While the most abundant and dominant species within a particular ecosy

Question

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 
 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

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

(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 
 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.

4 
 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.
(E) Such an identification can best be made by observing predation behavior.

RC00556-03
RC00556-04
RC00556-05
RC00556-06

GMATNinja · Accepted Answer

In this passage, it is tricky to apply the techniques recommended in the  Ultimate RC Guide for Beginners  because we only have one paragraph!

In this case, you might break down the passage into logical chunks and ask yourself WHY the author has written each chunk:

Okay, so it looks like the author is trying to introduce/define the concept of a "keystone" species and to tell us that identifying/studying a keystone species using a seemingly simple approach is actually problematic. So you might jot down: "introduce/define "keystone" species; identifying/studying K.S. by removing/observing is problematic".

Great, that gives us an idea of WHY the author wrote that first chunk. Even though we might not completely understand every detail, let's move on to the next chunk:

The purpose of this chunk is described in the first sentence. The author wants us to realize that even though it might  seem  that certain traits would clearly define a species as a keystone species, those traits do not necessarily indicate that we have a keystone species. So you might jot down: "cannot determine K.S. solely by identifying certain traits".

The same species can be keystone in some habitats and not keystone in other habitats. Keystone status depends not solely on certain traits but rather on context (i.e. geography and community diversity). The purpose of this chunk is to explain that "keystone status depends on habitat/geography, community diversity, and other factors".

Okay, so we have

Chunk 1: "introduce/define "keystone" species; identifying/studying K.S. by removing/observing is problematic"
Chunk 2: "cannot determine K.S. solely by identifying certain traits"
Chunk 3: "keystone status depends on habitat/geography, community diversity, and other factors"

Now, what is the purpose of the passage as a whole? It seems like the author wants to introduce the concept of a keystone species and then explain and provide evidence to support that determining keystone status is not a simple task. So, for the purpose of the passage, you might write: "introduce/define "keystone" species; show that determining keystone status is complex".

Hopefully that helps you get your head around this complicated passage without getting bogged down in the details!

See if question #3 makes more sense in light of this analysis. As a starting point, remind yourself of the purpose of that chunk (chunk #3).

I hope that helps!

jacquerc · Answer

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   Correct. In more wave-sheltered habitats the impact of P. ochraceus is weak or nonexistent. 
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? 
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.    Correct. A problematic way to identify a keystone is to move the species and oberve changes in the ecosystem. 
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? 
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 reflection of a particular geography, community diversity or length of species interaction. 
 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.   Correct. It refers to the community diversity. 
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. Not related 
E. A species of moth that occupies a keystone role in a prairie habitat develops coloration patterns that camouflage it from potential predators. Not related

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.  Author mentioned this method is problematic. 
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.   Predation behavior also occurs in systems that do not have keystones.

GMATNinja · Answer

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 
 
 Question 1
 
  Part 1  
  Part 2    
  Question 2  
 Question 3
 
  Part 1  
  Part 2    
  Question 4

KyleWiddison · Answer

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.

JarvisR · Answer

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
(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?

(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?

(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.
(E) Such an identification can best be made by observing predation behavior.

adkikani · Answer

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 your post: https://gmatclub.com/forum/experts-topi ... l#p1857560

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,
Arpit

BillyZ · Answer

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.”

Step 3: Predict an Answer

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.

BillyZ · Answer

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). ”

Step 3: Predict an Answer

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.

proabhinav · Answer

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?

proabhinav · Answer

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 ?

GMATNinja · Answer

Question 2 
 
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.

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).

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 
 
That's not quite what choice (C) says. Here's (C) again:
 
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):
 
(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!

Maddy16 · Answer

HI GMAT NINJA, kindly clarify the following wrt Q1

"The passage mentions which of the following as a factor that affects the role of P. ochraceus as a keystone species within different habitats?"

Reworded: factors that affect PO's role as keystone species within different habitats?

The correct answer A, assumes that the degree of waves have an affect the role of PO?

jack0997 · Answer

GMATNinja ,

Q4: The passage suggests which of the following about the identification of a species as a keystone species?

Why is option B: Such an identification can best be made by removing the species from a particular ecosystem and observing changes that occur in the ecosystem.

is wrong? I get that it is problematic, but option B uses superlative word BEST, an ideal scenario. If that BEST (albeit hypothetical) condition is served, the purpose is solved.

Pl. explain. Thank you.

GMATNinja · Answer

Question 1 
 
Question 1 asks us which answer choice contains a factor that affects the role of P. ochraceus as a keystone species within different habitats.

