The "balance of nature" is not an empty phrase. Nature provides a population to occupy a suitable environment and cuts down surplus population to fit the available food supply. One means of reducing surplus population is predators(食肉动物); others are parasites(寄生虫)and diseases. Also, population density produces nervous disorders and even drives animals to mass migrations, like the lemmings(旅鼠)of Norway who plunge into(跳入)the sea.

That predators populations increase to control other animals has long been known. Many years ago, the Hudson's Bay Company records revealed that the fox population went up and down about a year after the rabbit population had gone up and down.

Sometimes a situation occurs in which the predators population is reduced to a level below that which nature can readily replace. On Valcour Island in Lake Champlain(New York), a costly campaign resulted in the elimination of predatory animals only to have birds and small animals—including grouse(松鸡)and hares, popular game(猎物)—increase for four years afterward. Then lacking predator control, nature resorted to cut down these populations.

Jamaica had an example of nature's persistence in providing animals for existing habits. Sugar planters, about 75 years ago, imported mongooses to control rats. The mongooses killed off the rats and, with plentiful food, multiplied. Rats became scarce and the mongooses ate lambs, kids, puppies(小狗), and wildlife. Eventually, food became scarce and the mongoose's population declined.

26. According to the article, the phrase "balance of nature" means ______.

A. the relation of wildlife to man

B. the adequacy of the food supply to support its animal population

C. the ratio of small game to predators

D. the destruction of predators

答案：B

27. The effect of predator control over other animal populations ______.

A. has just been learned

B. is not recent knowledge

C. is not important now

D. was learned in Norway

答案：B

28. When predators control is not available, nature brings animal population into balance by resorting to ______.

A. hunters

B. diseases

C. storms and flood

D. forest fires

答案：B

29. When the mongooses in Jamaica killed off the rats, they ______.

A. quickly died for lack of food

B. attacked humans

C. became problems themselves

D. ate the sugar crop

答案：C

30. Implied but not stated: ______.

A. Sugar planters imported mongooses to control rats

B. Man should never tamper(损害)with nature

C. To upset the balance of nature can be troublesome

D. Man has complete control over nature

答案：C

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