IELTSwithJurabek

PASSAGE 2

Read the text and answer questions 14-26

The Return of the Black-footed Ferret

The rise, fall, and rise of one of North America's rarest mammals

A The black-footed ferret (Mustela nigripes) is a small carnivorous mammal, similar in length to a domestic cat, and is the only ferret species native to North America. For at least 100,000 years they lived in an area that extended almost uninterrupted from southern Canada to northern Mexico. But numbers fell dramatically throughout the 19th and 20th centuries, to such an extent that the black-footed ferret was declared 'possibly extinct'. Then, almost miraculously, it returned from the dead.

B Within the genus Mustela, the ferret belongs to the subgenus Putorius, of which there are only three extant species: the European polecat; the Siberian polecat; and the black-footed ferret. The European polecat lives in open forests and meadows, and is thought to be the ancestor of the domestic ferret. The Siberian polecat looks more similar to the black-footed ferret, and leads a similar life on open grasslands and semi-desert regions across Russia, China and Siberia.

C Ferrets probably evolved in Europe between three and four million years ago, and dispersed into North America across an ice-age land bridge at least two million years ago. They then advanced south-eastward to the Great Plains through ice-free passageways. The black-footed ferret's history in North America is inextricably linked to the creature which became almost its sole prey - the prairie dog. These small rodents are a type of ground squirrel which live in networks of underground tunnels, known as burrows. The black-footed ferret's short legs and long, slim body make it superbly adapted for life in the burrow systems of its prey.

D Spending virtually all their time underground, black-footed ferrets have always been elusive. Ferrets are never mentioned in accounts of the early explorers or pioneers who crossed the Great Plains by wagon train during the early to mid-1800s. They were, however, occasionally listed in records kept by fur companies in the upper Missouri River basin during that period. Black-footed ferrets were not officially recognized by scientists until 1851, when a description and drawing of one featured in a book by the renowned naturalist John James Audubon and Reverend John Bachman. For over two decades, no other specimens were seen, and scientists even began to question their existence. Then, in 1874, a Dr Elliott Coues issued a request for specimens through the popular magazine The American Sportsman, and was soon rewarded. With these he was able to add to Audubon's description, and confirm the existence of the black-footed ferret.

E In the decades that followed, European settlement across North America changed the landscape dramatically, with vast areas being transformed into crop and grazing land. As agriculture and ranching expanded, the black-footed ferret's main prey, the prairie dog, became a target for extermination, and was eventually eradicated from 98% of its original range. The result was devastating for black-footed ferret populations throughout the continent. Even today, the prairie dog is still trapped and poisoned, because ranchers believe that it competes with livestock for grass - a notion that belies recent research showing that prairie dogs and cattle feed on different plants.

F As late as the 1950s, black-footed ferrets were still thought to occur in low densities throughout most of their historic range. However, by the 1960s, the only known population was a small colony in south-western South Dakota. In 1967, the US Fish and Wildlife Service classified the black-footed ferret as an endangered species, and in 1974 the South Dakota colony disappeared. Finally, in January 1979, the last ferret in a captive-breeding programme died - making the black-footed ferret the first mammal to be declared extinct in the Northern Hemisphere in the 20th century.

G But that was not the end of the story. In 1981, a farm dog called Shep carried an odd-looking creature to its owner's ranch house in Meeteetse, Wyoming. The rancher put the carcass in his fridge and alerted state officials. The dog had found a black-footed ferret - which led to efforts again being made to save the species. An intensive search in the region led to the discovery of a 130-strong population. This wild colony thrived for a while, but disaster struck again and the number of animals was drastically reduced by the disease distemper - probably contracted from domestic dogs. In 1985, in a final bid to save the species, the remaining 18 individuals in the colony were trapped for a new captive-breeding programme. This project was successful, and there are now more than 2,000 black-footed ferrets in captivity across the US and Canada.

H A reintroduction programme began in 1991. Before they can be reintroduced into their natural habitat, captive ferrets have to learn to hunt and escape from predators. They are first held in pens on the prairie, where they are fed prairie dog meat. Once the ferrets associate these animals with food, keepers unleash live prey into the pens, and they soon realise that they must hunt or go hungry. Then, to get the ferrets used to living underground, they have to be tempted into the holes with food.

