GWD阅读的一道疑问

Joshwu

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 appearance, also play a vital role. But because complex species interactions may be involved, identifying a keystone species by removing the species and observing changes in the ecosystem is problematic.

It might seem that certain traits would clearly define 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 systems 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 headlands, in more wave-sheltered habitats the impact of P. ochraceus predation is weak or nonexistent, and at certain sites sand burial is responsible for eliminating mussels. Keystone status appears to depend on context, whether of particular geography or of such factors as community 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 ecosystem).

Q33:

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 particularecosystem 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.

E.Such an identification can best be made by observing predation behavior.

正确答案为D. 但我认为B是正确的。文中第一段 a “keystone” species, here defined as one whose effects are much larger than would be predicted from its appearance, also play a vital role. But because complex species interactions may be involved, identifying a keystone species by removing the species and observing changes in the ecosystem is problematic. 用一个虚拟语气表达，如果这种species对于ecosystem的作用比它存在时要大，那么就是说它是keystone，只是由于如果人为removal这种species会带来problems，所以采用其他的方法去define keystone。

Q33:

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 particularecosystem 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.

E.Such an identification can best be made by observing predation behavior.

正确答案为D. 但我认为B是正确的。文中第一段 a “keystone” species, here defined as one whose effects are much larger than would be predicted from its appearance, also play a vital role. But because complex species interactions may be involved, identifying a keystone species by removing the species and observing changes in the ecosystem is problematic. 用一个虚拟语气表达，如果这种species对于ecosystem的作用比它存在时要大，那么就是说它是keystone，只是由于如果人为removal这种species会带来problems，所以采用其他的方法去define keystone。