GWD2 2 :I suspect I am right!!!

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 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

The key is D! But I choose E.

Help!!!

the first paragraph introduces the definition of "key stone", then the first sentence of second paragraph states the TS of the passage: It might seem that certain traits would clearly define a species as a keystone  species. Giving the example of P, second P describes some detailed factors. Finally the passage summarizes all factor that Keystone status appears to depend on .

So the answer D is right.  

唉，还是不懂！

sorry,I made a mistake.I will replay your Q later.

hehe

agree!

Further Help Needed!!

呵呵

我也说不至于这么荒谬

顶一下，这篇阅读有没有标准的答案？？

楼上的问题我也有同感？第四题为什么是c??而不是D?