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 wolves.
When experimental populations are set up under simple laboratory conditions, the predator often exterminates its pre 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 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 snow shoe 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 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 year 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 fall, so do lynx numbers, as their food supply depleted.
What causes the predator-prey oscillations? Do increasing number of hares lead to overharvesting of plants, which in turn results in reduced hare populations, or do increasing numbers of lynx lead to overharvesting 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 those 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 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 stand point of diversity, it is usually a mistake to eliminate a major predator from a community.
Paragraph 1: 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 wolves.
1.In paragraph 1, why does the author discuss the moose and wolves on Isle Royale?
O To provide an example of predators moving to new habitats by following migrating prey
O To show that the interactions between predator populations and prey populations are not always might be expected
O To suggest that prey populations are more influenced by predation than food availability and disease
O To argue that studies of geographically isolated populations tend not to be useful to naturalists
Paragraph 2: When experimental populations are set up under simple laboratory conditions, the predator often exterminates its pre 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 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.
Paragraph 3: 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 snow shoe 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.
2. The word “rebound” in the passage is closest in meaning to
O escape
O recover
O survive
O resist
3.Paragraph 2 implies which of the following about experimental environments in which predators become extinct?
O They may yield results that do not accurate predict changes of populations in the wild.
O In these environments, the prey species is better adapted than the predator species.
O These environments are appropriate only for studying small populations of predators and prey.
O They are unrealistic because some predators are also the prey of other predators.
4.Which of the following can be inferred from paragraphs 2 and 3 about the small mammals that experience population cycles?
O Their population cycles are not affected by predators.
O Their predators’ populations periodically disappear.
O They typically undergo ten-year cycles.
O They have access to places safe from predators.
5. The word “roughly” in the passage is closest in meaning to
O usually
O repeating
O approximately
O observable
6. The word “generating” in the passage is closest in meaning to
O producing
O changing
O speeding up
O smoothing out
Paragraph 4: 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 year for the quantity of twigs to recover.
7.According to paragraph 4, all of the following are true of the food of snowshoe hares EXCEPT
O The preferred food fore hares consists of willow and birch twigs.
O High fiber food is the most nutritious for hares.
O Depletion of the supply of willow and birch twigs cause low birth and growth rates.
O The food supply takes two or three years to recover after a peak in hare population density.
8. The word “conjunction” in the passage is closest in meaning to
O determination
O combination
O alternation
O transformation
Paragraph 5: 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 fall, so do lynx numbers, as their food supply depleted.
9.According to paragraph 5, which of the following statements best characterizes the abundance cycle of the Canada lynx?
O It closely follows the cycle the snowshoe hare.
O When the numbers of lynx fall, the numbers of snowshoe hares soon decrease.
O When hare numbers decrease, lynx numbers increase.
O It is not clearly related to the availability of lynx food.
Paragraph 6: What causes the predator-prey oscillations? Do increasing number of hares lead to overharvesting of plants, which in turn results in reduced hare populations, or do increasing numbers of lynx lead to overharvesting 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 those 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 conjunction of the two factors.
10.According to paragraph 6, which of the following was true of the hare population cycle in Krebs’s experiment?
O The effects of providing food while at the same time introducing predators cancelled each other, so there was no cycle.
O The cycle existed when either the food supply was limited or there were predators.
O There was a cycle when there were no predators and food was supplied.
O If the hares had places to hide from the lynx, the hare population increased tenfold and then remained at that level.
Paragraph 7: 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 stand point of diversity, it is usually a mistake to eliminate a major predator from a community.
11.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?
O Bivalve population are kept low, allowing species that compete with bivalves to survive.
O The numbers of most species of bivalves are greatly reduced, leaving the bivalve species that is the strongest competitor to dominate among the survivors.
O Biological diversity begins to decrease because many bivalve species disappear.
O Sea stars dominate at first but then die off because of the depleted food supply.
12.According to paragraph 7, which of the following is true of the phenomenon of competitor exclusion?
O It results in more diverse communities.
O It requires the presence of predators.
O It affects all competitions equally.
O It happens only when there is a dominant competitor.
13. 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 competitions to monopolize the environment’s resources.
Where would the sentence best fit?
14 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.
The relationships between predators and prey are complex.
●
●
●
Answer Choices
O Studies of the interactions between wolves and moose on Isle Royale in Lake Superior reveal that wolf predation is not the primary factor controlling the moose population.
O Predators help maintain biological diversity by limiting populations of a dominant competitor species, thereby preventing that species from excluding others.
O A species’ population tends to rise and falls in a cycle pattern if the food supply for the population is limited, or if the population has a major predator.
O Ecologists are interested in studying predator-prey population cycles because understanding how predators and prey interact will allow better wildlife management programs.
O In predator-prey population cycles, predator populations increase or decrease following similar population changes in the species they prey on.
O The removal of sea stars reduces the diversity of the community in which they are predators, and is therefore a bad idea.
