人教高中英语高二下UNIT16-2（在线收听）

  THE BISON ON THE PLAINS OF AMERICA
        The first settlers on the plains of America were Native Americans. They arrived more than 30,000 years ago by crossing a land bridge from Asia to America. The Native Americans lived by gathering roots, nuts and wild fruit and hunting wild animals.
        They hunted bison, a type of wild animals which used to exist in huge numbers on the plains of America. The bison grows to a shoulder-height of 1.5 metres and can weigh 1,100 kilogrammes. It was an important part of Native Americans' life. Bison were killed for their meat, while their fur provided warm clothing during cold winters. The Native Americans made tents and water containers from the skins and tools from the bones. The teeth were used to make necklaces.
        From about 1830 onwards in the USA, European settlers began to move westwards. Large groups of Native Americans were forced to move away from their old hunting grounds. When they resisted, they were killed. There were many fierce wars between Native Americans and European settlers. The American government made agreements with Native American chiefs but always broke them afterwards. In this way, Native Americans were forced onto poor land that the settlers did not want.
       The settlers built railways across the plains and began to hunt even more bison. While early settlers had killed bison for food, now the killing became more widespread. They killed the bison, cut off the skins and left the bodies behind to rot. The bison skins were sent by rail to cities to be sold. Between 1850 and 1910 the bison population is thought to have fallen from 60 million to just a few
hundred.
       The killing of the bison changed the whole wildlife of the plains. With fewer bison, grass shoots were not eaten, so grass did not grow as strongly. Bison waste no longer fell on the ground to improve the soil, which as a result became less good for growing plants. The ground supported fewer plants , and the insects which lived on these plants died out. There was less food for birds and also for the prairie dog, a kind of animal which lives in holes in the ground. This in turn had an effect on the food supply for wolves. Thus one simple fact, a change in the number of bison, had an effect on the whole wildlife chain of the plains.