1、Passage 13_by 申友 Alice 老师While the most abundant and dominant species within a particular ecosystem is often crucial in perpetuating the ecosystem, a “keystone“ species ,here defined as one whose effects are much larger than would be predicted from its abundance（插入语，解释说明 a keystone“ species）,can als
2、o play a vital role./介绍 a keystone species 的意义。But because complex species interactions may be involved, identifying a keystone species by removing the species and observing changes in the ecosystem is problematic./通过移动或者观察的方法很难确定 a keystone species。 It might seem that certain traits would clearly d
3、efine a species as a keystone species; for example, Pisaster ochraceus is often a keystone predator because it consumes and suppresses mussel populations, which in the absence of this starfish can be a dominant species. But such predation on a dominant or potentially dominant species occurs in syste
4、ms that do as well as in systems that do not have species that play keystone roles. Moreover, whereas P.ochraceus occupies an unambiguous keystone role on wave-exposed rocky headland, in more wave-sheltered habitats the impact of P.ochraceus predation is weak or nonexistent, and at certain sites a s
5、and burial is responsible for eliminating mussels./通过特性来鉴定 a keystone species，举例 Pisaster ochraceus消耗抑制蚌数量的特性确定为 a keystone species；但抑制的作用在没有统治种类的生态系统上也发生，并且受环境中 wave-exposed 或 wave-sheltered 的影响。Keystone status appears to depend on context, whether of particular geography or of such factors as comm
6、unity diversity (for example, a reduction in species diversity may thrust more of the remaining species into keystone roles)and length of species interaction(since newly arrived species in particular may dramatically affect ecosystems)./提出观点判断 keystone status 的地位依据环境来确定。 逻辑简图第一层：介绍 a keystone specie
7、s 的意义。第二层：通过移动或者观察的方法很难确定 a keystone species。第三层：通过特性来鉴定 a keystone species，举例 Pisaster ochraceus 消耗抑制蚌数量的特性确定为 a keystone species；但抑制的作用在没有统治种类的生态系统上也发生，并且受环境中 wave-exposed 或 wave-sheltered 的影响。第四层：提出观点判断 keystone status 的地位依据环境来确定。1. The passage mentions which of the following as a factor that aff
8、ects the role of P.ochraceus as a keystone species within different habitats? （细节题，定位到文章的第三层）(A) The degree to which the habitat is sheltered from waves（符合原文）(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
9、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.ochraceus2. Which of the following hypothetical experiments most clearly exemplifies the method of identifying species ro
10、les that the author considers problematic?（类推题，定位到第二层: identifying a keystone species by removing the species and observing changes in the ecosystem is problematic.）(A) A population of seals in an Arctic habitat is counted in order to determine whether it is the dominant species in that ecosystem. (
11、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 ar
12、e removed from that ecosystem and the ecosystem is then studied.（符合原文信息）(D) Different mountain ecosystems are compared to determine how geography affects a particular species ability to dominate its ecosystem(E) In a grassland experiencing a changing climate,patterns of species extinction are traced
13、 in order to evaluate the effect of climate changes on keystone species in that grassland3. Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage(line24-31)?（类推题，定位到最后一层：Keystone status appears to depend on context
14、）(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(B) A species of iguana occupies a keystone role on certain tropical islands, but does not play that role on
15、adjacent tropical islands that are inhabited by a greater number of animal species（符合原文依环境而定的信息）(C) Close observation of savannah 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 abund
16、ant 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 patterns that camouflage it from potential predators.4. The passage suggests which of the following about the identification of a species as a keystone
17、species?(A) Such an identification depends primarily on the species relationship to the dominant species（新说法）(B) Such an identification can best be made by removing the species from a particular ecosystem and observing changes that occur in the ecosystem（错误表述）(C) Such an identification is likely to be less reliable as an ecosystem becomes less diverse（新说法）(D) Such an identification seems to depend on various factors within the ecosystem（符合原文）(E) Such an identification can best be made by observing predation behavior.（错误表述）
