4．How do predators （猎食动物） affect populations of the prey （猎物） animals？The answer is not as simple as might be thought．The Moose（麋鹿） reached Isle Royale in Lake Superior by crossing over winter ice and bred freely there in isolation without predators．When wolves later reached the island，naturalists widely assumed that the wolves would play a key role in controlling the moose population．Careful studies have demonstrated，however，that this is not the case．The wolves eat mostly old or diseased animals that would not survive long anyway．In general，the moose population is controlled by food availability，disease and other factors rather than by wolves．
When experimental populations are set up under simple laboratory conditions，the predator often wipes out its prey and then becomes extinct itself．However，if safe areas like those prey animals have in the wild are provided，the prey population drops to low level but not extinction．Low prey population levels then provide inadequate food for the predators，causing the predator population to decrease．When this occurs，the prey population can rebound．In this situation the predator and prey population may continue in this cyclical pattern for some time．
Population cycles are characteristic of small mammals，and they sometimes appear to be brought about by predators．Ecologists studying hare populations have found that the North American snowshoe hare follows a roughly ten-year cycle．Its numbers fall tenfold to thirty in a typical cycle，and a hundredfold change can occur．Two factors appear to be generating the cycle：food plants and predators．
The preferred foods of snowshoe hares are tender willow branches．As the hare population increases，the quantity of these branches decreases，forcing the hares to feed on low-quality high-fiber food．Lower birth rates and low growth rates follow，so there is a corresponding decline in hare abundance．Once the hare population has declined，it takes two to three year for the quantity of branches to recover．
A key predator of the snowshoe hare is the Canada lynx．The Canada lynx shows a ten-year cycle of abundance（大量） that parallels the abundance cycle of hares．As hare numbers fall，so do lynx numbers，as their food supply decreased．
Predators are an essential factor in maintaining communities that are rich and diverse in species．Without predators，the species that is the best competitor for food，shelter，and other environmental resources tends to dominate and exclude the species with which it competes．This phenomenon is known as"competitor exclusion"．However，if the community contains a predator of the strongest competitor species，then the population of that competitor is controlled．Thus even the less competitive species are able to survive．From the stand point of diversity（多样性），it is usually a mistake to eliminate a major predator from a community．
73．The author uses the example of the moose and wolves on Isle Royale toB．
A．provide evidence that predators influence prey populations
B．question the belief in the effect of predators on prey populations
C．demonstrate predator population grows faster than that of the prey
D．prove that studies of isolated populations tend to be useful
74．The word"rebound"in the passage is closest in meaning toD．
A．React
B．Resist
C．Remain
D．Recover
75．Which of the following is true according to the passage？A
A．Laboratory results can't explain the changes in predator and prey populations of the wild．
B．The growth of hare population may lead to a corresponding increase in its birth rates．
C．The experimental environments can promote the growth of predator and prey populations．
D．The existence of a major predator in a community is a threat to the diversity of species．
76．What can we conclude from the passage about the cycle of the Canada lynx？C
A．When hare numbers decrease，lynx numbers increase．
B．It has a great effect on the number of snowshoe hare．
C．It closely follows the cycle of the snowshoe hare．
D．It is not directly related to the availability of lynx food．
77．What is the best title of this passage？B
A．What role predators play in keeping competitor species．
B．Whether predators have an impact on prey populations．
C．A study of the populations of the predator and prey animals．
D．Contributing factors of the changes in animal populations．
分析 本文主要说明了猎食动物数量与被猎食动物数量的关系，并举出山猫和雪兔，狼和麋鹿等例子来具体说明．
解答 73．B．写作意图题．根据第一段"Careful studies have demonstrated，however，that this is not the case．""In general，the moose population is controlled by food availability，disease and other factors rather than by wolves．"可知作者之所以用麋鹿和狼的例子就是为了说明："并不像人们认为的那样，猎食动物影响猎物的数量，猎物的数量受食物、疾病等其他因素的影响．"，故选B．
74．D．词意猜测题．根据第二段"Low prey population levels then provide inadequate food for the predators，causing the predator population to decrease．When this occurs，the prey population can rebound．"可知："猎物数量减少，猎食动物的食物就不够吃，数量也会减少，这样的话，猎物数量就会恢复．"，故选D．
75．A．细节理解题．根据第二段"When experimental populations are set up under simple laboratory conditions，the predator often wipes out its prey and then becomes extinct itself．However，if safe areas like those prey animals have in the wild are provided，the prey population drops to low level but not extinction．"可知在实验室里，猎食动物会吃完猎物最后自己饿死，而在野外，猎物有安全区的话就不会灭绝，只是减少，所以实验室的研究结果不能解释野外的猎食动物和猎物的数量变化，故选A．
76．C..推理判断题．根据倒数第二段"A key predator of the snowshoe hare is the Canada lynx．The Canada lynx shows a ten-year cycle of abundance（大量） that parallels the abundance cycle of hares．As hare numbers fall，so do lynx numbers，as their food supply decreased．"对雪兔的关键捕食者是加拿大山猫．加拿大天猫显示丰富的10年周期（大量）与野兔的丰度周期．随着兔数下降，天猫的数量也减少了，因为他们的食物供应减少了，可以推出山猫随雪兔数量变化而变化，故选C．
77．B．主旨大意题．根据第一段的"How do predators （猎食动物） affect populations of the prey （猎物） animals？The answer is not as simple as might be thought．"可知本文的话题是猎食动物和猎物的数量的关系，接下来以麋鹿和狼的例子，the North American snowshoe hare和willow branches以及 the Canada lynx的数量变化关系加以说明．可以看出本文主要讲的是猎食动物对猎物的数量影响，故选B．
点评 科教类阅读难度比一般阅读较大，它存在的生词比较多，因此考生应当综合略读、跳读和查读，猜词等阅读技巧来做题，平时也应加强练习．
