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While the most abundant and dominant species within a particular ecosystem(生态系统) is often crucial in perpetuating（使长存） the ecosystem（从句）, a “keystone”（重点） species, here defined as one whose effects are much larger than would be predicted（预测、预示） from its abundance（插入语，暂不看）, can also play a vital role. But because complex species interactions may be involved（从句）, identifying a keystone species by removing the species and observing changes in the ecosystem（修饰成分，先略过） is problematic. It might seem that certain traits（特点） would clearly define a species as a keystone species; for example, Pisaster ochraceus（PO不知道是什么） is often a keystone predator because it consumes and suppresses mussel（蚌类） populations（原因从句）, which in the absence of this starfish（海星） can be a dominant species（只有当PO不在的时候，starfish才是统治者）. But（大转折） such predation（捕食） on a dominant or potentially dominant species occurs in systems that do as well as in （=）systems that do not have species that play keystone roles（与没有关键物种的系统相比这种捕食行为的出现是一样的）. Moreover（进一步）, whereas（小对比） P.ochraceus occupies an unambiguous keystone role on wave-exposed rocky headlands（向水中突出的陆地，先不管它是什么，把它WRH当做一个地点，PO占据了WE）, in more wave-sheltered habitats the impact of P.ochraceus predation is weak or nonexistent（在WS，PO占据力弱）, and at certain sites sand burial（沙埋） is responsible for eliminating mussels（并列，沙埋是蚌消失的原因）. Keystone status appears to depend on context（环境）, whether of particular geography or of such factors as community diversity(生态群落多样性) (for example, a reduction in species diversity may thrust（猛推、强迫） more of the remaining species into keystone roles) and length of species interaction (since newly arrived species in particular may dramatically affect ecosystems).
【文章逻辑结构】 1.数量最多物种 V 关键物种——维持生态稳定起作用 2.But 识别关键物种很难 举例：关键捕食者PO：在有统治地位 Vs. 无统治地位时，捕食行为同 3.Moreover, PO 占据wave-exposure Vs. 不占据wave-shelter ,且M的消失由于沙埋导致 4.关键地位的出现依靠环境：（1）地理因素（2）群落多样性（3）物种交流
Questions 111-114 refer to the passage above. 111. The passage mentions which of the following as a factor that affects the role of P.ochraceus as a keystone species within different habitats? A. The degree to which the habitat is sheltered from waves B. The degree to which other animals within a habitat prey on mussels C. The fact that mussel populations are often not dominant within some habitats occupied by P.ochraceus D. The size of the P. ochraceus population within the habitat E. The fact that there is great species diversity within some habitats occupied by P.ochraceus 112. Which of the following hypothetical（假定的） experiments most clearly exemplifies（举例证明） the method of identifying species' roles that the author considers problematic?（作者认为有问题的） A. A population of seals in an Arctic habitat is counted in order to determine whether it is the dominant species in that ecosystem. B. A species of fish that is a keystone species in one marine ecosystem is introduced into another marine ecosystem to see whether the species will come to occupy a keystone role. C. In order to determine whether a species of monkey is a keystone species within a particular ecosystem, the monkeys are removed from that ecosystem and the ecosystem is then studied.（原文细节定位） D. Different mountain ecosystems are compared to determine how geography affects a particular species' ability to dominate its ecosystem.（定位到最后一句） E. In a grassland experiencing a changing climate, patterns of species extinction are traced in order to evaluate the effect of climate changes on keystone species in that grassland. 113. Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage (lines 24-31)?（加强题） A. A species of bat is primarily responsible for keeping insect populations within an ecosystem low, and the size of the insect population in turn affects bird species within that ecosystem. （种间关系）在同一系统中，蝙蝠数量—昆虫数量—鸟的数量。OG解释：This scenario(方案) does not indicate anything about keystone status; this is simply a description of how species population in a single ecosystem affect one another. 此选项错误 B. A species of iguana occupies a keystone role on certain tropical（热带） islands, but does not play that role on adjacent（邻近的） tropical islands that are inhabited by a greater number of animal species.（ig在一个地方是key，但在另一个地方由于当地的diversity多，而不是key,涵盖了geo和diversity两个因素） 该选项为正确答案。 C. Close observation of a savannah（大草原） ecosystem reveals that more species occupy keystone roles within that ecosystem than biologists had previously believed. D. As a keystone species of bee becomes more abundant, it has a larger effect on the ecosystem it inhabits. E. A species of moth that occupies a keystone role in a prairie habitat develops coloration（染色） patterns that camouflage（掩盖） it from potential predators. 114. The passage suggests which of the following about the identification of a species as a keystone species? A. Such an identification depends primarily on the species' relationship to the dominant species. B. Such an identification can best be made by removing the species from a particular ecosystem and observing changes that occur in the ecosystem. C. Such an identification is likely to be less reliable as an ecosystem becomes less diverse. D. Such an identification seems to depend on various factors within the ecosystem.（最后一句的归纳总结） E. Such an identification can best be made by observing predation behavior.
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发表于 2016-3-12 21:30:26
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