How do predatorsaffectpopulations of thepreyanimals? The answer is not as simple as might be thought. Moose reached Isle Royale in LakeSuperiorbycrossingover winter ice and multiplied 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 moosepopulation. Careful studies have demonstrated, however, that this is not the case. The wolves eat mostly old or diseased animals that would notsurvivelong anyway. In general, the moosepopulationis controlled by foodavailability,diseaseand other factors rather than by wolves.

Whenexperimentalpopulations are set up under simplelaboratoryconditions, thepredatoroften exterminates its pre and then becomesextinctitself, having nothing left to eat. However, if safe areas like thosepreyanimals have in the wild are provided, thepreypopulationdrops to lowlevelbut notextinction. Lowpreypopulationlevels then provide inadequate food for the predators, causing thepredatorpopulationtodecrease. When this occurs, thepreypopulationcan rebound. In this situation thepredatorandpreypopulationmay continue in this cyclicalpatternfor some time.

Populationcycles arecharacteristicof small mammals, and they sometimesappearto be brought about by predators. Ecologists studying hare populations have found that the North American snow shoe hare follows aroughlyten-yearcycle. Its numbers fall tenfold to thirty in atypicalcycle, and a hundredfold change canoccur. Two factorsappearto be generating the cycle: food plants and predators.

The preferred foods of snowshoe hares arewillowandbirchtwigs. As haredensityincreases, thequantityof these twigs decreases, forcing the hares to feed on low-quality high-fiber food. Lowerbirth rates, lowjuvenilesurvivorship, and low growth rates follow, so there is a correspondingdeclinein hareabundance. Once the harepopulationhas declined, it takes two to three year for thequantityof twigs torecover.

A keypredatorof the snowshoe hare is the Canada lynx. The Canada lynx shows a ten-yearcycleofabundancethat parallels theabundancecycleof hares. As hare numbers fall, so do lynx numbers, as their foodsupplydepleted.

What causes the predator-prey oscillations? Do increasing number of haresleadto overharvesting of plants, which in turn results in reduced hare populations, or do increasing numbers of lynxleadto overharvesting hares? Fieldexperiments carried out by Charles Krebs and coworkers in 1992 provide an answer. Krebs investigatedexperimentalplots in Canada’s Yukonterritorythat contained hare populations. When food was added to those plots (no food effect) and predators were excluded (nopredatoreffect) from anexperimentalarea, hare numbers increased tenfold and stayed there—thecyclewas lost. However, thecyclewas retained if either of the factors was allowed tooperatealone: if predators were excluded but food was not added (food effect alone), or if food was added in the presence of predators (predatoreffect alone). Thus both factors canaffectthecycle, which, in practice, seems to be generated byconjunctionof the two factors.

Predators are anessentialfactorin maintaining communities that are rich anddiverseinspecies. Without predators, thespeciesthat is the best competitor for food,shelter, nesting sites, and otherenvironmentalresources tends todominateandexcludethespecieswith which it competes. Thisphenomenonis known as “competitor exclusion”. However, if thecommunitycontains apredatorof the strongest competitorspecies, then thepopulationof that competitor is controlled. Thuseventhe lesscompetitivespeciesare able tosurvive. For example, sea starspreyon avarietyof bivalve mollusks and prevent these bivalves from monopolizing habitats on the sea floor. This opens upspacefor many other organisms. When sea stars are removed,speciesdiversityfallssharply. Therefore, from the stand point ofdiversity, it is usually a mistake toeliminateamajorpredatorfrom acommunity.

