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题库 / OG2016-RC-113
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
文章大意：
提出一个概念“keystone” → 辨别方法难→ 举例子 P.o → 环境对keystone影响大。
题目分析：
文章细节题：在不同的栖息地，以下哪点影响P作为keystone的角色？
原文：在wave-exposed的多岩石岬，P绝对是keystone，但在wave-sheltered栖息地，P的作用就很弱或者不连续，并且……
选项分析：
A选项：正确。栖息地被waves遮盖的程度：原文对比了在exposed和在sheltered的情况。
B选项：栖息地中其他动物捕猎mussel的程度：无关。
C选项：在一些栖息地，Mussel数量不由P主导：无关。
D选项：栖息地P的数量：无关。
E选项：P主导的栖息地中生物很多样：无关。
定位到后面的wave exposed就可以知道wave的冲击可以影响P的地位，选A
rc
suppresses 抑制压制
定位：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.
within different habitats 注意仔细阅读问题
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.
这篇总体还算简单，中心句就一句： a "keystone" species can also play a vital role. 前面让步提到了 the most abundant and dominant species ，所以后面有 the most abundant and dominant species 和keystone species的对比，但不是本文重点。本文重点在于解释怎么分辨keystone species。先提出不能通过remove the species and observing changes，因为生态环境有其复杂性。可以用一些特征来分辨，举了个P predation的例子，总结却说predation也不是完全靠谱，可能有K species，也可能没有。又用P说明k species的属性会因环境改变（w-expose和w-shelter），最终得出结论 Keystone status appears to depend on context,，并且合格context是受很多因素影响的
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）.这种对于dominant species的predation也发生在那些没有keystone的系统里
文章中具体的例子也要具体看
文章中总结性的话一定要认真读
Specific; 定位：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.
细节题，定位错了定到最后两个factors去了，但是题目问的就是P这个生物，文中有具体的原因。
比较了 wave-exposed和more wave-sheltered地点的P.o的情况，所以答案是a
定位词：Moreover
阐述了P在两种情况下的主导性 wave-exposed 和 wave-sheltered
答案A，错选B
细节题定位： Moreover, whereas P. ochraceus occupies an unambiguous keystone role on wave-exposed rocky headlands, in more wave-sheltered habitats the impact of P.
答案是a
B无法定位，答案应该是A吧……
这题的答案是A吧？答案错了