Official 3 Passage 3
答题结果:0/14
答题时间:0''
Question 14 of 14

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.


The process of succession and the stability of a climax community can change over time.
Answer Choices:

A.

The changes that occur in an ecosystem from the pioneer to the climax community can be seen in one human generation.

B.

Ecologists agree that climax communities are the most stable types of ecosystems.

C.

A high degree of species diversity does not always result in a stable ecosystem.

D.

Disagreements over the meaning of the term "stability" make it difficult to identify the most stable ecosystems.

E.

The level of resilience in a plant community contributes to its long-term stability.

F.

The resilience of climax communities makes them resistant to destruction caused by humans.

我的答案:

正确答案:CDE

译文

The Long-Term Stability of Ecosystems

[#paragraph1]Plant communities assemble themselves flexibly, and their [#highlight1]particular[/highlight1] 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 climax 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. Climax communities themselves change but over periods of time greater than about 500 years.
 

[#paragraph2]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.
 

[#paragraph3]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 or pests.
 

[#paragraph4]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.
 

[#paragraph5]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 [#highlight8]guarantee[/highlight8] ecosystem stability—just the opposite, in fact. A more complicated system is, in general, more likely than a simple system to break down. [#highlight9](A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle.)[/highlight9]
 

[#paragraph6][#insert1]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. [#insert2] The destruction caused by the volcanic explosion of Mount St. Helens, in the northwestern United States, for example, [#highlight10]pales[/highlight10] in comparison to the destruction caused by humans. [#insert3] We need to know what aspects of a community are most important to the community’s resistance to destruction, as well as its recovery. [#insert4]
 

[#paragraph7][#highlight11]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.[/highlight11] A local population that goes extinct is quickly replaced by immigrants from an [#highlight12]adjacent[/highlight12] 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.