资源描述
Extinction is for ever! 专家指出，地球上的动植物物种正以前所未有的速度消失，而无论是它们栖息地的丧失、污染、捕猎，还是全球变暖带来的间接影响，祸根都在于人类。,Before mans arrival on Earth，species became extinct quite naturally. Natural extinction happens when a species decreases in numbers gradually but steadily at the end of its evolutionary (进化) period on Earth. The length of this period depends on how well a species can get used to changes around it.This process of extinction can take a very long timesometimes several million yearsand the extinction of one species is immediately followed by the appearance of another in a continuous cycle.,The case of the dinosaurs is the most wellknown example of natural extinction.They appeared on Earth about 200 million years ago and dominated both land and sea for almost 100 million years.It is not certain why the dinosaurs became extinct，but their disappearance was a natural one and new species of animals evolved to replace them.,The rate of extinction has speeded up unnaturally over the last 400 years，rising sharply since 1900.This increase in the rate of extinctioh is directly related to the increase in the human population over the same period of time.The great number of humans has caused great damage to the planet.As wild habitats have been taken over，animals and plants have been forced into smaller and smaller areas，until some of them have become extinct.We have also polluted some habitats with chemicals and waste，making them unfit for wildlife.These causes of extinction are known as indirect (间接的) destruction.,Animals may also become extinct through direct destruction.This includes the hunting and catching of animals.Man has always hunted and killed wildlife but early humans lived more in harmony with naturethey killed animals for essential food and clothing.When firearms (火器) were invented，great destruction of species was possible.Animals have been，and still are，killed for meat，clothing，medicines，feathers，eggsand sometimes just for amusement.Some species are still caught in the wild for the live pet trade，even though their numbers are decreasing.,The extinction of at least 500 species of animals has been caused by man.Today there are about 5,000 endangered species and at least one species dies out every year.There are probably many more which become extinct without anyone knowing.,同学们,来学校和回家的路上要注意安全,同学们,来学校和回家的路上要注意安全,
展开阅读全文