托福TPO3阅读Passage3原文文本+题目+答案解析.pdf
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1、 为了帮助大家高效备考托福,为大家带来托福 TPO3 阅读 Passage3 原文文本+题目+答案 解析,希望对大家备考有所帮助。 托福 TPO3 阅读 Passage3 原文文本: The Long-Term Stability of Ecosystems Plant communities assemble themselves flexibly, and their particular structure depends on the specific history of the area. Ecologists use the term “succession” to refer
2、to the changes that happen in plant communities and ecosystems over time. The first community in a succession is called a pioneer community, while the long- lived community at the end of succession is called a climax community. Pioneer and successional plant communities are said to change over perio
3、ds from 1 to 500 years. These changesin plant numbers and the mix of speciesare cumulative. Climax communities themselves change but over periods of time greater than about 500 years. An ecologist who studies a pond today may well find it relatively unchanged in a years time. Individual fish may be
4、replaced, but the number of fish will tend to be the same from one year to the next. We can say that the properties of an ecosystem are more stable than the individual organisms that compose the ecosystem. At one time, ecologists believed that species diversity made ecosystems stable. They believed
5、that the greater the diversity the more stable the ecosystem. Support for this idea came from the observation that long-lasting climax communities usually have more complex food webs and more species diversity than pioneer communities. Ecologists concluded that the apparent stability of climax ecosy
6、stems depended on their complexity. To take an extreme example, farmlands dominated by a single crop are so unstable that one year of bad weather or the invasion of a single pest can destroy the entire crop. In contrast, a complex climax community, such as a temperate forest, will tolerate considera
7、ble damage from weather to pests. The question of ecosystem stability is complicated, however. The first problem is that ecologists do not all agree what “stability” means. Stability can be defined as simply lack of change. In that case, the climax community would be considered the most stable, sinc
8、e, by definition, it changes the least over time. Alternatively, stability can be defined as the speed with which an ecosystem returns to a particular form following a major disturbance, such as a fire. This kind of stability is also called resilience. In that case, climax communities would be the m
9、ost fragile and the least stable, since they can require hundreds of years to return to the climax state. Even the kind of stability defined as simple lack of change is not always associated with maximum diversity. At least in temperate zones, maximum diversity is often found in mid-successional sta
10、ges, not in the climax community. Once a redwood forest matures, for example, the kinds of species and the number of individuals growing on the forest floor are reduced. In general, diversity, by itself, does not ensure stability. Mathematical models of ecosystems likewise suggest that diversity doe
11、s not guarantee ecosystem stabilityjust the opposite, in fact. A more complicated system is, in general, more likely than a simple system to break down. A fifteen-speed racing bicycle is more likely to break down than a childs tricycle. Ecologists are especially interested to know what factors contr
12、ibute to the resilience of communities because climax communities all over the world are being severely damaged or destroyed by human activities. The destruction caused by the volcanic explosion of Mount St. Helens, in the northwestern United States, for example, pales in comparison to the destructi
13、on caused by humans. We need to know what aspects of a community are most important to the communitys resistance to destruction, as well as its recovery. Many ecologists now think that the relative long-term stability of climax communities comes not from diversity but from the “patchiness” of the en
14、vironment, an environment that varies from place to place supports more kinds of organisms than an environment that is uniform. A local population that goes extinct is quickly replaced by immigrants from an adjacent community. Even if the new population is of a different species, it can approximatel
15、y fill the niche vacated by the extinct population and keep the food web intact. 托福 TPO3 阅读 Passage3 题目: Question 1 of 14 The word “particular ” in the passage is closest in meaning to A. natural. B. final. C. specific. D. complex. Question 2 of 14 According to paragraph 1, which of the following is
16、 NOT true of climax communities? A. They occur at the end of a succession B. They last longer than any other type of community C. The numbers of plants in them and the mix of species do not change D. They remain stable for at least 500 years at a time Question 3 of 14 According to paragraph 2, which
17、 of the following principles of ecosystems can be learned by studying a pond? A. Ecosystem properties change more slowly than individuals in the system B. The stability of an ecosystem tends to change as individuals are replaced C. Individual organisms are stable from one year to the next D. A chang
18、e in the members of an organism does not affect an ecosystems properties Question 4 of 14 According to paragraph 3, ecologists once believed that which of the following illustrated the most stable ecosystems? A. Pioneer communities. B. Climax communities. C. Single-crop farmlands. D. Successional pl
19、ant communities. Question 5 of 14 According to paragraph 4, why is the question of ecosystem stability complicated? A. The reasons for ecosystem change are not always clear. B. Ecologists often confuse the word “stability” with the word “resilience.”. C. The exact meaning of the word “stability” is
20、debated by ecologists D. There are many different answers to ecological questions Question 6 of 14 According to paragraph 4, which of the following is true of climax communities? A. They are more resilient than pioneer communities B. They can be considered both the most and the least stable communit
21、ies C. They are stable because they recover quickly after major disturbances D. They are the most resilient communities because they change the least over time Question 7 of 14 Which of the following can be inferred from paragraph 5 about redwood forests? A. They become less stable as they mature B.
22、 They support many species when they reach climax C. They are found in temperate zones D. They have reduced diversity during mid-successional stages Question 8 of 14 The word “guarantee ” in the passage is closest in meaning to A. increase. B. ensure. C. favor. D. complicate. Question 9 of 14 In par
23、agraph 5, why does the author provide the information that “A fifteen-speed racing bicycle is more likely to break down than a childs tricycle ”? A. To illustrate a general principle about the stability of systems by using an everyday example. B. To demonstrate that an understanding of stability in
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