Gwd 2-4

qiqisu

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

Question 4:

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.

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

答案是B，我觉得应该是D，因为B中内容是原文明显反对的观点（一段末），请大家帮忙看看。。谢谢。。

xealot

Totally agree with you.

whoami2

2-4赞成 D

fly_zhao123

Pisaster ochraceus isoften a keystone predator becauseit                            8
consumes and suppressesmussel populations, which in the absence of thisstarfish              9

can be a dominant species.
根据这句话，我怎么觉得答案是A啊，认为D的能给说一下为啥吗？
谢了！