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

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While the most abundant and dominant species within a particular ecosy

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ag153

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06 Apr 2022, 22:00

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Q1- C- They have cited 2 factors that affect P's status as dominant species: 1) Wave exposure and 2) Elimination of mussells in certain areas due to sand burials- (less mussells in certain areas)

Now choice C is in line with the second reason. Why is it wrong? We are told that number of mussells are not dominant as per C so we can say that yes that could affect the key stone status. Pls clarify.

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ag153

Note what option (C) says:

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?

(C) The fact that mussel populations are often not dominant within some habitats occupied by P. ochraceus

The passage does not mention that mussel populations are often not dominant within some habitats. It mentions that "at certain sites sand burial is responsible for eliminating mussels" just like at other sites where mussels are dominant, P. ochraceus eliminates mussels.
So at sand burial sites too, mussels could be the dominant species. Their method of elimination is sand burial and not P. ochraceus.
The passage doesn't say that at some sites mussels are not dominant.

Hence (C) is not correct.

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Updated on: 10 Apr 2022, 00:36

Originally posted by ag153 on 08 Apr 2022, 02:47.
Last edited by ag153 on 10 Apr 2022, 00:36, edited 2 times in total.

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KarishmaB

1. Yes but that does not stop it from being a factor. As from the passage, we jsut need to determine the factor not say whether factor holds true as per passage or not?

2. Can you pls elaborate more on your point in C by relating to passage?

3. What do these 2 sentences mean? Pls explain the sentence by breaking the sentence structure up and what roles do they play in the passage or wat insight do they provide:
a. 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.

b.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.--- Doesn't this imply that impact of P's predation is weak at both wave sheltered habits AND at sites where sand burial is responsible for eliminating mussells?

BillyZ

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While the most abundant and dominant species within a particular ecosystem is often crucial in perpetuating the ecosystem, a “keystone” species, here defined as one whose effects are much larger than would be predicted from its abundance, can also play a vital role. But because complex species interactions may be involved, identifying a keystone species by removing the species and observing changes in the ecosystem is problematic. It might seem that certain traits would clearly define a species as a keystone species; for example, Pisaster ochraceus is often a keystone predator because it consumes and suppresses mussel populations, which in the absence of this starfish can be a dominant species. But such predation on a dominant or potentially dominant species occurs in systems that do as well as in systems that do not have species that play keystone roles. Moreover, whereas P. ochraceus occupies an unambiguous keystone role on wave-exposed rocky headlands, in more wave-sheltered habitats the impact of P. ochraceus predation is weak or nonexistent, and at certain sites sand burial is responsible for eliminating mussels. Keystone status appears to depend on context, whether of particular geography or of such factors as community diversity (for example, a reduction in species diversity may thrust more of the remaining species into keystone roles) and length of species interaction (since newly arrived species in particular may dramatically affect ecosystems).

Show Spoiler

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.”

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.

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ag153

The passage MENTIONS ONLY "wave exposure" as a factor -
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 (next is an independent clause joined with comma and 'and' so it doesn't have any connection to the previous clause)

P. ochraceus is keystone in wave exposed regions but in more wave sheltered, it isn't.

"at certain sites sand burial is responsible for eliminating mussels." only means that at some places, sand burial eliminated mussels. We don't even know whether P. ochraceus is present there or not, whether mussels ae dominant species there or not etc. We cannot assume anything.

Look at the sentences you mentioned in context. Here is the SIMPLIFIED PASSAGE:

The dominant species plays a crucial role in making an ecosystem what it is. Also, a keystone species (whose effect is much larger despite a relatively small population) play a crucial role too.
We can't remove the keystone species and then observe changes in ecosystem to establish that it is indeed a keystone species. So how do we come to know which species is keystone, if there is one?

Certain traits define a species as a keystone species e.g. P. ochraceus eliminates mussels which will become dominant without P. ochraceus. But that doesn't mean all predators of dominant or would be dominant species are keystone. Such predators exist in ecosystems that have keystone species as well as in ecosystems that do not have keystone species.
Also P. ochraceus is keystone on wave exposed rocks but not on wave sheltered. Also, there are other ways to eliminate mussels in other regions.

So what makes a species keystone? It depends on geography, community diversity, length of species interaction etc.

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Question 1

sagarsangani123

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!

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

Quote:

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?

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):

Quote:

(B) The degree to which other animals within a habitat prey on mussels

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!

GMATNinja

Thank you for your thorough explanation. I have a few follow-up questions below after reading through your explanations.

For question 576 (the passage mentions which of the following as a factor that affects the role of P. ochraceus as a keystone species within different habitats), can someone please further clarify why Choice B is incorrect? I see why Choice C is correct. However, the passage does mention 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." So, P. ochraceus does seem to depend on the degree to which other animals prey on mussels (based on its relation to when starfish are present or absent). What am I missing?

For question 578 (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), why doesn't Choice A fall under "community diversity" in that bats managing the size of the insect population and the size of the insect population manages the bird population?

Thank you so much again for all of your time, help, and dedication.

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Question 1

woohoo921

Thank you for your thorough explanation. I have a few follow-up questions below after reading through your explanations.

For question 576 (the passage mentions which of the following as a factor that affects the role of P. ochraceus as a keystone species within different habitats), can someone please further clarify why Choice B is incorrect? I see why Choice C is correct. However, the passage does mention 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." So, P. ochraceus does seem to depend on the degree to which other animals prey on mussels (based on its relation to when starfish are present or absent). What am I missing?

For question 578 (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), why doesn't Choice A fall under "community diversity" in that bats managing the size of the insect population and the size of the insect population manages the bird population?

Thank you so much again for all of your time, help, and dedication.

Take another look t that line of the passage: "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."

"This starfish" refers to P. ochraceus. In other words, P. ochraceus IS the starfish. So, there's no interaction between P. ochraceus and other starfish. Eliminate (B) for question 1.

Question 3

The hypothesis in the last line of the passage states that context is important for determining keystone status. For example, a change in species diversity could strip a species of its keystone status, or thrusts a non-keystone species into a keystone role.

Answer choice (A) really doesn't tell us anything about species diversity. Sure, we've got bats, birds, and insects, but is that a diverse ecosystem? There's no way of knowing. Also, it really doesn't capture the idea that a change in species diversity could change keystone status. It could be that these animals have existed just like this for a long time, and the keystone species have also remained the same (instead of shifting when the context changes). Because (A) doesn't support the hypothesis that keystone status is dependent on context, you can eliminate (A).

I hope that helps!

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Question 1

woohoo921

Question 3

GMATNinja
Thank you!
I have one follow-up question. For question 3, you noted the importance of "change" in community diversity.

For future passages, should the takeaway be that it is important to read what is in the parenthesis, as community diversity is qualified to be essentially a change in community diversity... and not broadly speaking diversity (how I understood it)? I thought that was in the parentheses after community diversity was just an example. Thank you again.

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Question 1

woohoo921

Question 3

You're right that the author uses "a reduction in species diversity" as an example. But what does this example support? It supports the hypothesis that keystone status depends on context. The example supports this hypothesis because as species diversity changes (i.e., the context changes), keystone status changes.

So, it's not enough just to connect to species diversity -- you have to connect to the actual hypothesis about context impacting keystone status.

In terms of takeaways for future passages: it's important to read every word of the passage, even the bits in parentheses. Even if you don't memorize the details on your first read though (and you absolutely should not do that!), you should definitely know why the author has included the example -- in this case, to support the hypothesis about context. Then, when you get to something like question 1, go back to the relevant piece of the passage and really investigate the details before diving into the answer choices.

I hope that helps!

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For Q1, why E is incorrect?
(E) The fact that there is great species diversity within some habitats occupied by P. ochraceus
Author has mentioned in the last sentence that species diversity could impact the keystone species status. Why E is incorrect then. I agree that community diversity could affect when a reduction in species diversity occur but in broader terms, diversity in general could affect.

In Q4, why C is incorrect?
(C) Such an identification is likely to be less reliable as an ecosystem becomes less diverse.
Acc to last line, identification depends on a diverse environment. So a less diverse environment could affect keystone status. Why can't we infer C from this sentence.

Thank you for your time

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Question 1

Sneha2021

url=[https://gmatclub.com:443/forum/memberlist.php?mode=viewprofile&un=KarishmaB]KarishmaB[/url] url=[https://gmatclub.com:443/forum/memberlist.php?mode=viewprofile&un=GMATNinja]GMATNinja[/url]

For Q1, why E is incorrect?
(E) The fact that there is great species diversity within some habitats occupied by P. ochraceus
Author has mentioned in the last sentence that species diversity could impact the keystone species status. Why E is incorrect then. I agree that community diversity could affect when a reduction in species diversity occur but in broader terms, diversity in general could affect.

