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While the most abundant and dominant species within a

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Intern
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While the most abundant and dominant species within a [#permalink] New post 21 Nov 2012, 09:34
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).
1. 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
[Reveal] Spoiler:
E


2. Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage?
A. A species of bat is primarily responsible for keeping insect populations within an ecosystem low, and the size of the insect population in turn affects bird species within that ecosystem.
B. A species of iguana occupies a keystone role on certain tropical islands, but does not play that role on adjacent tropical islands that are inhabited by a greater number of animal species.
C. Close observation of a savannah ecosystem reveals that more species occupy keystone roles within that ecosystem than biologists had previously believed.
D. As a keystone species of bee becomes more abundant it has a larger effect on the ecosystem it inhabits.
E. A species of moth that occupies a keystone role in a prairie habitat develops coloration patterns that camouflage it from potential predators.
[Reveal] Spoiler:
B


3. The passage suggests which of the following about the identification of a species as a keystone species?
A. Such an identification depends primarily on the species’ relationship to the dominant species.
B. Such an identification can best be made by removing the species from a particular ecosystem and observing changes that occur in the ecosystem.
C. Such an identification is likely to be less reliable as an ecosystem becomes less diverse.
D. Such an identification seems to depend on various factors within the ecosystem.
E. Such an identification can best be made by observing predation behavior.
[Reveal] Spoiler:
D


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Re: While the most abundant and dominant species [#permalink] New post 21 Nov 2012, 09:34
Can someone explain why (B) in Q2?
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Re: While the most abundant and dominant species within a [#permalink] New post 24 Nov 2012, 06:41
Expert's post
First, I have to say that the OA for the first question is A, not E.

As for question 2, you must answer the question in relation to the information provided in the last sentence. When you see these questions that relate to specific parts of the passage, you must go to those parts and read them carefully. This question becomes difficult because the last sentence has a pretty crazy structure with a leading clause followed by some modifying text with two parenthetical statements. You have to focus on the key points: keystone status depends on the context - geography, diversity, and length of interaction.

2. Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage?
A. A species of bat is primarily responsible for keeping insect populations within an ecosystem low, and the size of the insect population in turn affects bird species within that ecosystem. No discussion of context here.
B. A species of iguana occupies a keystone role on certain tropical islands, but does not play that role on adjacent tropical islands that are inhabited by a greater number of animal species. This shows how the geographical context (different tropical islands with different species diversities) impacts keystone status. This fits perfectly with the argument in the last sentence.
C. Close observation of a savannah ecosystem reveals that more species occupy keystone roles within that ecosystem than biologists had previously believed.While this discusses keystone roles, it doesn't talk about differences in context impacting keystone status.
D. As a keystone species of bee becomes more abundant it has a larger effect on the ecosystem it inhabits. No discussion of how context impacts keystone status.
E. A species of moth that occupies a keystone role in a prairie habitat develops coloration patterns that camouflage it from potential predators. Again, no discussion of context impacts on keystone status.
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Re: While the most abundant and dominant species within a   [#permalink] 24 Nov 2012, 06:41
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