While the most abundant and dominant species within a particular ecosystem is often crucial in perpetuating the ecosystem, a "keystone"...
GMAT Reading Comprehension : (RC) Questions
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).
The passage mentions which of the following as a factor that affects the role of P. ochraceus as a keystone species within different habitats?
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| 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. | What it says: Most ecosystems have dominant species (lots of them) that keep the ecosystem going, but there are also "keystone" species that have huge effects even though there aren't many of them. What it does: Introduces the main concept and sets up a contrast between two types of important species Source/Type: Author's exposition/definition Connection to Previous Sentences: This is our foundation sentence - establishes the key distinction we'll be exploring Visualization: Ecosystem A has 10,000 rabbits (dominant) vs. 50 wolves (keystone) - but those 50 wolves control the entire ecosystem's balance Reading Strategy Insight: The author immediately defines "keystone species" for us - this is helpful, not intimidating. Store this definition as our anchor point. What We Know So Far: Two types of important species exist - abundant/dominant ones and rare but powerful keystone ones |
| But because complex species interactions may be involved, identifying a keystone species by removing the species and observing changes in the ecosystem is problematic. | What it says: It's hard to figure out which species are keystone species because ecosystems are complicated, and the obvious method (remove the species and see what happens) doesn't work well. What it does: Introduces the central problem/challenge Source/Type: Author's explanation of scientific difficulty Connection to Previous Sentences: This builds directly on sentence 1 by saying "OK, keystone species exist, BUT here's the problem with identifying them." The "But" signals this challenge. Visualization: Scientists want to remove those 50 wolves to test their importance, but the ecosystem has hundreds of interconnected relationships, so the results would be messy and unclear Reading Strategy Insight: This is still setup - the author is explaining why this topic is tricky before giving us examples. Don't panic about "complex interactions" - they're about to clarify. What We Know So Far: Keystone species exist but are hard to identify through direct testing What We Don't Know Yet: What makes identification so difficult in practice |
| 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. | What it says: You might think some obvious signs would tell us what's a keystone species. Here's an example: a starfish called Pisaster ochraceus eats mussels and keeps their population down - without the starfish, mussels would take over. What it does: Provides a concrete example to illustrate the keystone concept Source/Type: Scientific example/evidence Connection to Previous Sentences: This directly illustrates the keystone definition from sentence 1. The starfish (low abundance) has huge effects (controlling mussels). This seems to solve the identification problem from sentence 2. Visualization: Rocky shore: 200 starfish keep 50,000 mussels in check. Remove starfish → mussels explode to 200,000 and cover all the rocks Reading Strategy Insight: Feel relieved here! The author just gave us a clear, concrete example after the abstract setup. This makes the keystone concept tangible. What We Know So Far: Keystone species have disproportionate effects; starfish example shows predator controlling potential dominant prey |
| 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. | What it says: However, this kind of predation (predators eating dominant species) happens in ecosystems both WITH and WITHOUT keystone species. What it does: Complicates the simple example by showing it's not a reliable identifier Source/Type: Author's analytical observation Connection to Previous Sentences: This directly challenges the easy solution from sentence 3. The "But" signals: "That example seemed clear, but actually it's not that simple." Visualization: Ecosystem 1: Starfish are keystone (huge impact). Ecosystem 2: Different starfish eat dominant prey but aren't keystone (minor impact). Same behavior, different keystone status. Reading Strategy Insight: This shows WHY identification is problematic (from sentence 2) - similar behaviors don't guarantee keystone status. What We Know So Far: Keystone species exist, are hard to identify, and apparent keystone behaviors can be misleading What We Don't Know Yet: What actually determines keystone status |
