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　　托福阅读真题：

　　The Long-Term Stability of Ecosystems

　　Plant #munities 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 #munities and ecosystems over time. The first #munity in a succession is called a pioneer #munity, while the long-lived #munity at the end of succession is called a climax #munity. Pioneer and successional plant #munities are said to change over periods from 1 to 500 years. These changes—in plant numbers and the mix of species—are cumulative. Climax #munities 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 year’s time. Individual fish may 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 organisms that #pose the ecosystem.

　　At one time, ecologists believed that species diversity made 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 climax #munities usually have more #plex food webs and more species diversity than pioneer #munities. Ecologists concluded that the apparent stability of climax ecosystems depended on their #plexity. 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 #plex climax #munity, such as a temperate forest, will tolerate considerable damage from weather to pests.

　　The question of ecosystem stability is #plicated, 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 #munity 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 major disturbance, such as a fire. This kind of stability is also called resilience. In that case, climax #munities would be the most 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 stages, not in the climax #munity. 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 does not guarantee ecosystem stability—just the opposite, in fact. A more #plicated 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 child’s tricycle.

　　Ecologists are especially interested to know what factors contribute to the resilience of #munities because climax #munities 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 #parison to the destruction caused by humans. We need to know what aspects of a #munity are most important to the #munity’s resistance to destruction, as well as its recovery.

　　Many ecologists now think that the relative long-term stability of climax #munities #es not from diversity but from the “patchiness” of the environment, 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 #munity. Even if the new population is of a different species, it can approximately fill the niche vacated by the extinct population and keep the food web intact.

　　Paragraph 1: Plant #munities 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 #munities and ecosystems over time. The first #munity in a succession is called a pioneer #munity, while the long-lived #munity at the end of succession is called a climax #munity. Pioneer and successional plant #munities are said to change over periods from 1 to 500 years. These changes—in plant numbers and the mix of species—are cumulative. Climax #munities 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 climax #munities?

　　○They occur at the end of a succession.

　　○They last longer than any other type of #munity.

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

　　○They remain stable for at least 500 years at a time.

　　Paragraph 2: An ecologist who studies a pond today may well find it relatively unchanged in a year’s time. Individual fish may 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 organisms that #pose 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 organisms are stable from one year to the next.

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

　　Paragraph 3: At one time, ecologists believed that species diversity made 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 climax #munities usually have more #plex food webs and more species diversity than pioneer #munities. Ecologists concluded that the apparent stability of climax ecosystems depended on their #plexity. 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 #plex climax #munity, such as a temperate forest, will tolerate considerable damage from weather of pests.

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

　　○Pioneer #munities

　　○Climax #munities

　　○Single-crop farmlands

　　○Successional plant #munities

　　Paragraph 4: The question of ecosystem stability is #plicated, 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 #munity 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 major disturbance, such as a fire. This kind of stability is also called resilience. In that case, climax #munities would be the most fragile and the least stable, since they can require hundreds of years to return to the climax state.

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

　　○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 many different answers to ecological questions.

　　6. According to paragraph 4, which of the following is true of climax #munities?

　　○They are more resilient than pioneer #munities.

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

　　○They are stable because they recover quickly after major disturbances.

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

　　Paragraph 5: 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 stages, not in the climax #munity. 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 does not guarantee ecosystem stability—just the opposite, in fact. A more #plicated 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 child’s tricycle.)

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

　　○They be#e less stable as they mature.

　　○They support many species when they reach climax.

　　○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 information 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 make a #parison that supports the claim that, in general, stability increases with diversity

　　○To provide an example that contradicts mathematical models of ecosystems

　　Paragraph 6: Ecologists are especially interested to know what factors contribute to the resilience of #munities because climax #munities 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 #parison to the destruction caused by humans. We need to know what aspects of a #munity are most important to the #munity’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 #mon

　　Paragraph 7：Many ecologists now think that the relative long-term stability of climax #munities #es not from diversity but from the “patchiness” of the environment, 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 #munity. Even if the new population is of a different species, it can approximately fill the niche vacated by the extinct population and keep the food web intact.