文勇的新托福黄金&ccedil

Out of human needs and aspirations it

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Unformatted text preview: amage 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 127 Mail@liuwenyong.com 在在在在在在 www.liuwenyong.com 在在在在在在在在在在在在在在在,在在在在在在在在在在 ○Single­crop 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 mid­successional 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 fifteen­speed 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 mid­successional 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 informat...
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