TPO托福阅读真题答案及解析

TPO托福阅读真题答案及解析

摘要：托福阅读文段的结构和出题点都有一定的规律，比如说托福阅读文段一般是总分总的

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托福阅读文段的结构和出题点都有一定的规律，比如说托福阅读文段一般是总分总的格式，考生重点关注首段，尾段，转折句，开头句，基本上就能找到主题句，从这个方面来说，练习托福阅读真题，从中总结托福阅读考试经验是十分必要的，下面新东方网为大家整理一份“TPO托福阅读

真题答案及解析”，希望对大家有所帮助。

托福阅读真题：

The Long-T erm Stability of Ecosystems

Plant communities 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 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.

An ecologist who studies a pond today may well find it relatively unchanged in a year’s time. Inpidual 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 inpidual organisms that compose the ecosystem.

At one time, ecologists believed that species persity made ecosystems stable. They believed that the greater the persity 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 persity than pioneer communities. Ecologists concluded that the apparent stability of climax ecosystems depended on their complexity. T o 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 to pests.

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.

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Even the kind of stability defined as simple lack of change is not always associated with maximum persity. At least in temperate zones, maximum persity 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 inpiduals growing on the forest floor are reduced. In general, persity, by itself, does not ensure stability. Mathematical models of ecosystems likewise suggest that persity 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.

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.

Many ecologists now think that the relative long-term stability of climax communities comes not from persity 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.

Paragraph 1: Plant communities 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 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.

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.

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○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 year …… 此处隐藏：14034字，全部文档内容请下载后查看。喜欢就下载吧 ……