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# While the most abundant and dominant species within a particular ecosy

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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
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Passage breakdown

In the first sentence, the author defines a term:

• A "keystone" species is one "whose effects are much larger than would be predicted from its abundance."

Then, the author discusses a problem:

• It's difficult to identify keystone species
• Why is it difficult? Because simply removing a species to see what happens is problematic. Also, certain traits seem like they'd be useful, but aren't. Finally, specific locations can impact whether a species is a keystone or not.

In the last sentence, the author argues that identifying a keystone species depends on context.

For more on the process of breaking down RC passages, check out this article and our live RC videos.

Explanations for individual questions

General Discussion
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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 particular ecosy [#permalink]
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NVM just saw that post above mentioned same problem I mentioned. Disregard my original post!

Can the OA be modified to reflect the correct answer? I didn't see Kyle's post until after I had posted about this issue. Don't want others to overlook our posts and experience the same confusion.

I only googled and stumbled upon the right answer because I couldn't understand why I was wrong (chose A). But of course, afterwards I found out Kyle already stated that in the post above mine. Thanks.
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
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This one was tough for me. Time taken 10 mins.

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
>>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.

B. The degree to which other animals within a habitat prey on mussels
>>certain sites sand burial is responsible for eliminating 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

2. Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage?
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).
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.
>>community diversity. What troubles me in this is "affects bird species within that ecosystem." Does this mean, BS r thriving .

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.
>>I don't see mentioned anything like that in para. As per OG this is OA. Can anyone elucidate this.. I believe PO habitat is an example for the GC. I am correct?
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.

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.
>>Correct. Keystone status appears to depend on context .....

E. Such an identification can best be made by observing predation behavior.
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
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katzzzz wrote:
can someone pls explain Why option c is wrong? as the passage mentioned that "mussel population, which in the absence of starfish can be a dominant species"...Thanks

The question is asking what factors affect the keystone status of the starfish. The fact that mussels are not dominant in some areas is an EFFECT of the starfish in the keystone role, not something that AFFECTS the keystone status. The sheltering from waves does affect the keystone status - making choice A correct.

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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
GMATNinja
I am finding it difficult to stay engaged in passage, especially ones like this.
Can you help implementation of taking notes by identifying passage structure based on
Can you explain last sentence of passage and Q3 in light in your understanding?
In playing more safely to not deviate from author's view I am trying to do exactly what is not recommended: facts - facts - facts; and hence missing crucial flow.
WR,
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
KyleWiddison wrote:
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.

IMO E in Q1 is wrong because of "within some habitats occupied by P . Orchraceus". Occupy here means take control

Posted from my mobile device
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
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omidsa wrote:
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

Passage: Keystone Species

Question: Detail Factor

The Simple Story

First, the passage defines a particular term: a keystone species is one whose effects are much larger than would be predicted from its abundance. In other words, this type of species has a larger impact on the ecosystem than would normally be expected.

How do you know for sure that a species is a keystone species? According to the passage, you can’t just remove that keystone species from the environment to see what happens. “It might seem” that a certain characteristic would define a keystone species, but the language it might seem signals that the author is going to disagree. Sure enough, the author provides a certain example about a possible keystone predator and then shows how that predator may or may not be a keystone species.

The author finally concedes that this question depends on two things: context (with an example given) and length of species interaction (with further elaboration given).

Sample Passage Map

This passage contains just one paragraph, so think about how to delineate different parts of it on your Map. Here is one way to map this passage. (Note: abbreviate as desired!)

KS: >> effect than expctd

Mid: How to ID KS? Hard.

2 things: context + interaction length

The “Mid” designation means “the whole middle of the passage is about this.” KS is an abbreviation for keystone species and ID is an abbreviation for identify.

Step 1: Identify the Question

The language mentions which of the following in the question stem indicates that this is a Detail question. Your job is to find this detail in the passage: what factor affects the role of the P thing as a KS in different habitats?

Step 2: Find the Support

Scan for the P thing; it’s introduced in line 12.

“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.”

The first two sentences talk about P as a KS, but the example doesn’t discuss different habitats until the third sentence, beginning moreover. In one location, P has an unambiguous keystone role, but in another area, P’s predation is weak or nonexistent, so it can’t be playing a keystone role there.

