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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).
【OG20-P449-579题】
The passage suggests which of the following about the identification of a species as a keystone species?
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A错误，primarily带有明显标志
B错误，best带有明显标志
C错误，没有出现这个比较
D正确，文章后半部分开始讨论影响基石物种的各种因素
E错误，best使该选项带有明显标志
美美全错，哎
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2021-10-17 10:28:42
A项：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.
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2021-10-02 22:48:50
Keystone status appears to depend on context
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2020-02-25 17:56:21
B选项漏看了best
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2019-09-27 09:18:41
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