GWD 2-2

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).(19)     

Question 2:The primary purpose of the passage is to

A.                 point out some of the differences between dominant and keystone species

B.                 emphasize the complexity of the interactions that occur between two particular species

C.                 detail the effect of a particular habitat on the role occupied by a certain keystone species

D.                 illustrate the importance of community diversity for the perpetuation of an ecosystem

E.                  explain some considerations involved in determining whether a species occupies a keystone role   

    MINE: E

ANS: D

HELP!

really?

I almost can't believe it! My reading is so weak.

楼主，2-2我也选的是E，当看见“标准答案”的时候我简直莫名其妙。嗯，这下总算找到知音了