Question 2 – 4:

While the most abundant and dominant species within a particular ecosystem 1

is often crucial in perpetuating the ecosystem, a “keystone” species, here defined 2

as one whose effects are much larger than would be predicted from its appearance, 3

also play a vital role. But because complex species interactions may be involved, 4

identifying a keystone species by removing the species and observing changes in 5

the ecosystem is problematic. 6

It might seem that certain traits would clearly define a species as a keystone 7

species; for example, Pisaster ochraceus is often a keystone predator because it 8

consumes and suppresses mussel populations, which in the absence of this starfish 9

can be a dominant species. But such predation on a dominant or potentially dominant 10

species occurs in systems that do as well as in systems that do not have species that 11

play keystone roles. Moreover, whereas P. ochraceus occupies an unambiguous 12

keystone role on wave-exposed rocky headlands, in more wave-sheltered habitats 13

the impact of P. ochraceus predation is weak or nonexistent, and at certain sites sand 14

burial is responsible for eliminating mussels. Keystone status appears to depend on 15

context, whether of particular geography or of such factors as community diversity 16

(for example, a reduction in species diversity may thrust more of the remaining species 17

into keystone roles) and length of species interaction (since newly arrived species 18

in particular may dramatically affect ecosystem). 19

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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

这题为何选 D. 我选的是E, 为什么不对呢?