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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-P449-578题】
Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage?
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答案点：for example, a reduction in species diversity may thrust more of the remaining species into keystone roles
说明diversity↓ → ↑specie成为keystone
B ————邻岛species多，diversity大，所以不是keystone；本岛之所以有keystone，是species较少的原因。
原文意思：关键地位似乎取决于环境，无论是特定地理环境还是社区多样性等因素（例如，物种多样性的减少可能会将更多的剩余物种推到关键地位）和物种相互作用的长度（因为新到达的物种尤其可能会显著影响生态系统）。 选项B：鬣蜥物种在某些热带岛屿上占据着关键地位，但在邻近的热带岛屿上却没有扮演这一角色，这些岛屿上居住着大量的动物物种。 所以鬣蜥物种在隔壁岛屿上没有取得关键作用是因为另一个岛的生物多样性更为丰富，从而推回来，鬣蜥物种在自己岛上取得关键作用的原因可能是盖岛生物多样性没有那么丰富，从而也符合原文关于生物多样性影响关键地位的观点
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2021-11-20 12:21:54
by a greater number of animal species 是diversity的restatement
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2020-12-30 16:52:31
选项意思没读懂。context与geography，community diversity和length of species interaction有关。A是两个物种之间的interaction。B是同一物种在不同的环境的地位不同-geography。C人类观察 D keystone越多，对环境影响越大。Emoth会掩饰自己是potential。
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2019-09-27 09:16:28
最后一句想表达的就是各种因素变化对role的影响，只有B变现出来因为某些（地理）因素的改变，而造成role的改变；
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2018-10-08 19:05:34
。。。最后一句说影响一个物种作为keystones有可能是一个特定的地理环境，也可能是物种的密度。 B选项说i在一个地方是ks,但在另一个地方由于受到另一种群数量过大的影响，而不能成为ks。 虽然我觉得A好像也在说种群密度的事。。。难道是因为A没有说KS？？
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2017-10-22 19:39:33
四圈回复河边柳青青
最后一段说的diversity是多样性的意思呀，是种类的问题。密度是density，两个词有点像
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2018-01-02 11:54:52
逻辑支持题，作者的论点为是否是基石物种要看环境 A错误，没提及上述因素 B正确 C错误，没提及上述因素 D错误，没提及上述因素 E错误，没提及上述因素
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2015-11-10 20:59:43
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