托福阅读题目解析：生态系统的长期稳定

　　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 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 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 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 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 communities usually have more complex food webs and more species diversity than pioneer communities. Ecologists concluded that the apparent stability of climax 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 climax community, 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 communities

　　○Climax 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 climax 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 major disturbance, such as a fire. This kind of stability is also called resilience. In that case, climax communities 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 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 many different answers to ecological questions.

　　6. According to paragraph 4, which of the following is true of climax 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 major disturbances.

　　○They are the most resilient communities 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 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 does not guarantee ecosystem stability—just 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 child’s tricycle.)

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

　　○They become 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