查看正確答案和解析
版權(quán)聲明：本原創(chuàng)文章版權(quán)歸“新通外語(yǔ)網(wǎng)（）”所有，未經(jīng)書面許可不得轉(zhuǎn)貼、轉(zhuǎn)載。否則，新通教育網(wǎng)將追究其相關(guān)法律責(zé)任。
參考答案
1.○2
2.○2
3.○1
4.○4
5.○3
6.○1
7.○2
8.○1
9.○2
10.○2
11.○1
12.○4
13.○3
14. A species’ population tends…
Ecologists are interested in…
In predator-prey population cycles…
-被捕食者循環(huán)
捕食者是怎樣影響被捕食者的數(shù)量呢？答案并不是想象中那么簡(jiǎn)單。麋鹿通過(guò)穿越冬天的冰層到達(dá)了在蘇必略湖的羅亞爾島，并由于沒(méi)有捕食者而自由繁殖。當(dāng)狼在晚一點(diǎn)的時(shí)候到達(dá)那座島時(shí)，自然學(xué)家都認(rèn)為，狼對(duì)控制麋鹿的數(shù)量將起到關(guān)鍵作用。但是，嚴(yán)密的研究說(shuō)明并不是這樣。狼吃掉的大部分是年老的，或生病的動(dòng)物，他們本身就不會(huì)存活很久。一般地，麋鹿的數(shù)量是由食物、疾病、和其它的一些條件，而不是狼控制的。
在簡(jiǎn)單的實(shí)驗(yàn)條件下，捕食者經(jīng)常吃掉了所有的實(shí)驗(yàn)設(shè)定的被捕食者，然后因?yàn)槭澄锶狈Χ约簻缃^了。但是，如果能給被捕食動(dòng)物提供如同在野外的安全的區(qū)域，被捕食動(dòng)物的數(shù)量會(huì)降低到很低的數(shù)值，但不會(huì)滅絕。被捕食者的數(shù)量的降低造成了捕食者的食物不足，使捕食者的數(shù)量下降。此時(shí)，被捕食者的數(shù)量又會(huì)反彈。這樣，一定時(shí)間內(nèi)捕食者和被捕食者的數(shù)量會(huì)持續(xù)地循環(huán)。
這種數(shù)量的循環(huán)是小型哺乳動(dòng)物的特性，有時(shí)候這種循環(huán)是由捕食者帶來(lái)的。生態(tài)學(xué)家對(duì)野兔數(shù)量的研究發(fā)現(xiàn)，北美白靴兔一直遵守著大約以十年為周期的循環(huán)。在一個(gè)典型的循環(huán)中，其數(shù)量會(huì)以十倍到三十倍的減少，甚至?xí)霈F(xiàn)一百倍的改變。有兩種因素會(huì)導(dǎo)致這種循環(huán)：食物和捕食者。
白靴兔比較喜歡的食物是柳木和樺樹(shù)樹(shù)枝。野兔的密度增加時(shí)，這些樹(shù)枝的數(shù)量就減少，迫使野兔去吃一些低質(zhì)量的，高纖維食物。隨之而來(lái)的是低生育率，低成活率，低生長(zhǎng)率，所以野兔數(shù)量隨之減少。一旦野兔數(shù)量減少，樹(shù)枝的數(shù)量需要兩到三年恢復(fù)。
白靴兔的主要捕食者是加拿大山貓。加拿大山貓的數(shù)量呈現(xiàn)了平行于野兔的十年為周期的循環(huán)。野兔數(shù)量下降時(shí)，山貓的食物供給減少，數(shù)量也隨之減少。
什么造成了捕食者和被捕食者的周期振動(dòng)呢？是野兔數(shù)量的增長(zhǎng)使得植物被過(guò)度采食，隨之導(dǎo)致了野兔的減少還是山貓的增加導(dǎo)致了野兔被過(guò)度捕食？在1992年，Charles Krebs和其合作者的野外試驗(yàn)提供了答案。Krebs研究了在加拿大Yuhon地區(qū)有野兔種群的試驗(yàn)田。當(dāng)食物被加到這片試驗(yàn)田（沒(méi)有食品因素影響）并且將捕食者移去（也沒(méi)有捕食者因素影響）。野兔的數(shù)量增加了十倍并且保持穩(wěn)定----不再循環(huán)。但是，在任意一個(gè)因素單獨(dú)存在時(shí)，循環(huán)都會(huì)出現(xiàn)：不管是捕食者被移除，食物不添加（也就是只有食物影響）；還是食物在捕食者存在的情況下被添加（也就是只有捕食者影響）。因此，兩個(gè)因素都可以影響這個(gè)循環(huán)，即在現(xiàn)實(shí)中，循環(huán)是兩個(gè)因素同時(shí)作用的結(jié)果。
捕食者是保持群體多樣性和數(shù)量的必要條件。沒(méi)有捕食者時(shí)，那種在對(duì)食物、庇護(hù)所、筑巢點(diǎn)和其它環(huán)境資源的爭(zhēng)奪中勝出的競(jìng)爭(zhēng)者，趨向于統(tǒng)治，并且滅絕與其競(jìng)爭(zhēng)的其它物種。這種現(xiàn)象被稱作“驅(qū)逐競(jìng)爭(zhēng)者”。但是，如果種群中的具有最強(qiáng)競(jìng)爭(zhēng)力的物種存在天敵，那這一物種數(shù)量就被控制。因此，競(jìng)爭(zhēng)力比較弱的物種都會(huì)得以生存。比如，海星捕食各種雙殼的軟體動(dòng)物，防止這些雙殼動(dòng)物壟斷海底。這使得其它的很多生物有了生存空間。海星被移除后，物種多樣性大幅度降低。因此，從多樣性的角度說(shuō)，從一個(gè)種群中消除一個(gè)主要捕食者通常是錯(cuò)誤的決定。
