Plant communitiesassemblethemselves flexibly, and theirparticularstructure depends on thespecifichistoryof the area. Ecologists use theterm“succession” toreferto the changes that happen in plant communities and ecosystems over time. The firstcommunityin asuccessionis called apioneercommunity, while the long-livedcommunityat the end ofsuccessionis called aclimaxcommunity. Pioneerand successional plant communities are said to change over periods from 1 to 500 years. These changes—in plant numbers and the mix ofspecies—arecumulative. Climaxcommunities themselves change but over periods of time greater than about 500 years.

An ecologist who studies a pond today may well find itrelativelyunchanged in a year’s time. Individualfish may be replaced, but the number of fish willtendto be the same from one year to the next. We can say that the properties of anecosystemare morestablethan theindividualorganisms thatcomposetheecosystem.

At one time, ecologists believed thatspeciesdiversitymade ecosystemsstable. They believed that the greater thediversitythe morestabletheecosystem. Support for this idea came from theobservationthat long-lastingclimaxcommunities usually have morecomplexfood webs and morespeciesdiversitythanpioneercommunities. Ecologists concluded that theapparentstabilityofclimaxecosystems depended on theircomplexity. To take anextremeexample, farmlands dominated by a single crop are so unstable that one year of bad weather or theinvasionof a singlepestcan destroy the entire crop. Incontrast, acomplexclimaxcommunity, such as atemperateforest, willtolerateconsiderabledamage from weather to pests.

The question ofecosystemstabilityiscomplicated, however. The first problem is that ecologists do not all agree what “stability” means. Stabilitycan be defined assimplylackof change. In that case, theclimaxcommunitywould be considered the moststable, since, bydefinition, it changes theleastover time. Alternatively,stabilitycan be defined as the speed with which anecosystemreturns to aparticularform following amajordisturbance, such as a fire. This kind ofstabilityis also calledresilience. In that case,climaxcommunities would be the mostfragileand theleaststable, since they canrequirehundreds of years to return to theclimaxstate.

Eventhe kind ofstabilitydefined as simplelackof change is not always associated withmaximumdiversity. Atleastintemperatezones,maximumdiversityis often found in mid-successional stages, not in theclimaxcommunity. Once a redwood forest matures, for example, the kinds ofspeciesand the number of individuals growing on the forest floor are reduced. In general,diversity, by itself, does not ensurestability. Mathematical models of ecosystemslikewisesuggest thatdiversitydoes notguaranteeecosystemstability—just theopposite, in fact. A morecomplicatedsystem is, in general, more likely than a simple system tobreakdown. A fifteen-speed racing bicycle is more likely tobreakdown than a child's tricycle.

Ecologists are especially interested to know what factorscontributeto theresilienceof communities becauseclimaxcommunities all over the world are beingseverelydamaged or destroyed by human activities. Thedestructioncaused by thevolcanicexplosionofMountSt. Helens, in the northwestern United States, for example, pales incomparisonto thedestructioncaused by humans. We need to know what aspects of acommunityare most important to thecommunity’sresistancetodestruction, as well as its recovery.

Many ecologists now think that therelativelong-termstabilityofclimaxcommunities comes not fromdiversitybut from the “patchiness” of the environment, an environment that varies from place to place supports more kinds of organisms than an environment that isuniform. Alocalpopulationthat goesextinctis quickly replaced by immigrants from anadjacentcommunity. Evenif the newpopulationis of a differentspecies, it canapproximatelyfill thenichevacated by theextinctpopulationand keep the food webintact.

