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托福阅读模拟题之生态系统的长期稳定(附译文)

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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 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 cli*x community. Pioneer and successional plant communities are said to change over periods from 1 to 500 years. These changes—in plant numbers and the mix of species—are cumulative. Cli*x communities themselves change but over periods of time greater than about 500 years.

An ecologist who stu*s a pond today *y well find it relatively unchanged in a year’s time. Individual fish *y be 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 organi*s that compose the ecosystem.

At one time, ecologists believed that species diversity *de ecosystems stable. They believed that the greater the diversity the more stable the ecosystem. Support for this idea came from the observation that long-lasting cli*x communities usually have more complex food webs and more species diversity than pioneer communities. Ecologists concluded that the apparent stability of cli*x ecosystems 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 cli*x community, such as a temperate forest, will tolerate considerable da*ge 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 cli*x community would be considered the most stable, since, 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 *jor disturbance, such as a fire. This kind of stability is also called resilience. In that case, cli*x communities would be the most fragile and the least stable, since they can require hundreds of years to return to the cli*x state.

Even the kind of stability defined as * lack of change is not always associated with *ximum diversity. At least in temperate zones, *ximum diversity is often found in mid-successional stages, not in the cli*x community. Once a redwood forest *tures, 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. Mathe*tical models of ecosystems likewise suggest that diversity does not guarantee ecosystem stability—just the opposite, in fact. A more complicated system is, in general, more likely than a * system to break down. A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle.

Ecologists are especially interested to know what factors contribute to the resilience of communities because cli*x communities all over the world are being severely da*ged or destroyed by hu*n activities. The destruction caused by the volcanic explosion of Mount St. Helens, in the northwestern United States, for example, pales in comparison to the destruction caused by hu*ns. We need to know what aspects of a community are most important to the community’s resistance to destruction, as well as its recovery.

Many ecologists now think that the relative long-term stability of cli*x communities comes not from diversity but from the “patchiness” of the environment, an environment that varies from place to place supports more kinds of organi*s 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 approxi*tely fill the niche vacated by the extinct population and keep the food web intact.

Paragraph 1: Plant communities assemble themselves flexibly, and their particular structure depends on the specific history of the area. Ecologists use the term “succession” to refer 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 cli*x community. Pioneer and successional plant communities are said to change over periods from 1 to 500 years. These changes—in plant numbers and the mix of species—are cumulative. Cli*x communities themselves change but over periods of time greater than about 500 years.

1. The word “particular” in the passage is closest in meaning to

○Natural

○Final

○Specific

○Complex

2. According to paragraph 1, which of the following is NOT true of cli*x communities?

○They occur at the end of a succession.

○They last longer than any other type of community.

○The numbers of plants in them and the mix of species do not change.

○They re*in stable for at least 500 years at a time.

Paragraph 2: An ecologist who stu*s a pond today *y well find it relatively unchanged in a year’s time. Individual fish *y be 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 organi*s that compose the ecosystem.

3. According to paragraph 2, which of the following principles of ecosystems can be learned by studying a pond?

○Ecosystem properties change more slowly than individuals in the system.

○The stability of an ecosystem tends to change as individuals are replaced.

○Individual organi*s are stable from one year to the next.

○A change in the members of an organi* does not affect an ecosystem’s properties

Paragraph 3: At one time, ecologists believed that species diversity *de ecosystems stable. They believed that the greater the diversity the more stable the ecosystem. Support for this idea came from the observation that long-lasting cli*x communities usually have more complex food webs and more species diversity than pioneer communities. Ecologists concluded that the apparent stability of cli*x ecosystems 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 cli*x community, such as a temperate forest, will tolerate considerable da*ge from weather of pests.

4. According to paragraph 3, ecologists once believed that which of the following illustrated the most stable ecosystems?

○Pioneer communities

○Cli*x communities

○Single-crop farmlands

○Successional plant communities

Paragraph 4: 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 cli*x community would be considered the most stable, since, 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 *jor disturbance, such as a fire. This kind of stability is also called resilience. In that case, cli*x communities would be the most fragile and the least stable, since they can require hundreds of years to return to the cli*x state.

5. According to paragraph 4, why is the question of ecosystem stability complicated?

○The reasons for ecosystem change are not always clear.

○Ecologists often confuse the word “stability” with the word “resilience.”

○The exact meaning of the word “stability” is debated by ecologists.

○There are *ny different answers to ecological questions.

6. According to paragraph 4, which of the following is true of cli*x communities?

○They are more resilient than pioneer communities.

○They can be considered both the most and the least stable communities.

○They are stable because they recover quickly after *jor disturbances.

○They are the most resilient communities because they change the least over time.

Paragraph 5: Even the kind of stability defined as * lack of change is not always associated with *ximum diversity. At least in temperate zones, *ximum diversity is often found in mid-successional stages, not in the cli*x community. Once a redwood forest *tures, 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. Mathe*tical models of ecosystems likewise suggest that diversity does not guarantee ecosystem stability—just the opposite, in fact. A more complicated system is, in general, more likely than a * system to break down. (A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle.)

7. Which of the following can be inferred from paragraph 5 about redwood forests?

○They become less stable as they *ture.

○They support *ny species when they reach cli*x.

○They are found in temperate zones.

○They have reduced diversity during mid-successional stages.

8. The word “guarantee” in the passage is closest in meaning to

○Increase

○Ensure

○Favor

○Complicate

9. In paragraph 5, why does the author provide the infor*tion that “(A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle)”?

○To illustrate a general principle about the stability of systems by using an everyday example

○To demonstrate that an understanding of stability in ecosystems can be applied to help understand stability in other situations

○To *ke a comparison that supports the claim that, in general, stability increases with diversity

○To provide an example that contradicts *the*tical models of ecosystems

Paragraph 6: Ecologists are especially interested to know what factors contribute to the resilience of communities because cli*x communities all over the world are being severely da*ged or destroyed by hu*n activities. The destruction caused by the volcanic explosion of Mount St. Helens, in the northwestern United States, for example, pales in comparison to the destruction caused by hu*ns. We need to know what aspects of a community are most important to the community’s resistance to destruction, as well as its recovery.

10. The word “pales” in the passage is closest in meaning to

○Increases proportionally

○Differs

○Loses significance

○Is common

Paragraph 7:Many ecologists now think that the relative long-term stability of cli*x communities comes not from diversity but from the “patchiness” of the environment, an environment that varies from place to place supports more kinds of organi*s 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 approxi*tely fill the niche vacated by the extinct population and keep the food web intact.

11.Which of the sentences below best expresses the essential infor*tion in the highlighted sentence in the passage? Incurred choices change the meaning in important ways or leave out essential infor*tion.

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