(A) tells us:
 
This doesn't say that the  degree of the waves  has an effect on the role of P. ochraceus. This tells us that we need to look at the  amount of protection from the waves  that a habitat has to determine whether  P. ochraceus  will play a keystone role in that habitat.

In the third chunk of the passage, we're told:

"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"

This tells us that the role of P. ochraceus varies depending on whether it is in a wave-exposed headland or a wave-sheltered habitat -- in other words, how protected from the waves the habitat is. This is what (A) is saying, so (A) is the answer to this question.

Question 4 
 
Let's take a closer look at (B):
 
(B) does not talk about an ideal, hypothetical scenario where the purpose is solved -- it tells us that the BEST  method  to identify a keystone species is to remove it from the ecosystem and observe any changes.

Comparing (B) to this part of the paragraph gives you enough reason to eliminate (B):

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

There is no mention in the passage of anything that would imply that removing a species would somehow be BEST in some context. The  only  thing we know about species removal is that it is problematic.

For this reason, (B) cannot be the answer to this question.

Compare this to (D):
 
The final sentence of the passage tells us:

"Keystone status appears to depend on context, whether of particular geography or of such factors as community diversity...and length of species interaction"

This part of the passage gives us some of the  "various factors within the ecosystem"  we need to consider in identifying a keystone species. A further illustration of this in the case of  P. ochraceus  is given earlier in the passage to outline the various factors in that particular case.

The passage does not say there is any one best way to identify a keystone species. We're told that various factors affect this identification and, from this information, we can say that (D) is the best answer to this question.

I hope that helps!

homersimpsons · Answer

OE 
Q1. 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 wavesB. 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
 Supporting idea 
This question depends on recognizing what the passage states about the factors affecting P. ochraceus’s role as a keystone species, which is different in different habitats. According to the passage, P. ochraceus consumes and suppresses mussel populations in some habitats—specifically, those that are wave-exposed —making it a keystone predator in those habitats. But in wave-sheltered habitats, P. ochraceus does not play the same role in suppressing mussel populations.
A.  Correct . The passage clearly states that P. ochraceus’s role in wave-exposed habitats differs from its role in wave-sheltered habitats.
B. The passage says that the impact of P. ochraceus predation on mussels is not strong in wavesheltered habitats, but this is not—at least not at all sites—because other animals are preying on the mussels; rather, at least at some sites, it is because mussels are controlled by sand burial.
C. The passage does not suggest that mussel populations are dominant in any habitats occupied by P. ochraceus.
D. The size of the P. ochraceus population affects the size of the mussel population within waveexposed habitats, but the passage does not suggest that P. ochraceus’s role as a keystone species depends on the size of its population within those habitats.
E. The only other species the passage mentions in conjunction with P. ochraceus habitats is the mussel; the passage does not address species diversity in these habitats.
 The correct answer is A.

Q2. Which of the following hypothetical experiments most clearly exemplifies the method of identifying species’ roles that the author considers problematic?
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.
 Application
 Answering this question depends on recognizing what the author says about identifying species’ roles in habitats and then extending that to another situation. The author considers a particular method of studying keystone species problematic: removing a suspected keystone species from its habitat and observing what happens to the ecosystem. The author finds this problematic because interactions among species are complex.
A. The author does not discuss counting the members of a population as a problematic way of determining whether that population is a dominant species.
B. The method that the author finds problematic has to do with observing what happens to anecosystem when a keystone species is removed from it, not with observing what happens to a different ecosystem when the species is introduced into it.
C.  Correct . The author states explicitly that removing a species from a habitat in order to determine its keystone status is problematic. Removing the monkeys from their habitat is a clear example of this problematic practice.
D. Comparison of habitats in order to determine geography’s effect on a particular species’ dominance would most likely find favor with the author, for this is the approach the author seems to advocate in investigating P. ochraceus’s keystone status.
E. The author does not discuss tracing patterns of extinction or changing climates in the passage.
 The correct answer is C.

Q3. Which of the following, if true, would most clearly support the argument about keystone status advanced in the last sentence of the passage (lines 24–31)?
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.
 Evaluation 
To answer this question, focus on the argument advanced in the last sentence of the passage and identify what information would support that argument. In the last sentence of the passage, the author claims that keystone status depends on context. The author then offers three contextual factors that may affect a species’ keystone status: geography, community diversity (i.e., the number of species in a given habitat), and length of species interaction. Evidence supporting this argument would show that context is important to a species’ keystone status.
A. This scenario does not indicate anything about keystone status; this is simply a description of how species populations in a single ecosystem affect one another.
B.  Correct . That the iguana is a keystone species in a location that has limited species diversity but not a keystone species in a location that has greater species diversity suggests that keystone status does indeed depend on context. Thus, this example supports the author’s argument in the last sentence of the passage.
C. That biologists were mistaken about keystone species in a particular ecosystem does not have a bearing on whether keystone status is context dependent.
D. It is not surprising that an increase in a species’ population would lead to that species having a larger effect on its ecosystem—but this does not speak directly to the question of whether keystone status itself depends on context.
E. A keystone species enhancing its ability to survive in a single ecosystem does not lend any support to the idea that keystone status depends on context. The moth’s keystone status would have to undergo some change for this to have a bearing on the question of context.
 The correct answer is B.