Questions:
question_stem: The word “particular” in the passage is closest in meaning to
choices: tag: A
text: natural

tag: B
text: final

tag: C
text: specific

tag: D
text: complex
answer_key: C
explanation: C particular: 特别的，特定的。A 是自然地；B 是最终地；C 是特定的；D 是复杂的。根据词意，答案是 C 的 specific。对应原文 Plant communities assemble themselves flexibly, and their particular structure depends on the specific history of the area.前文说 assemble them flexibly，多变的，就是每个群落都不一样，强调特殊性。而且后半句话说了 particular 的结构取决于 specific 的历史，particular 和 specific 呼应，因此答案很明显是 C。

question_stem: According to paragraph 1, which of the following is NOT true of climax communities?
choices: tag: A
text: They occur at the end of a succession.

tag: B
text: They last longer than any other type of community.

tag: C
text: The numbers of plants in them and the mix of species do not change.

tag: D
text: They remain stable for at least 500 years at a time.
answer_key: C
explanation: B 以 climax communities 做关键词定位至原文第三句和最后一句： 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. Climax communities themselves change but over periods of time greater than about 500 years.第三句说明 A 对，不选；最后一句说明 D 对，不选；B 没直说，说其他 communities 变化周期在1-500年，climax 最少是500年，说明 climax 比较长，B 对，不选。而且最后一句也说了 climax 是变化的，C 说反了，选。

question_stem: According to paragraph 2, which of the following principles of ecosystems can be learned by studying a pond?
choices: tag: A
text: Ecosystem properties change more slowly than individuals in the system.

tag: B
text: The stability of an ecosystem tends to change as individuals are replaced.

tag: C
text: Individual organisms are stable from one year to the next.

tag: D
text: A change in the members of an organism does not affect an ecosystem’s properties.
answer_key: A
explanation: A, 本段非常短，迅速看完之后发现最后一句是结论： We can say that the properties of an ecosystem are more stable than the individual organisms that compose the ecosystem.说生态系统比组成生态系统的任意单一物种都稳定，A 是答案。其他三个选项都未提及。

question_stem: According to paragraph 3, ecologists once believed that which of the following illustrated the most stable ecosystems?
choices: tag: A
text: Pioneer communities

tag: B
text: Climax communities

tag: C
text: Single-crop farmlands

tag: D
text: Successional plant communities
answer_key: B
explanation: B 以 ecologists 做关键词定位至本段第一二句，说生态学家认为生态系统中物种越多越稳定。接着在举例子的时候作者举到了 climax communities： 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.说明生态学家认为 climax communities 最稳定，答案 B。

question_stem: According to paragraph 4, why is the question of ecosystem stability complicated?
choices: tag: A
text: The reasons for ecosystem change are not always clear.

tag: B
text: Ecologists often confuse the word “stability” with the word “resilience.”

tag: C
text: The exact meaning of the word “stability” is debated by ecologists.

tag: D
text: There are many different answers to ecological questions.
answer_key: C
explanation: C 以 question of ecosystem stability 为关键词定位至本段头两句： The first problem is that ecologists do not all agree what "stability" means.说生态学家对于什么是生态系统的稳定性依然存在争议，C 是原文的忠实改写。

question_stem: According to paragraph 4, which of the following is true of climax communities?
choices: tag: A
text: They are more resilient than pioneer communities.

tag: B
text: They can be considered both the most and the least stable communities.

tag: C
text: They are stable because they recover quickly after major disturbances.

tag: D
text: They are the most resilient communities because they change the least over time.
answer_key: B
explanation: B 关于 climax communities 原文有两个地方提到： In that case, the climax community would be considered the most stable, since, by definition, it changes the least over time. 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.因为生态学家对什么是稳定的定义存在两个观点：以不变这种观点看，climax 是最稳定的；以快速恢复这种观点看，climax 是最不稳定的，所以 climax 既稳定也不稳定。B 明确说到两个方面，其它选项都只说到一个方面。

question_stem: Which of the following can be inferred from paragraph 5 about redwood forests?
choices: tag: A
text: They become less stable as they mature.

tag: B
text: They support many species when they reach climax.

tag: C
text: They are found in temperate zones.

