go much beyond that and pilots pass out. Go further still and they suffer serious internal damage. So Stengel decided that the maximum vertical acceleration for the public should be 6g, and then only for a second or so. What is more, he put firm restrictions on the rate at which acceleration can increase - you will never go down a 45-degree ramp into a tight circular loop, for instance.

28C. But stricter safety limits only intensify the need to search for novel ways to thrill customers. Part of the problem is that no matter how exciting an attraction is, after a few rides the passengers will have some idea of what to expect. The next stage in designing rides, however, could throw predictability out of the window. This step has already been taken in the most recent waltzers, or tea-cup rides. Ride a waltzer and you sit in a car that spins on its own axis. The car is on a huge platform that also rotates. In the past you could take comfort from the fact that the spin was tightly controlled by gears that turned your car at a rate determined by the rotation speed of the whole ride. But the latest generation of waltzer cars spin freely, at a rate determined by the weight and position of the people in them. So you never have the same experience twice. "People seem to like these chaotic rides," says Stengel.

29D. Although seemingly a passport to endless thrills, chaos does have one rather obvious drawback: it is unpredictable. Despite complex calculations, designers can never be completely sure that something odd will not happen, especially since freely turning systems occasionally hit a resonance frequency. For example, if pushed at a particular frequency, a child on a swing would go over the top of the swing's frame. Similarly, if you drive a revolving waltzer car at its resonance frequency it could speed up uncontrollably. This could be very hazardous, according to Stengel. If a ride is subjected to unforeseen stresses, no one can guarantee that it will be able to cope.

30E. No one even knows what the safe limits of rotational force are, let alone its effect on the human body. Stengel has worked with the German Air Force, rotating volunteers head over heels while also making them cartwheel or pirouette like a ballet dancer. It emerged that if the pilots were turned on all three axes simultaneously, they became so nauseous they almost blacked out, and when they got off they could not walk. But what Stengel found particularly puzzling was that they also developed headaches and other problems about two days later. Since these effects are not understood, he tries to limit how people on his rides are rotated. We want to provide fun, not pain.

31F. With that goal in mind, Stengel feels that flinging people around in ever more chaotic machines is no longer the way forward. He believes that the sequence of accelerations, not their size, is what counts and that the way to make rides more fun is to put people through a carefully designed succession of relatively small accelerations. Other experts in this field agree, and it seems likely that designers could formulate profiles even for existing attractions that would lead to higher amusement value. Recent experiments testing the tolerances of Dutch military pilots to a range of accelerations have shown that tumbling around in machines does not have to be unpleasant. When the force is kept low, the subjects actually enjoy the experience.

32G. The fun seems to come from the unforeseen, particularly when an effect called the Coriolis Illusion comes into play. This is an agreeable tumbling feeling which occurs, for example, when the head is suddenly tilted while the subject is spinning with eyes closed. It appears that a roll which includes, for instance, an unexpected change of acceleration from a small negative - a feeling of weightlessness - to a small positive - a slight crushing sensation - has an extraordinary effect on people. If the theories of Stengel and other experts really do work, fairground fun might one day be measured in smiles, not screams.