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。。 | 