tag: D
text: They have reduced diversity during mid-successional stages.
answer_key: C
explanation: C 以 redwood forest 做关键词定位至原文第三句，事实上这个 redwood 是前句的一个具体例子： At least in temperate zones, maximum diversity is often found in mid-successional stages, not in the climax community.前面句说至少在温带，mid 是最稳定的，不是 climax，紧接着就举了一个 redwood 的例子，可以推断 redwood 是温带植物，C 是答案。A/B/D 都与原文意思相反，而且 B 和 D 基本上意思一样，都不对。

question_stem: The word “guarantee” in the passage is closest in meaning to
choices: tag: A
text: increase

tag: B
text: ensure

tag: C
text: favor

tag: D
text: complicate
answer_key: B
explanation: B guarantee: 保证，确保。A 是增加；B 是确保；C 是赞成，喜欢；D 是使…复杂。根据词意，B 是正确答案。对应原文 Mathematical models of ecosystems likewise suggest that diversity does not guarantee ecosystem stability—just the opposite, in fact.本句中有个 likewise，说明它与前面句子构成类比，前句说 diversity does not ensure stability, 后文的内容几乎和前句一模一样，除了动词被替换，因此 guarantee 的意思就是 ensure。

question_stem: In paragraph 5, why does the author provide the information that “A fifteen-speed racing bicycle is more likely to break down than a child's tricycle”?
choices: tag: A
text: To illustrate a general principle about the stability of systems by using an everyday example

tag: B
text: To demonstrate that an understanding of stability in ecosystems can be applied to help understand stability in other situations

tag: C
text: To make a comparison that supports the claim that, in general, stability increases with diversity

tag: D
text: To provide an example that contradicts mathematical models of ecosystems
answer_key: A
explanation: A 括号的作用是解释括号之前的文字： A more complicated system is, in general, more likely than a simple system to break down.前文说系统越复杂越容易坏，后面举了十五速赛车和小孩子的三轮车的例子，三轮车明显比赛车简单，所以就是支持前文，A 正确，C 正好说反，其他都没说。

question_stem: The word “pales” in the passage is closest in meaning to
choices: tag: A
text: increases proportionally

tag: B
text: differs

tag: C
text: loses significance

tag: D
text: is common
answer_key: C
explanation: C pale: 苍白，使失色。A 是成比例增长；B 是不同；C 是失去重要性；D 是常见，普通。根据词意，C 是正确答案。对应原文 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.单词所在句之前说很多 climax communities 都被人类破坏了，也就是强调人的破话很严重。后面举了个例子，说圣海伦斯火山爆发造船的破坏还没有人类活动的破坏严重，所以 C 选项 lose significance 正确。

question_stem: 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.
choices: tag: A
text: Ecologists now think that the stability of an environment is a result of diversity rather than patchiness.

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

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

tag: D
text: A patchy environment is thought to increase stability because it is able to support a wide variety of organisms.
answer_key: D
explanation: D : 原文的结构是生态系统的长期稳定性不是因为 diversity，而是因为 patchiness，又对这个进行了解释，说物种多。A 说反；B 错，因为原文没有说 patchiness 和 diversity 是矛盾的；C 错，因为原文比较的是 patchiness 和 uniform，而 C 直接将 patchiness 和 climax 进行比较，偷换概念；D 正确，注意原文貌似没有明显的因果关系，因此可能单看句子间关系的话会排除 D，但原文后半句整个是对 patchiness 的一种解释，所以存在不明显的因果关系。

question_stem: The word “adjacent” in the passage is closest in meaning to
choices: tag: A
text: foreign

tag: B
text: stable

tag: C
text: fluid

tag: D
text: neighboring
answer_key: D
explanation: D，adjacent: 相邻。A 是外国的；B 是稳定的；C 是流动的；D 是相邻的。根据词意，D 是正确答案。对应原文 A local population that goes extinct is quickly replaced by immigrants from an adjacent community.原文说本地的 population 灭绝的话会被哪里来的群落补充，可能的只有 foreign 和 neighboring，这两个选项代入原文都是可以的，除知道这个单词的意思之外，还可以认为相邻的群落更有可能迅速补充，而外来的范围太大，不对。