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).
The passage mentions which of the following as a factor that affects the role of P. ochraceus as a keystone species within different habitats?
The degree to which the habitat is sheltered from waves
The degree to which other animals within a habitat prey on mussels
The fact that mussel populations are often not dominant within some habitats occupied by P. ochraceus
The size of the P. ochraceus population within the habitat
The fact that there is great species diversity within some habitats occupied by P. ochraceus
提出一个概念“keystone” → 辨别方法难→ 举例子 P.o → 环境对keystone影响大。