What aspect of the habitats influence this difference? In the first location, there are a lot of waves. In the second, the area is sheltered from the waves.

Step 4: Eliminate and Find a Match

(A) CORRECT. This choice reflects the circumstances described in the passage. When the area is exposed to the waves, P can be a keystone species. When the area is sheltered from the waves, P is much less central to the system.

(B) The example does not address what other animals are preying on the mussels or how that would affect P’s status as a KS.

(C) The passage does state that P consumes and suppresses mussel populations, which could be dominant in the absence of this starfish, suggesting that mussels are not dominant when P is present. However, the next sentence indicates that such predation on a potentially dominant species in not a guarantee of status as a KS in a given habitat.

(D) This choice is a trap. While it makes logical sense that a large population would have a significant impact on an ecosystem, the first sentence specifically defines a KS as one whose effects are much larger than would be predicted from its abundance. In other words, the species is having a much larger impact than you would expect based on its population size—but the effects don’t necessarily vary based on population size itself.

(E) The passage mentions species diversity (line 27) only as a general example of how a reduction in diversity could impact keystone roles in an ecosystem. It does not address species diversity specifically with respect to the P examples.
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
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omidsa wrote:
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).
3. 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.

Passage: Keystone Species

Question: Strengthen

The Simple Story

First, the passage defines a particular term: a keystone species is one whose effects are much larger than would be predicted from its abundance. In other words, this type of species has a larger impact on the ecosystem than would normally be expected.

How do you know for sure that a species is a keystone species? According to the passage, you can’t just remove that keystone species from the environment to see what happens. “It might seem” that a certain characteristic would define a keystone species, but the language it might seem signals that the author is going to disagree. Sure enough, the author provides a certain example about a possible keystone predator and then shows how that predator may or may not be a keystone species.

The author finally concedes that this question depends on two things: context (with an example given) and length of species interaction (with further elaboration given).

Sample Passage Map

This passage contains just one paragraph, so think about how to delineate different parts of it on your Map. Here is one way to map this passage. (Note: abbreviate as desired!)

KS: >> effect than expctd

Mid: How to ID KS? Hard.

2 things: context + interaction length

The “Mid” designation means “the whole middle of the passage is about this.” KS is an abbreviation for keystone species and ID is an abbreviation for identify.

Step 1: Identify the Question

The words if true and most clearly support indicate that this is a Strengthen question. You can answer this one in the same way that you answer Strengthen the Argument Critical Reasoning questions.

Step 2: Find the Support

The question provides a specific line reference:

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 author claims that keystone status depends on two things: context and length of species interaction. What new information would support this claim?

Step 4: Eliminate and Find a Match

(A) These examples do not specifically address geography, community diversity, or the length of interaction among different species.

(B) CORRECT. This example discusses two matters of context mentioned in the passage: geography (different islands) and community diversity (some islands have a greater number of species).

(C) This choice states only that some scientists were wrong about some previous conclusions; it does not address how one can try to determine whether a species has earned keystone status.

(D) This example does not specifically address geography, community diversity, or the length of interaction among different species.

(E) This example does not specifically address geography, community diversity, or the length of interaction among different species.
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
1. From the passage we have " in more wave-sheltered habitats the impact of P. ochraceus predation is weak or nonexistent " This means in other habitats it doesn't act as a keystone. Hence a habitat plays a role in determining whether a species is a keystone. HEnce in this case in wave - sheltered habitats P. ochraceus is not a keystone.
2. From the passage, we have : But because complex species interactions may be involved, identifying a keystone species by removing the species and observing changes in the ecosystem is problematic
3. From the passage, we have "Keystone status appears to depend on context, whether of particular geography or of such factors as community diversity and length of species interaction"
Essentially this means that a species that acts as a keystone in one particular habitat may not act as the same in other habitats. The status depends on 2 factors as mentioned at the end of the passage.
Hence the answer should be B
4. same as 3.
Keystone status appears to depend on context, whether of particular geography or of such factors as community diversity and length of species interaction
Hence the answer should be D
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
Dear All ; For Q2, I need your help in understanding why B&E may not be best answers. Though I chose the correct answer, I had a lot to convince myself for the right choice. What' the key point in B&E that helps eliminate it with the POE process ?

