OG16 RC113 keystone那篇阅读

dudu122

While the most abundant and dominant species within a particular ecosystem
is often crucial in perpetuating the ecosystem, a “keystone” species, here
defined as one whose effects are much larger than would be predicted from its
abundance, can also play a vital role. But because complex species interactions
may be involved, identifying a keystone species by removing the species and
observing changes in the ecosystem is problematic. It might seem that certain
traits would clearly define a species as a keystone species; for example,
Pisaster ochraceus is often a keystone predator because it consumes and
suppresses mussel populations, which in the absence of this starfish can be a
dominant species. 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. Moreover, whereas P. ochraceus occupies an
unambiguous keystone role on wave-exposed rocky headlands, in more wave-sheltered
habitats the impact of P. ochraceus predation is weak or
nonexistent, and at certain sites sand burial is responsible for eliminating
mussels. Keystone status appears to depend on context, whether of particular
geography or of such factors as community diversity (for example, a reduction
in species diversity may thrust more of the remaining species into keystone
roles) and length of species interaction (since newly arrived species in
particular may dramatically affect ecosystems).

113.  The passagementions which of the following as a factor that affects the role of P.
ochraceus as a keystone species within different habitats?
(A) The degree to which the habitat is sheltered from waves
(B) The degree to which other animals within a habitat prey on mussels
(C) The fact that mussel populations are often not dominant within some
habitats occupied by P. ochraceus
(D) The size of the P. ochraceus population within the habitat
(E) The fact that there is great species diversity within some habitats
occupied by P. ochraceus

关于A选项和B选项 想听听大家的见解~谢谢~