2017年托福阅读真题答案及解析
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,下面是小编为大家搜索整理的托福阅读真题答案及解析,希望大家能有所收获,更多精彩内容请及时关注我们应届毕业生考试网!
托福阅读真题:
The Long-Term 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. 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.
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 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.
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.
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 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.
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.
○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.
相關文章
-
2017年新托福阅读真题及答案解析
凡事欲其成功,必要付出代价:奋斗。以下是小编为大家搜整理的2017年新托福阅读真题及答案解析,希望能给大家带来帮助!更多精彩内容请及时关注我们应届毕业生考试网!Before 1815 manufacturing in the United States had b -
2017年托福阅读真题预测及答案解析
天才不是别的,而是辛劳和勤奋。以下是小编为大家搜索整理的2017年托福阅读真题预测及答案解析,希望对正在关注的您有所帮助!更多精彩内容请及时关注我们应届毕业生考试网! 词汇题:subtly=slightlyeventually=in the e -
2016年3月19日托福阅读真题及答案解析
在托福备考的四个项目中,相对而言,阅读还是比较难的,就2016年3月19日托福阅读,下面YJBYS小编为大家搜索整理了相关的真题及答案解析,欢迎参考阅读,希望对大家有所帮助!想了解更多相关信息请持续关注我们应届毕业生培训网! -
2017年托福考试真题阅读及答案
我的箴言始终是:无日不动笔;如果我有时让艺术之神瞌睡,也只为要使它醒后更兴奋。以下是小编为大家搜索整理的2017年托福考试真题阅读及答案,希望能给大家带来帮助!更多精彩内容请及时关注我们应届毕业生考试网! 托福 -
2017年托福写作真题练习及答案解析
人天天都学到一点东西,而往往所学到的是发现昨日学到的是错的。以下是小编为大家搜索整理的2017年托福写作真题练习及答案解析,希望能给大家带来帮助!更多精彩内容请及时关注我们应届毕业生考试网! 综合写作Topic议 -
2017年托福阅读真题回顾解析
学习既要有铁棒磨成针的精神,又要有举一反三的;读书既要有勇于攀登的志气,又要有乐于思考的'精神。以下是小编为大家搜索整理的2017年托福阅读真题回顾解析,希望能给大家带来帮助!更多精彩内容请及时关注我们应届毕业生 -
2017年公共英语三级阅读理解真题及答案解析
没有人富有得可以不要别人的帮助,也没有人穷得不能在某方面给他人帮助。以下是小编为大家搜索整理的2017年公共英语三级阅读理解真题及答案解析,希望能给大家带来帮助!更多精彩内容请及时关注我们应届毕业生考试网! -
2017年英语四级真题阅读答案及解析
英语四六级考试的2017年上半年考试即将到来,为帮助各位更好的进行知识点的巩固,以下是小编为大家搜索整理的2017年英语四级真题阅读答案及解析,希望能给大家带来帮助!更多精彩内容请及时关注我们应届毕业生考试网! 选 -
2017年托福考试阅读理解模拟题及答案
未来的汽车是充气的,只要按一下按钮,它就能自动充气,变成一辆高级汽车,不用时它可以折叠起来。下面由小编为您整理出的未来的汽车作文5篇300字,一起来看看吧。 篇一:未来的汽车未来的汽车是什么样子呢?你不知道吧!那就让我 -
2015年托福阅读考试真题及答案(6月27日)
第一篇考生回忆:文章一开始说在海里的某个m区域,阳光很稀缺所以动物需要有特殊的技巧来生存。比如说有超级敏感(sensitive)的眼睛和可以探测光的器官(light detective organs)。还有一种身体的面积可以扩大,从而可以