Questions:
question_stem: In paragraph 1, why does the author discuss the moose and wolves on Isle Royal?
choices: tag: A
text: To provide an example of predators moving to new habitats by following migrating prey

tag: B
text: To show that the interactions between predator populations and prey populations are not always might be expected

tag: C
text: To suggest that prey populations are more influenced by predation than food availability and disease

tag: D
text: To argue that studies of geographically isolated populations tend not to be useful to naturalists
answer_key: B
explanation: B 对应文章的第四句，Careful studies…not the case.这句话前面说人们觉得狼对控制鹿的数量控制起到了重要作用，这句话对其进行了否定，对应了 B 选项。A 与原文没有关系。C 与原文第一段最后一句话矛盾。D 也和原文无关。

question_stem: The word “rebound”in the passage (paragraph 2) is closest in meaning to
choices: tag: A
text: escape

tag: B
text: recover

tag: C
text: survive

tag: D
text: resist
answer_key: B
explanation: B  rebound: 回弹，回升。A 是逃走，B 是恢复，C 是存活，D 是反抗。根据词意，选择 B。

question_stem: Paragraph 2 implies which of the following about experimental environments in which predators become extinct?
choices: tag: A
text: They may yield results that do not accurate predict changes of populations in the wild.

tag: B
text: In these environments, the prey species is better adapted than the predator species.

tag: C
text: These environments are appropriate only for studying small populations of predators and prey.

tag: D
text: They are unrealistic because some predators are also the prey of other predators.
answer_key: A
explanation: A 用 experimental environments 定位到原文第一句，第一句说了 experimental environments 的结果是捕食者在消灭了所有猎物之后，自己最终也灭绝了。但后面立刻出现了 however。证明出现了转折，后文在一定程度上否定了前面的观点，说在自然界中并不会灭绝。所以选 A。B 和原文主题没关系，C 虽然没有明显错误，但没有表现出真正的主题，重点是在真实生活中的情况。D 原文也没提到。

question_stem: Which of the following can be inferred from paragraphs 2 and 3 about the small mammals that experience population cycles?
choices: tag: A
text: Their population cycles are not affected by predators.

tag: B
text: Their predators’populations periodically disappear.

tag: C
text: They typically undergo ten-year cycles.

tag: D
text: They have access to places safe from predators.
answer_key: D
explanation: D 从两个段落中的内容进行推断，有点难度，也没有特别好的定位词，可以用选项去原文进行匹配。需要大量阅读。 A 选项对应第三段最后一句话：Two factors appear to be generating the cycle: food plants and predators. 这句话说了一项 population cycle 的两个因素之一就是：捕食者。所以，A 错误。B 对应第二段的第二句话后面：However, if safe areas like those prey animals have in the wild are provided, the prey population drops to low levels but not to extinction. 这里说了，在正常情况下捕食者不会消失，所以 B 选项错误。C 对应第三段第二句：Ecologists studying hare populations have found that the North American snow shoe hare follows a roughly ten-year cycle. 这句话出现了 ten-year cycle，但是仅仅是对野兔的描写，并不适用于全部哺乳动物。D 对应第二段第二句：However, if safe areas like those prey animals have in the wild are provided, the prey population drops to low levels but not to extinction. 说被捕食者在现实生活中可以居于安全的远离捕食者的地方。所以选 D。

question_stem: The word “roughly”in the passage is closest in meaning to
choices: tag: A
text: usually

tag: B
text: repeating

tag: C
text: approximately

tag: D
text: observable
answer_key: C
explanation: C roughly: 粗略地。A 是通常的，B 是重复的，C 是大约的，D 是可观察的。根据词义选 C。

question_stem: The word “generating”in the passage is closest in meaning to
choices: tag: A
text: producing

tag: B
text: changing

tag: C
text: speeding up

tag: D
text: smoothing out
answer_key: A
explanation: A  generate: 使形成，发生，产生。A 是产生，B 是改变，C 是加速，D 是渐出。根据意思选择 A。

question_stem: According to paragraph 4, all of the following are true of the food of snowshoe hares EXCEPT
choices: tag: A
text: The preferred food fore hares consists of willow and birch twigs.

tag: B
text: High fiber food is the most nutritious for hares.

tag: C
text: Depletion of the supply of willow and birch twigs cause low birth and growth rates.

