4．How do predators （獵食動(dòng)物） affect populations of the prey （獵物） 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（多樣性），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 本文主要說(shuō)明了獵食動(dòng)物數(shù)量與被獵食動(dòng)物數(shù)量的關(guān)系，并舉出山貓和雪兔，狼和麋鹿等例子來(lái)具體說(shuō)明．
解答 73．B．寫作意圖題．根據(jù)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為了說(shuō)明："并不像人們認(rèn)為的那樣，獵食動(dòng)物影響獵物的數(shù)量，獵物的數(shù)量受食物、疾病等其他因素的影響．"，故選B．
74．D．詞意猜測(cè)題．根據(jù)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："獵物數(shù)量減少，獵食動(dòng)物的食物就不夠吃，數(shù)量也會(huì)減少，這樣的話，獵物數(shù)量就會(huì)恢復(fù)．"，故選D．
75．A．細(xì)節(jié)理解題．根據(jù)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．"可知在實(shí)驗(yàn)室里，獵食動(dòng)物會(huì)吃完獵物最后自己餓死，而在野外，獵物有安全區(qū)的話就不會(huì)滅絕，只是減少，所以實(shí)驗(yàn)室的研究結(jié)果不能解釋野外的獵食動(dòng)物和獵物的數(shù)量變化，故選A．
76．C..推理判斷題．根據(jù)倒數(shù)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．"對(duì)雪兔的關(guān)鍵捕食者是加拿大山貓．加拿大天貓顯示豐富的10年周期（大量）與野兔的豐度周期．隨著兔數(shù)下降，天貓的數(shù)量也減少了，因?yàn)樗麄兊氖澄锕⿷?yīng)減少了，可以推出山貓隨雪兔數(shù)量變化而變化，故選C．
77．B．主旨大意題．根據(jù)第一段的"How do predators （獵食動(dòng)物） affect populations of the prey （獵物） animals？The answer is not as simple as might be thought．"可知本文的話題是獵食動(dòng)物和獵物的數(shù)量的關(guān)系，接下來(lái)以麋鹿和狼的例子，the North American snowshoe hare和willow branches以及 the Canada lynx的數(shù)量變化關(guān)系加以說(shuō)明．可以看出本文主要講的是獵食動(dòng)物對(duì)獵物的數(shù)量影響，故選B．
點(diǎn)評(píng) 科教類閱讀難度比一般閱讀較大，它存在的生詞比較多，因此考生應(yīng)當(dāng)綜合略讀、跳讀和查讀，猜詞等閱讀技巧來(lái)做題，平時(shí)也應(yīng)加強(qiáng)練習(xí)．
