托福阅读模拟试题及答案：生态系统的长期稳定

　　11.Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incurred choices change the meaning in important ways or leave out essential information.

　　A.Ecologists now think that the stability of an environment is a result of diversity rather than patchiness.

　　B.Patchy environments that vary from place to place do not often have high species diversity.

　　C.Uniform environments cannot be climax communities because they do not support as many types of organisms as patchy environments.

　　D.A patchy environment is thought to increase stability because it is able to support a wide variety of organisms.

　　12.The word “adjacent” in the passage is closest in meaning to

　　A.Foreign

　　B.Stable

　　C.Fluid

　　D.Neighboring

　　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.

　　13.Look at the four squares [ ] that indicate where the following sentence could be added to the passage.

　　In fact, damage to the environment by humans is often much more severe than damage by natural events and processes.

　　Where would the sentence best fit? Click on a square to add the sentence to the passage.

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

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

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

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

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

　　托福阅读真题参考答案：

　　1. C　　2. C　　3. A　　4. B　　5. C　　6. B　　7. C　　8. B　　9. A　　10. C　　11. D　　12. D　　13. B　　14. BCD

　　托福阅读真题译文：

　　生态系统的长期稳定

　　植物群体可以自由地聚集，他们特殊的结构取决于聚集区域的具体历史。生态学家使用“演替”来诠 释植物群落和生态系统随着时间推移所发生的变化。演替中的第一个群落被称作先锋群落，而处于演替最后那个长期生存的群落被称为顶极群落。先锋群落和紧接着 的植物群落的变化周期是从1到500年不等，植物数量和混合种类数量的变化是慢慢积累的。顶极群落本身也改变，但其变化周期超过500年。

　　现代一个研究池塘的生态学会发现池塘在一年当中相对而言是不变的。个别鱼类可能被替换，但一年一年鱼的总数都趋于一致。也就是说，生态系统自身的性质比组成生态系统的单个生物体更为稳定。

　　生态学家们一度认为物种的多样性使生态系统稳定，生态系统物种越多样则生态系统越稳定。通过观 察得出的结论支持了这个观点，长期持久的顶极群落通常要比先锋群落具备更为复杂的食物网和更多的物种。生态学家家们得出的结论是：顶点生态系统的稳定性明 显取决于他们的复杂化程度。举个极端的例子，在单一作物的农田中，一年的恶劣天气或单一害虫的入侵就可以摧毁所有作物。与此相反，在一个复杂的顶极群落 里，如温带森林，他们便可以抵御来自气候和害虫的入侵。

　　不管怎样，生态系统稳定性的问题非常复杂。首先，不是所有的生态学家都赞同“稳定”的含义。稳 定性可以简单地定义为缺乏变化。如果是这样的话，顶极群落将被视为最稳定的，因为根据定义，他们随着时间推移而变化是最少。另外，稳定性也可以界定为生态 系统在经历了严重破坏之后回复原貌的速度，比如火灾。这种稳定性也被称作弹性。在这种情况下，顶极群落将是最脆弱和最不稳定的，因为他们可能需要数百年时 间才能恢复到顶点状态。

　　即使是这种被定义为简单地缺乏变化的稳定性并非总是与最多样的物种联系起来。至少在温带地区， 会经常在演替过程中发现最多物种，而不是在顶极群落中。例如，红树林一旦成熟，其中的物种数量以及单个物种的数量都会减少。总的来说，多样性本身并不能保 证稳定性，生态系统的数学模型也可以得出同样的结论。一般来说，一个更复杂的系统可能比一个简单的系统更容易被破坏(一个十五速的赛车比一个孩子的三轮车 更容易损坏)。

　　生态学家们更想弄清楚到底哪些因素有助于促成群落的恢复，因为世界各地的顶极群落都因为人类活 动而遭受到严重的损坏或毁坏。就像美国西北部圣海伦火山的猛烈喷发所造成的破坏，在人类活动对环境造成的的破坏面前也相形见绌。我们必须了解对群落抵抗破 坏和恢复来说哪些是最重要的。

　　现在的很多生态学家们认为，顶极群落相对长期的稳定性并非来于自多样性，而是来自环境的“补 缀”，随处变化的环境比统一的环境更有利于多种有机体的生存。当地物种灭亡后，马上就会被相邻群落的移民取代。即便是另一种不同的物种，他们也可以填补那 些已灭绝生物的空缺，并保持食物网的完整。