单项选择题

  Line      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
    (5)    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
    (10)    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
    (15)    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
    (20)    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
    (25)    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
    (30)    interaction (since newly arrived species in particular
            may dramatically affect ecosystems).   The passage mentions 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