高中英语人教版高三1 13

[06:24.42]It is thought that they arrived more than 30,000 years ago
[06:29.77]by crossing a land bridge from Asia to America.
[06:34.81]They lived by hunting and by gathering roots,nuts and wild fruits.
[06:41.29]They also hunted and killed wild animals.
[06:45.97]Around the year 1600
[06:49.73]the first horses were brought to the American contiment by the Spanish.
[06:55.58]Soon there were many wild horses across the country.
[07:00.73]The Native Americans caught and trained them
[07:05.09]and were then able to use them to carry their goods
[07:09.53]when they travelled from one campilng ground to another.
[07:13.97]Now that they could ride horses,it became easier to hunt the bison,
[07:19.93]a type of cattle which used to exist in huge numbers on the plains of America.
[07:26.28]The bison grows to a shoulder-height of 1.5 metres and can weigh 1100 kilograms.
[07:35.55]It was an important part of Native Americans'existence.
[07:40.70]Bison were killed for their meat,
[07:44.46]while their fur provided warm clothing during cold winters.
[07:50.05]The skins were used for making tents and water containers.
[07:55.38]Bones were carved into needles and tools,
[08:00.10]and the teeth were used to make necklaces.
[08:04.18]From 1830 onwards in the USA and from about 1870 in Canada,
[08:11.94]settlers began to move westwards and to take possession of the plains as their own.
[08:18.89]Large groups of Native Americans were forced to move away from their old hunting grounds.
[08:24.22]When they objected,they were killed.
[08:28.37]Fierce wars between Native Americans and European settlers broke out.
[08:34.25]Settlers made agreements with Native American chiefs
[08:39.21]but always broke them afterwards.
[08:43.05]In this way Native Americans were forced
[08:47.91]onto poor land that the settlers did not want.
[08:52.98]The settlers built railways across the plains and began to hunt even more bison.
[08:59.74]While early settlers had killed bison for food,
[09:04.10]now the killing became more widespread.
[09:08.18]They killed the bison,cut off the skins and left the bodies behind to rot.
[09:15.02]The bison skins were sent by rail to cities to be sold.
[09:20.90]It was also thought that by removing their main supply of food,
[09:25.86]the Native Americans would be forced to give in.
[09:30.54]Between 1850 and 1910 the bison population is thought to have fallen
[09:38.19]from 60 million to just a few hundred.
[09:43.76]The killing of the bison changed the whole wildlife of the plains.
[09:49.32]With fewer bison,grass shoots were not eaten so grass did not grow as strongly.
[09:56.17]Bison waste no longer fell on the ground to improve the soil,
[10:01.44]which as a result became less good for growing plants.
[10:06.80]The ground supported fewer plants,
[10:10.95]and the insects which lived on these plants died out.
[10:16.10]There was less food for birds and also for the prairie dog,
[10:21.87]a kind of animal which lives in holes in the ground.
[10:26.91]This in turn had an effect on the food supply for wolves.
[10:33.16]Thus one simple fact,a change in the number of bison,
[10:39.79]had an effect on the whole wildlife chain of the plains.