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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
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 comparison 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 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 destruction caused by humans. 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 climax communities comes 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 community. 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.
| 重点单词 | 查看全部解释 | |||
| illustrate | ['iləstreit] | 联想记忆 | ||
| illustrated | ['iləstreitid] | |||
| pond | [pɔnd] | |||
| volcanic | [vɔl'kænik] | |||
| guarantee | [.gærən'ti:] | |||
| affect | [ə'fekt] | 联想记忆 | ||
| temperate | ['tempərit] | 联想记忆 | ||
| diversity | [dai'və:siti] | 联想记忆 | ||
| tolerate | ['tɔləreit] | |||
| demonstrate | ['demənstreit] | 联想记忆 |
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