While the most abundant and dominant species within a particular ecosy

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?

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.

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!

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.

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!

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!

I hold the opinion that some of the passages that GMAT writes are not particularly well-done...I have seen quite a few instances now where the writing was simply unclear so not sure if that is intentional or not. Give me a kudos if you agree.

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, the second last sentence mentions this explicitly
(B) The degree to which other animals within a habitat prey on mussels
-no, there is no mention of ‘other animals’ that prey on mussels
(C) The fact that mussel populations are often not dominant within some habitats occupied by P. ochraceus
-no, while the passage does mention that mussels can be a dominant population in the absence of the starfish, but this does not suggest that the mussels ARE NOT dominant, merely that they are preyed on by the keystone species
(D) The size of the P. ochraceus population within the habitat
-no, the passage does not mention this
(E) The fact that there is great species diversity within some habitats occupied by P. ochraceus
-no, species diversity in a habitat affects the keystone status of the remaining species

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,
-no, the problem occurs when there is a removal of a particular species that is involved; this choice fails to capture that

(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.
-no, the problem occurs when there is a removal of a particular species that is involved; this choice fails to capture that

(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
(D) Different mountain ecosystems are compared to determine how geography affects a particular species, ability to dominate its ecosystem.
-no, the passage never cites geography as one of the reasons why identifying keystone species is a problem

(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.
-no, species extinction is irrelevant

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, this choice fails to capture any notions about geography, community diversity, or length of interactions

(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; one of the factors that affect keystone status is the diversity of species within a particular area; the claim is that low/a reduction in diversity leads to the emergence of a keystone species so we would expect that high diversity prevents that from happening

(C) Close observation of a savannah ecosystem reveals that more species occupy keystone roles within that ecosystem than biologists had previously believed.
-no, this choice fails to capture any notions about geography, community diversity, or length of interactions

(D) As a keystone species of bee becomes more abundant it has a larger effect on the ecosystem it inhabits.
-no, this choice fails to capture any notions about geography, community diversity, or length of interactions

(E) A species of moth that occupies a keystone role in a prairie habitat develops coloration patterns that camouflage it from potential predators.
-no, this choice fails to capture any notions about geography, community diversity, or length of interactions

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.
-no, we don’t know whether the species’s relationship with dominant species is the PRIMARY factor

(B) Such an identification can best be made by removing the species from a particular ecosystem and observing changes that occur in the ecosystem.
-no, this is antithetical to the entire passage which suggests that by removing them it would be DIFFICULT to observe changes

(C) Such an identification is likely to be less reliable as an ecosystem becomes less diverse.
-no, this is the opposite of what the last sentence says

(D) Such an identification seems to depend on various factors within the ecosystem.
-Correct, a safe, accurate statement that is captured in the last sentence of the passage

(E) Such an identification can best be made by observing predation behavior.
-no, the example of the starfish and mussels demonstrates that using predation to study keystone status is a murky way to identify keystone species

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.

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!

GMATNinja


Yes, it helped. Thank you.

The author states, or potentially dominant species occurs in systems that do as well as in systems that do not have species that play keystone roles, hinting species can be rendered not keystone depending on circumstances in an ecosystem. And later on, stating Keystone status appears to depend on context, and then 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, if the mussel population is not dominant within some habitat doesn't it make P.O (starfish) not Keystone?

However, it is worthy to note question says passage mentions not suggest(s) or inference based. I feel like what I wrote above is an inference based not something directly mentioned in the passage.
Anyone insight will be helpful. Or am I telling myself a story?

GMATNinja KarishmaB GMATNinjatwo mikemcgarry

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.

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.

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?

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.

1a. Alright I kind of interpreted it totally differently. So now to put everything in place, if we knew that P. ochraceous was also present in that (sand burial) area, could we then say that P ochraceous is a keystone species given that mussells population decreases in their presence without which, mussells would have been a dominant species?
b. If still not, pls tell why and what additional info would help us conclude that P ochraceous is a keystone species, given that mussells are not dominant when P ochraceous is present and at certain sites, sand burial is responsible for their elimination.

2. Also can we say that "while it could have been a factor that the species predating on dominant species gives the species a keystone status, it was later discarded in the passage because such predation on dominant species can also occur in a place where there are no keystone species? And hence predating on dominant species is NO MORE a factor of calling any species a keystone species?" So even if we know that P ochraceous is present and that mussells WOULD be dominant in that case (as per passage), then also we cannot say P ochraceous is a keystone species because a mere predation of a dominant species CANNOT lead to a keystone status and they could have been destroyed by sand burial too? And maybe this reasoning would be a second level reasoning to my point 1 and answer to my above doubt 1a would be NO? KarishmaB

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.