tag: D
text: The food supply takes two or three years to recover after a peak in hare population density.
answer_key: B
explanation: B 排除题，选项 A 对应第一句，正确。选项 B 对应第二句，原文说 high-fiber 意味着 low quality，所以 B 错误，选。C 对应第三句，正确，不选。D 对应最后一句，正确，不选。

question_stem: The word “conjunction”in the passage (paragraph 6) is closest in meaning to
choices: tag: A
text: determination

tag: B
text: combination

tag: C
text: alternation

tag: D
text: transformation
answer_key: B
explanation: B conjunction: 结合，同时发生。A 是决定，B 是结合，C 是交替，D 是转化。从词意上来看，B 正确。

question_stem: According to paragraph 5, which of the following statements best characterizes the abundance cycle of the Canada lynx?
choices: tag: A
text: It closely follows the cycle the snowshoe hare.

tag: B
text: When the numbers of lynx fall, the numbers of snowshoe hares soon decrease.

tag: C
text: When hare numbers decrease, lynx numbers increase.

tag: D
text: It is not clearly related to the availability of lynx food.
answer_key: A
explanation: A。原文说的是由于野兔是山猫的食物，所以野兔数量下降，山猫数量也会随之下降。所以选择 A。B 的逻辑关系不对，并非野兔跟着山猫变化。C 更不对，两者数量是同时增减的。D 也不符合原文。

question_stem: According to paragraph 6, which of the following was true of the hare population cycle in Krebs’s experiment?
choices: tag: A
text: The effects of providing food while at the same time introducing predators cancelled each other, so there was no cycle.

tag: B
text: The cycle existed when either the food supply was limited or there were predators.

tag: C
text: There was a cycle when there were no predators and food was supplied.

tag: D
text: If the hares had places to hide from the lynx, the hare population increased tenfold and then remained at that level.
answer_key: B
explanation: B 没有特别好的定位词，但是文章结构比较简单，段落中间描述实验的具体情况，随后引出实验的结论，答案出现在原文的倒数第二句话： However, the cycle was retained if either of the factors was allowed to operate alone: if predators were excluded but food was not added (food effect alone), or if food was added in the presence of predators (predator effect alone).总结就是：两个因素只要其中一个存在就会导致 cycle。对应 B 选项，所以 B 正确。两个同时都没有的时候，就不存在 cycle 了。A 选项的两个因素互相抵消，这样的关系在原文没有论证过，所以 A 错误；C 说两因素都不存在的时候，cycle 还存在，这和原文矛盾；D 选项跟两个因素都没关系，不选。

question_stem: According to paragraph 7, which of the following statements correctly characterizes the effect of sea stars on the ecosystem in which they are predators of bivalves?
choices: tag: A
text: Bivalve population are kept low, allowing species that compete with bivalves to survive.

tag: B
text: The numbers of most species of bivalves are greatly reduced, leaving the bivalve species that is the strongest competitor to dominate among the survivors.

tag: C
text: Biological diversity begins to decrease because many bivalve species disappear.

tag: D
text: Sea stars dominate at first but then die off because of the depleted food supply.
answer_key: A
explanation: A 用 sea stars 定位到 For example 开始的两句话。For example, sea stars prey on a variety of bivalve mollusks and prevent these bivalves from monopolizing habitats on the sea floor. This opens up space for many other organisms. When sea stars are removed, species diversity falls sharply.这两句话说海星的存在对双壳动物是一个限制，使得别的生物有了生存的空间。符合 A 选项。B 不对，B 和原文矛盾；C 不对，bivalve 控制后，diversity 应该是上升的；D 也不对，原文没有说 Sea stars 后来死了。

question_stem: According to paragraph 7, which of the following is true of the phenomenon of competitor exclusion?
choices: tag: A
text: It results in more diverse communities.

tag: B
text: It requires the presence of predators.

tag: C
text: It affects all competitions equally.

tag: D
text: It happens only when there is a dominant competitor.
answer_key: D
explanation: D  根据 competitor exclusion 定位，这句话说 this phenomenon，那么对于现象描写应该在前面： Without predators, the species that is the best competitor for food, shelter, nesting sites, and other environmental resources tends to dominate and exclude the species with which it competes.往前找可以看到这个现象是要在 “best competitor” 存在的情况，这个竞争者还 tend to dominate，符合 D。A 并不是这个现象的直接作用，其直接作用是对其他 competitor 的限制。B 虽然正确，但没指出其唯一性。C 没提到。