2. Which of the following hypothetical experiments most clearly exemplifies the method of identifying species, roles that the author considers problematic?

B. A species of fish that is a keystone species in one marine ecosystem is introduced into another marine ecosystem to see whether the species will come to occupy a keystone role. .. this explains wrt. Community diversity where the fish, if removed from one marine ecosystem will impact that ecosystem and KS status( if it was or some other species was KS because of fish, elimination of fish will impact the ecosystem. Same manner it will have it' effect on new ecosystem). I had eliminated this option because no reference of fish being KS was mentioned and hence, out of context. Is there anything else ?

E. In a grassland experiencing a changing climate, patterns of species extinction are traced in order to evaluate the effect of climate changes on keystone species in that grassland. - Though [i]I understand author/passage has only mentioned the community diversity and length of association as key and hence stuck of this option. Is there anything else to eliminate besides rationale shared ?[/i]
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
Dear All ; For Q2, I need your help in understanding why B&E may not be best answers. Though I chose the correct answer, I had a lot to convince myself for the right choice. What' the key point in B&E that helps eliminate it with the POE process ?

2. Which of the following hypothetical experiments most clearly exemplifies the method of identifying species, roles that the author considers problematic?

B. A species of fish that is a keystone species in one marine ecosystem is introduced into another marine ecosystem to see whether the species will come to occupy a keystone role. .. this explains wrt. Community diversity where the fish, if removed from one marine ecosystem will impact that ecosystem and KS status( if it was or some other species was KS because of fish, elimination of fish will impact the ecosystem. Same manner it will have it' effect on new ecosystem). I had eliminated this option because no reference of fish being KS was mentioned and hence, out of context. Is there anything else ?

E. In a grassland experiencing a changing climate, patterns of species extinction are traced in order to evaluate the effect of climate changes on keystone species in that grassland. - Though I understand author/passage has only mentioned the community diversity and length of association as key and hence stuck of this option. Is there anything else to eliminate besides rationale shared ?
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
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Question 2

proabhinav wrote:
Dear All ; For Q2, I need your help in understanding why B&E may not be best answers. Though I chose the correct answer, I had a lot to convince myself for the right choice. What' the key point in B&E that helps eliminate it with the POE process ?

2. Which of the following hypothetical experiments most clearly exemplifies the method of identifying species' roles that the author considers problematic?

We can eliminate choices most confidently when we are clear on what the question is asking us to do. This enables us to get rid of choices that simply aren't answering the question.

In this case, Question #2 presents five hypothetical experiments. The question stem asks us which of the experiments most clearly exemplifies the method of identifying species' roles that the author considers problematic.

According to the author's own words, what method does the author consider to be problematic?

"But because complex species interactions may be involved, identifying a keystone species by removing the species and observing changes in the ecosystem is problematic."

The method in question is "removing the species and observing changes in the ecosystem." So we're going to eliminate any choice that doesn't name this method of identifying the role of a given species.

Quote:
(B) A species of fish that is a keystone species in one marine ecosystem is introduced into another marine ecosystem to see whether the species will come to occupy a keystone role.

The method used in experiment (B) is introducing a species into an ecosystem. This is not the same as removing a species from its ecosystem. That's why we eliminate (B).

Quote:
(E) In a grassland experiencing a changing climate, patterns of species extinction are traced in order to evaluate the effect of climate changes on keystone species in that grassland.

The method used in experiment (E) is tracing patterns of species extinction. This has nothing to do with removing a species from its ecosystem. That's why we eliminate (E).

Question 3

csaluja wrote:
Could someone please explain the reasoning behind option B for Q3? I choose C instead based on this sentence from the passage: "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). Isn't the bold part what option C is saying?

That's not quite what choice (C) says. Here's (C) again:
Quote:
(C) Close observation of a savannah ecosystem reveals that more species occupy keystone roles within that ecosystem than biologists had previously believed.

There's no mention here of a reduction in species diversity, and definitely no mention of remaining species being thrust into keystone roles.

Instead, (C) describes a scenario where there is no change to the species, no change to the roles that species play, and no change to the species' setting. The only thing at work here is biologists' belief in the roles that species occupy (which change after close observation).

This doesn't line up with the passage's description of keystone status depending on context. Instead, it focuses on biologist's observation of keystone status. That's why we eliminate (C).

