GWD2 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).     

Question 3:

Which of the following, if true, would most clearly support the argument about keystone

status advanced in the last sentence of the passage (lines 15 – 19)?

A.     A species of bat is primarily responsible for keeping insect populations within an

ecosystem low, and the size of the insect population in turn affects bird species

within that ecosystem.

B.     A species of iguana occupies a keystone role on certain tropical islands, but does

not play that role on adjacent tropical islands that are inhabited by a greater number

of animal species.

C.     Close observation of a savannah ecosystem reveals that more species occupy

keystone roles within that ecosystem than biologists had previously believed.

D.     As a keystone species of bee becomes more abundant, it has a larger effect on

the ecosystem it habits.

E.      A species of mouse that occupies a keystone role in a prairie habitat develops

coloration patterns that camouflage it from potential predators.

为什么选A,文中有些句子不明白

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.  这句的成分是怎么样的?看不明白

agree

but    那句话的意思是什么呀

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).

打括号的就是并列成分，不知对否，讨论中

再就是谁帮我看看第二题的主旨题啊，我居然选C？

第二题选E吧,谢谢对成分的分析,现在才看到

But such predation on a dominant or potentially dominant  species occurs (in systems that do have species that  play keystone roles) as well as (in systems that do not have species that  play keystone roles).

括号内部分并列；红字部分省略。