Question 1 asks us which answer is specifically mentioned in the passage regarding P. ochraceus.

In that last sentence, the author isn't talking specifically about P. ochraceus any more. Instead, he/she is talking generally about what might keystone status for any species.

So, could great species diversity within some habitats occupied by P. ochraceus impact its keystone status? Sure! But the author doesn't mention that species diversity ACTUALLY impacts P. ochraceus's status. By contrast, the author DOES specifically mention that P. ochraceus's role is impacted by the degree to which its habitat is sheltered from waves.

(E) is out and (A) is the correct answer for question 1.

Question 4

Sneha2021

url=[https://gmatclub.com:443/forum/memberlist.php?mode=viewprofile&un=KarishmaB]KarishmaB[/url] url=[https://gmatclub.com:443/forum/memberlist.php?mode=viewprofile&un=GMATNinja]GMATNinja[/url]

In Q4, why C is incorrect?
(C) Such an identification is likely to be less reliable as an ecosystem becomes less diverse.
Acc to last line, identification depends on a diverse environment. So a less diverse environment could affect keystone status. Why can't we infer C from this sentence.

Thank you for your time

Here's the exact language of question 4:

Quote:

4. The passage suggests which of the following about the identification of a species as a keystone species?

(C) then says that "such an identification is likely to be less reliable as an ecosystem becomes less diverse."

Hmm. (C) isn't directly talking about how to tell whether something is a keystone species (and your'e absolutely right that a less diverse environment could affect keystone status). This is talking about how reliable the identification is. In other words, how sure are you that a certain species is or is not a keystone?

There's nothing in the passage implying that this identification is more or less reliable in one situation or another. You could be very confident about whether something is a keystone in a very diverse environment, and equally confident about whether its a keystone in a non-diverse environment.

We can eliminate (C) for question 4 because there's nothing implying that one situation makes identifying keystone species less reliable than another.

I hope that helps!

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Question 1

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Question 3

Dear GMAT Ninja, for Q3, in the paragraph, the author said that 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), so how about "A species of bat is primarily responsible for keeping insect populations within an ecosystem low", can we infer that this is also support for determining bad is the keystone species?

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Question 3

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Choice (B) works because it's evidence that context matters: in one geography, the iguana is a keystone species, but in another geography, it isn't.

Choice (A), on the other hand, does not illustrate how context matters. And keeping insect populations low is not the same as reducing species diversity. If you start with, say, 50 species and then reduce all of their populations, you'd still have 50 species -- so there would be no change in the species diversity.

If we were told that the bat became a keystone species ONLY AFTER there was a reduction in species diversity, then we'd have some support for the last sentence. But that's not what we get in choice (A).

For more on (A) vs (B), check out this post: https://gmatclub.com/forum/while-the-mo ... l#p2708389.

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10 Mar 2026, 21:03

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Why is D not the correct answer for #3?

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10 Mar 2026, 21:45

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teneturassumenda

Why is D not the correct answer for #3?

Question 3:

Because (D) does not support the last sentence’s claim that keystone status depends on context (geography, diversity, or length of interaction). It only says within the same context, if that bee becomes more abundant, its effect grows. That is just “more individuals, bigger impact,” not “same species is keystone here but not there because conditions differ.”

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10 Mar 2026, 22:15

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

The last sentence says keystone status depends on context, including geography, community diversity (lower diversity can push more remaining species into keystone roles), and how long species have been interacting (new arrivals can have big effects). So the best support should show the same species being keystone in one context but not in another due to those factors.

(A) A bat keeps insects low, which affects birds.

This describes a strong ecological effect, but it does not show context dependence. It could be keystone, but it does not support the “depends on diversity/geography/time” claim.

(B) An iguana is keystone on certain islands but not on adjacent islands with more animal species.

This directly supports context dependence tied to community diversity: more diverse islands do not give the iguana a keystone role, while less diverse ones do. That matches the last sentence most clearly.

(C) More species occupy keystone roles than previously believed.

This is about undercounting keystone roles, not about why keystone status depends on context.

(D) As a keystone bee becomes more abundant it has a larger effect.

That is about abundance scaling, not context dependence.

(E) A keystone moth develops camouflage.

That is about an evolutionary trait, not about context dependence of keystone status.

Answer: (B)

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