Compare this to choice (B):
Quote:
(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.

(B) remains the best answer choice because it does spell out an example of a keystone role depending on context. The iguana's keystone role on one island doesn't exist when the iguana inhabits a different island.

I hope this helps!
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
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main point - keystone species; example ; determining keystone.

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?

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.

(A) The degree to which the habitat is sheltered from waves -

-----------------------------------

2. Which of the following hypothetical experiments most clearly exemplifies the method of identifying species, roles that the author considers problematic?

(C) In order to determine whether a species of monkey is a keystone species within a particular ecosystem, the monkeys are removed from that ecosystem and the ecosystem is then studied.

-----------------------------------------

3. Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage?

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 one follows these two rules.

(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.
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4. 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. - this is not sure in all such cases.
(B) Such an identification can best be made by removing the species from a particular ecosystem and observing changes that occur in the ecosystem. - will it follows every such case, for sure no.
(C) Such an identification is likely to be less reliable as an ecosystem becomes less diverse. - this way one can not identify.
(D) Such an identification seems to depend on various factors within the ecosystem. - yes , two are defined in last sentence.
(E) Such an identification can best be made by observing predation behavior. - no
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
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I want to talk about how to answer the questions in RC question to ge highest accuracy.

the following explanation comes from my experience of a few years learning gmat. I hope it is good for non native.

we have 2 ways to answers questions in RC. prethinking and POE (power of elimination)

for easy questions, when we read a question, we can prethink an answer and then we go to answer choices and find a match. this way is good if the information prethought and the answer choice are the same.

but for harder question, after prethinking , we can not find out the match between the the prethough information and an answer choice. the reason for this case is because the information in the correct answer need generalization of information is the passage or because the wrong answers is wrong in one or 2 words.
if we face this case, we use POE. POE is wrong method which permit us realize wrong answer choices which is wrong in 1 or 2 words.

so my approach is
first, prethink. this step help us understand the passage better, and help us eliminate quickly 2 or 3 wrong answer choices. for hard questions, prethink dose not help us eliminate any answer choices.
second, POE. this step is help us eliminate wrong answers close to the correct one. this method can be applied to all 4 answer choices or can be applied after prethinking.

gmat makes hard questions by making questions wrong in one or 2 words and we have to use POE for this case.

back to passage above
for the first 3 questions, we can apply prethinking to find the correct question. we do not need apply POE.
for the 4rd questions, we need to apply POE to eliminate choice B " best studied ". this choice is wrong in just one word .
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
Very Nice Passage Indeed,

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?

Find the Portion 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

From the Above line we infer that wave shelter plays a role. Option A states that

(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

2. Which of the following hypothetical experiments most clearly exemplifies the method of identifying species, roles that the author considers problematic?

Look at the Excerpt from the passage ,

“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.

So basically The keystone Species has to be removed from the eco system and then needs to study the changes in the Ecosystem ,the Experiment which is a bit problematic. Option C says

(A) A population of seals in an Arctic habitat is counted in order to determine whether it is the dominant species in that ecosystem,

(B) A species of fish that is a keystone species in one marine ecosystem is introduced into another marine ecosystem to see whether the species will come to occupy a keystone role.

(C) In order to determine whether a species of monkey is a keystone species within a particular ecosystem, the monkeys are removed from that ecosystem and the ecosystem is then studied.

(D) Different mountain ecosystems are compared to determine how geography affects a particular species, ability to dominate its ecosystem.

(E) In a grassland experiencing a changing climate, patterns of species extinction are traced in order to evaluate the effect of climate changes on keystone species in that grassland.

3. Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage?

Last sentence is 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).

So author is basically trying to say that A species which is Keystone depends upon number of factors .

If we closely see Option B clearly states that

(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.

4.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.

Discussed in Question 3

(E) Such an identification can best be made by observing predation behavior.
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Re: While the most abundant and dominant species within a particular ecosy [#permalink]
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choice b and e in question 4 contain only 1 wrong word, "best".
in the specific question, the wrong answer contain some wrong words, and sometimes, as in this question, 1 wrong word.
so, read the answer choice and try to find out just 1 wrong word.

for main idea question, the tricky wrong answer mentions most of the passage while the correct answer mention all of passage. be ready to dederentiate these close answer choices to win.
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