Q4. 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.
 Inference 
Answering this question requires identifying how the passage suggests that keystone species should be identified. The passage identifies a particular way in which keystone status should not be determined: removing a species and observing what happens to the ecosystem. The passage also argues that keystone status depends strongly on context: that is, an ecosystem’s characteristics, including its geography and inhabitants, determine its keystone species.
A. While the passage uses an example of a keystone species, P. ochraceus, which preys on a species that would, in the keystone species’ absence, be dominant, there is nothing to suggest that a keystone species must have a particular relationship with the dominant, or potentially dominant, species in an ecosystem.
B. The passage explicitly states that this method of identification would be problematic.
C. A reduction in an ecosystem’s diversity might alter which species occupy keystone roles in that ecosystem, the passage suggests, but there is no indication that identifying such species would become more difficult.
D.  Correct . If, as the passage suggests, keystone status for any given species depends on the context of the ecosystem in which it lives, then it is likely that identifying keystone species depends strongly on understanding what factors of the ecosystem contribute to creating keystone status. The passage lists such factors as geography, community diversity, and species interaction.
E. While the passage uses a predator, P. ochraceus, as its example of a keystone species, there is no indication that predation is an essential component of the actual definition of keystone species (one whose effects are much larger than would be predicted from its abundance).
 The correct answer is D.

sagarsangani123 · Answer

Dear Experts,

Though for question 1), the OA is (A), I am confused between (A) and (B). The passage mentions that: "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".  . Can't answer choice (B) be inferred from the boldfaced part?

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

Kindly help resolve. 
Thanks!

GMATNinja · Answer

Question 1 
 
First, it’s important to clearly identify what exactly question 1 is asking for. Take a closer look:

The question does not ask us to find an answer choice that can be  inferred  on the basis of the passage. Rather, it asks for an answer choice that is  mentioned  in the passage. This subtle, but important, distinction means that we’re looking for an answer choice that is explicitly mentioned in the passage.

With that in mind, here’s (B):

The quote you referenced from the passage indicates that “at certain sites  sand burial  is responsible for eliminating mussels.” But sand burial is not an  animal . So, while we could  infer  that in certain habitats the degree to which other animals prey on mussels affects whether or not  P. ochraceus  is a keystone species, that is not  mentioned  in the passage. Eliminate (B).

I hope that helps!

Vegita · Answer

Hi  GMATNinja

I was down to option A and B. However, I went ahead with option A because I thought that option B addresses only the two given factors out the three factors mentioned by the author.

(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.

Option B talks about geography and diversity but fails to mention the lenght of interaction among the species. So I went with option A on impluse.

GMATNinja · Answer

Question 3 
 
Question 3 asks which answer choice would " most clearly support  the argument about keystone status advanced in the last sentence of the passage."

So, we're not looking for an answer choice that  perfectly  supports the argument in the last line -- instead, we're just looking for the answer choice that supports the argument  more clearly  than the other options.

Here is that last line:
 
There are several factors listed here, but the author doesn't say that ALL of these factors MUST be at play in order for a species to achieve keystone status. In fact, the author implies that they are NOT all at play at the same time. Keystone status depends on context, but that context could be "particular geography,"  or  it could be something like "community diversity" and "length of species interaction."

Here is (B):
 
The status of this species of iguanas depends on context -- specifically, community diversity seems to impact its keystone status. This clearly supports the argument in the last line of the passage.

And here's (A):
 
This answer choice is not about context at all, so it really doesn't support the argument in the last line of the passage. In fact, it seems to connect back to an idea presented earlier in the passage -- that perhaps keystone species can be identified by their predation habits. This line of reasoning is not explored at all in the last line of the passage, so (A) is not the right answer.

(A) is out, and (B) is the correct answer to question 3.

I hope that helps!

While the most abundant and dominant species within a particular ecosy

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Reading Comprehension (RC) Questions

While the most abundant and dominant species within a particular ecosy

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards