『新东方在线托福听力』为托福考生准备了ETS托福TPO/Official 27 Passage 3阅读练习，TPO/Official 27 Passage 3原文答案解析，TPO/Official 27 Passage 3阅读原文练习音频翻译。

托福TPO/Official 27 Passage 3阅读原文题目答案及解析-新东方托福官方网校手机版

How do predators affect populations of the prey animals?

答案并不像人们想象的那么简单。

The answer is not as simple as might be thought.

驼鹿越过冬季的冰层到达苏必利尔湖的罗亚尔岛，在那里自由繁殖，没有捕食者。

Moose reached Isle Royale in Lake Superior by crossing over winter ice and multiplied freely there in isolation without predators.

当狼群后来抵达该岛时，博物学家们普遍认为，狼群将在控制驼鹿数量方面发挥关键作用。

When wolves later reached the island, naturalists widely assumed that the wolves would play a key role in controlling the moose population.

然而，仔细的研究表明，情况并非如此。

Careful studies have demonstrated, however, that this is not the case.

狼吃的大多是年老或有病的动物，反正活不了多久。

The wolves eat mostly old or diseased animals that would not survive long anyway.

一般来说，驼鹿的数量是由食物、疾病和其他因素控制的，而不是由狼群控制的。

In general, the moose population is controlled by food availability, disease, and other factors rather than by the wolves.

当实验种群在简单的实验室条件下建立起来时，捕食者通常会消灭它的猎物，然后自己灭绝，没有留下任何食物。

When experimental populations are set up under simple laboratory conditions, the predator often exterminates its prey and then becomes extinct itself, having nothing left to eat.

然而，如果提供像野生猎物那样的安全区域，猎物数量会下降到低水平，但不会灭绝。

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.

较低的猎物数量为捕食者提供了不足的食物，导致捕食者数量减少。

Low prey population levels then provide inadequate food for the predators, causing the predator population to decrease.

当这种情况发生时，猎物的数量就会反弹。

When this occurs, the prey population can rebound.

在这种情况下，捕食者和猎物的数量可能会在一段时间内保持这种循环模式。

In this situation the predator and prey populations may continue in this cyclical pattern for some time.

种群循环是某些小型哺乳动物的特征，它们有时似乎是由捕食者带来的。

Population cycles are characteristic of some species of small mammals, and they sometimes appear to be brought about by predators.

生态学家在研究野兔种群时发现，北美的雪鞋野兔大约有一个10年的周期。

Ecologists studying hare populations have found that the North American snowshoe hare follows a roughly ten-year cycle.

在一个典型的周期中，它的数量会下降10倍至30倍，甚至可能发生100倍的变化。

Its numbers fall tenfold to thirtyfold in a typical cycle, and a hundredfold change can occur.

产生这种循环的因素有两个：食用植物和捕食者。

Two factors appear to be generating the cycle: food plants and predators.

雪兔最喜欢的食物是柳树和桦树的树枝。

The preferred foods of snowshoe hares are willow and birch twigs.

随着野兔密度的增加，这些细枝的数量减少，迫使野兔以低质量、高纤维的食物为食。

As hare density increases, the quantity of these twigs decreases, forcing the hares to feed on low-quality, high-fiber food.

较低的出生率，较低的幼兽存活率，以及随之而来的较低的生长速度，使得野兔数量相应减少。

Lower birth rates, low juvenile survivorship, and low growth rates follow, so there is a corresponding decline in hare abundance.

一旦野兔数量减少，需要两到三年的时间来恢复嫩枝的数量。

Once the hare population has declined, it takes two to three years for the quantity of twigs to recover.

雪鞋野兔的主要捕食者是加拿大猞猁。

A key predator of the snowshoe hare is the Canada lynx.

加拿大猞猁的丰度周期为10年，与野兔的丰度周期相似。

The Canada lynx shows a ten-year cycle of abundance that parallels the abundance cycle of hares.

随着野兔数量的增加，猞猁的数量也在增加，这是对猞猁食物供应增加的反应。

As hare numbers increase, lynx numbers do too, rising in response to the increased availability of lynx food.

当野兔数量减少时，猞猁数量也会减少，因为它们的食物供应已经耗尽。

When hare numbers fall, so do lynx numbers, as their food supply is depleted.

是什么导致了捕食者-被捕食者的振荡？野兔数量的增加会导致植物的过度捕猎，进而导致野兔数量的减少吗？还是猞猁数量的增加会导致野兔的过度捕猎呢？

What causes the predator-prey oscillations? Do increasing numbers of hares lead to overharvesting of plants, which in turn results in reduced hare populations, or do increasing numbers of lynx lead to overharvesting of hares?

克雷布斯（Charles Krebs）和他的同事在1992年进行的实地实验提供了答案。

Field experiments carried out by Charles Krebs and coworkers in 1992 provide an answer.

克雷布斯调查了加拿大育空地区含有野兔种群的试验田。

Krebs investigated experimental plots in Canada’s Yukon territory that contained hare populations.

当食物被放到这些试验区（没有食物效应），食肉动物被排除在试验区之外（没有捕食者效应），野兔的数量增加了十倍，并保持在那里——这样的循环就消失了。

When food was added to these plots (no food effect) and predators were excluded (no predator effect) from an experimental area, hare numbers increased tenfold and stayed there—the cycle was lost.

然而，如果任一个因素单独起作用，这个循环就会被保留下来：如果排除了捕食者，但不增加食物（只增加食物效应），或者在捕食者存在的情况下增加食物（只增加捕食者效应）。

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).

因此，这两个因素都可以影响周期，而实际上，周期似乎是由这两个因素的结合产生的。

Thus, both factors can affect the cycle, which, in practice, seems to be generated by the conjunction of the two factors.

捕食者是维持物种丰富多样的群落的一个重要因素。

Predators are an essential factor in maintaining communities that are rich and diverse in species.

没有捕食者，那些在食物、庇护所、巢穴和其他环境资源方面最具竞争力的物种往往会占据主导地位，并排斥那些与之竞争的物种。

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.

这种现象被称为“竞争对手排斥”。

This phenomenon is known as “competitor exclusion.”

然而，如果群落中有最强的竞争物种的捕食者，那么该竞争物种的数量就会受到控制。

However, if the community contains a predator of the strongest competitor species, then the population of that competitor is controlled.

因此，即使竞争力较弱的物种也能生存下来。

Thus even the less competitive species are able to survive.

例如，海星捕食各种双壳类软体动物，防止这些双壳类动物独占海底的栖息地。

For example, sea stars prey on a variety of bivalve mollusks and prevent these bivalves from monopolizing habitats on the sea floor.

这为许多其他生物开辟了空间。

This opens up space for many other organisms.

海星消失后，物种多样性急剧下降。

When sea stars are removed, species diversity falls sharply.

因此，从多样性的角度来看，将一个主要的捕食者从一个群落中消灭通常是一个错误。

Therefore, from the standpoint of diversity, it is usually a mistake to eliminate a major predator from a community.

In paragraph 1, why does the author discuss the moose and wolves on Isle Royale?

The word “rebound” in the passage is closest in meaning to

Paragraph 2 implies which of the following about experimental environments in which predators become extinct?

Which of the following can be inferred from paragraphs 2 and 3 about the small mammals that experience population cycles?

The word “roughly” in the passage is closest in meaning to

The word “generating” in the passage is closest in meaning to

According to paragraph 4, all of the following are true of the food of snowshoe hares EXCEPT:

According to paragraph 5, which of the following statements best characterizes the abundance cycle of the Canada lynx?

The word “conjunction” in the passage is closest in meaning to

According to paragraph 6, which of the following was true of the hare population cycle in Krebs’s experiment?

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?

According to paragraph 7, which of the following is true of the phenomenon of competitor exclusion?

Look at the four squares  that indicate where the following sentence could be added to the passage. As a result, there are not enough of the strong competitors to monopolize the environment’s resources.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Predator-Prey Cycles How do predators affect populations of the prey animals? The answer is not as simple as might be thought. Moose reached Isle Royale in Lake Superior by crossing over 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 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 the wolves. When experimental populations are set up under simple laboratory conditions, the predator often exterminates its prey and then becomes extinct itself, having nothing left to eat. 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. 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 populations may continue in this cyclical pattern for some time. Population cycles are characteristic of some species 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 thirtyfold 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 willow and birch twigs. As hare density increases, the quantity of these twigs decreases, forcing the hares to feed on low-quality, high-fiber food. Lower birth rates, low juvenile survivorship, 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 years for the quantity of twigs to recover. A key predator of the snowshoe hare is the Canada lynx. The Canada lynx shows a ten-year cycle of abundance that parallels the abundance cycle of hares. As hare numbers increase, lynx numbers do too, rising in response to the increased availability of lynx food. When hare numbers fall, so do lynx numbers, as their food supply is depleted. What causes the predator-prey oscillations? Do increasing numbers of hares lead to overharvesting of plants, which in turn results in reduced hare populations, or do increasing numbers of lynx lead to overharvesting of hares? Field experiments carried out by Charles Krebs and coworkers in 1992 provide an answer. Krebs investigated experimental plots in Canada’s Yukon territory that contained hare populations. When food was added to these plots (no food effect) and predators were excluded (no predator effect) from an experimental area, hare numbers increased tenfold and stayed there—the cycle was lost. 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). Thus, both factors can affect the cycle, which, in practice, seems to be generated by the conjunction of the two factors. 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, nesting sites, 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.  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. Therefore, from the standpoint of diversity, it is usually a mistake to eliminate a major predator from a community.