I Reintroduction programmes are now underway across central USA, but there is one location that is key. The Janos area on the US-Mexico border is the obvious place to reactivate the continent's wild black-footed ferret population, being one of the most important remaining grassland ecosystems in the Americas, and home to an abundance of prairie dogs. At the last report, the future of the black-footed ferret finally looks promising.

You should spend about 20 minutes on Questions 27-40, which are based on Reading Passage 3 below.

Living dunes

27A Armies of giant sand dunes are advancing across the world's deserts, engulfing anything that crosses their path. They are tens of metres tall and hundreds of metres long. Fortunately, they aren't going very fast. Even the smallest, speediest dunes only travel about 100 metres over the course of a year, while the bigger ones, which weigh something in the order of 10,000 tonnes, barely move one metre in that time. However, their insidious creep can have serious consequences if there is an oil installation or a railway line in their path.

28B About 47% of the world's land mass, including Antarctica, most of Australia and large areas of Africa, is classified as arid or semi-arid desert. Only around 20% of that is sand-covered, however, and over half of that is classified as 'linear' sand dunes. These form in a long curving wave, as a result of wind blowing strongly from several quarters, flipping them from side to side. Although linear dunes are static, sand blowing off them can cause problems for desert villages, burying crops and buildings.

29C Moving dunes make up just a small percentage of the rest, but they are of the most interest to scientists. They are known as 'barchans': heavy, crescent-shaped sand piles with a ridged crest and two elongated arms, one curving away to either side. 'Barchan dunes' only tend to form where you have one-directional winds on the edge of sandy deserts near coastal areas, says Giles Wiggs, a geomorphologist at Oxford University, who has been studying the formation and movement of sand dunes for more than a decade.

30D But even with strong winds, how can entire barchans move while retaining their form? That question was first answered in the mid-20th century by British explorer Ralph Alger Bagnold, and his answer hinges on the fact that dunes aren't solid, but granular. Bagnold figured out how barchan dunes are able to move grain by grain. Imagine a single grain of sand being blown up the back of a dune by the wind and deposited on the top. More grains follow the same pattern, until the accumulated weight of piled-up sand finally pushes the top down the dune face. The grain tumbles, then stops on the face until subsequent mini-avalanches bury it. Eventually, it reappears at the back of the dune, ready to repeat the process. As this happens to every grain of sand in the dune, the whole thing creeps in the direction of the prevailing wind.

31E The relationship between the wind and barchan dunes is complex. As a dune grows, it modifies the speed and course of the wind, which in turn alters how that dune and its neighbours evolve. 'Interestingly the dune can regulate its own shape, and maintain it as it moves,' says Dr Stephane Douady, a physicist at Ecole Normale Superieure (ENS) in France. 'Even when two dunes collide, they quickly take on their distinctive shapes again. It is like a living organism.'

32F Douady and his colleagues have also been studying an even odder phenomenon than moving dunes: some barchans actually sing. Local legends attributed the sounds to dangerous spirits which were trying to trap unwary travellers. Douady is more pragmatic. "It's a strong booming noise with a low frequency," he explains, making a noise like a foghorn to demonstrate. "It can last for a long time - up to several minutes. It's a very loud sound and you don't understand where it's coming from when you first hear it." There are about 50 dunes distributed across 35 deserts round the world that are known to sing. Douady says the sound is caused by the way sand avalanches down the faces of particular dunes. Rather than tumbling randomly, the sand grains flow in synchrony and set each other vibrating like the membrane on a gigantic loudspeaker. The synchronisation causes the air to move in and out between the grains, creating a powerful sound wave.

33G What really surprised the scientists, however, was that they were able to take samples of the singing sand back to France and replicate the sound at ENS, proving that it's the sand, not the dune shape, that causes the sound. Their studies show the grains are a uniform shape, wellrounded from years of striking each other, and that the variations in size affect the tone. Crucially, the grains are coated with a special veneer, which Douady calls 'desert glaze', made from a precise combination of minerals from surrounding rocks including iron, aluminium, manganese, silicon and calcium. The team found that after a month or so, the veneer wore off and the grains lost their 'voice'. "We managed to reproduce the desert glaze and then the grains started to sing again," says Douady. "We tried putting the coating onto different grains, but they weren't round enough and it didn't work. But some American colleagues made some artificial grains and managed to make them sing, after covering them in desert glaze." Douady has now made recordings of dunesong from all over the world which is to be made into a CD.