| 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. | What it says: Furthermore, the same starfish species (P. ochraceus) is definitely a keystone species in rough, wavy areas, but in calmer areas protected from waves, the starfish has little or no impact. In some places, sand covering the mussels kills them instead of the starfish doing it. What it does: Provides specific evidence that the same species can be keystone in one place but not another Source/Type: Scientific observation/evidence Connection to Previous Sentences: This gives concrete proof for the complexity mentioned in sentence 4. Same species, same behavior, but keystone status changes based on location. This is still building the "identification is hard" theme from sentence 2. Visualization: Rocky headland (rough waves): 200 starfish control 50,000 mussels = KEYSTONE. Sheltered bay (calm water): 200 starfish barely affect 50,000 mussels = NOT KEYSTONE. Sandy area: Sand buries mussels before starfish can eat them Reading Strategy Insight: This is still the same example deepening our understanding, not a completely new concept. The author is showing us WHY keystone status is context-dependent. What We Know So Far: Same species can be keystone in some environments but not others; environmental context matters |
| 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). | What it says: Whether a species is a keystone species depends on the situation - the specific location, how many different species live there (fewer species might make the remaining ones more important), and how long species have been interacting (especially new arrivals can have dramatic effects). What it does: Provides the main conclusion/takeaway by generalizing from all the examples Source/Type: Author's analytical conclusion Connection to Previous Sentences: This is the BIG PICTURE summary! This sentence takes all the complexity we've seen (sentences 2-5) and gives us the simple principle: "It depends on context." This REDUCES complexity by giving us the unifying rule. Visualization: Context 1 (rough coast + high diversity + long interaction): Species A = not keystone. Context 2 (calm bay + low diversity + recent arrival): Same Species A = keystone. Context 3 (medium waves + medium diversity + medium time): Species A = somewhat keystone. Reading Strategy Insight: MAJOR RELIEF POINT! The author just solved all the complexity with one simple rule: context determines keystone status. Everything before this was building to this conclusion. Final Takeaway: The passage argues that keystone species identification is difficult because keystone status is context-dependent, not an inherent species trait. |
2. Passage Summary:
Author's Purpose:
To explain why identifying keystone species in ecosystems is more complicated than it initially appears, showing that keystone status depends on environmental context rather than being a fixed trait of the species.
Summary of Passage Structure:
The author builds their argument by moving from simple concepts to complex realities:
- First, the author introduces keystone species as organisms that have huge impacts despite being rare, contrasting them with abundant dominant species
- Next, they point out that identifying these keystone species is actually quite difficult because ecosystems are complex
- Then, they provide a seemingly clear example of a starfish that controls mussel populations, but immediately show why this example is misleading by demonstrating that similar predation happens in systems both with and without keystone species
- Finally, they use the same starfish example to prove that one species can be keystone in some environments but not others, leading to their main conclusion that context determines keystone status
Main Point:
Keystone status is not an inherent characteristic of a species but depends entirely on environmental context, including factors like geography, community diversity, and how long species have been interacting with each other.
1. Question Analysis:
This question asks us to identify what the passage specifically mentions as a factor that affects P. ochraceus's keystone role in different habitats. We need to find direct evidence from the passage about what causes this starfish to be keystone in some places but not others.
Connecting to Our Passage Analysis:
Our passage analysis highlighted that sentence 5 provides the crucial evidence: "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 directly contrasts two different habitat types and their effect on the starfish's keystone status. The passage analysis emphasized that this sentence serves as concrete proof that environmental context determines keystone status.
Prethinking:
The passage explicitly contrasts wave-exposed areas (where P. ochraceus IS a keystone species) with wave-sheltered areas (where P. ochraceus is NOT a keystone species). This is the clearest example given of how habitat characteristics affect keystone status. We should look for an answer choice that directly references this wave exposure factor mentioned in the passage.
Why It's Right:
• The passage directly states that P. ochraceus "occupies an unambiguous keystone role on wave-exposed rocky headlands" but "in more wave-sheltered habitats the impact of P. ochraceus predation is weak or nonexistent"
• This explicitly contrasts two habitat types based on wave exposure, showing how this environmental factor determines keystone status
• This is the primary example used to demonstrate that keystone status depends on environmental context
Key Evidence: "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"
Why It's Wrong:
• The passage never mentions other animals preying on mussels as a factor affecting P. ochraceus's role
• The focus is entirely on P. ochraceus's own predation impact, not competition from other predators
• This introduces information not present in the passage
Common Student Mistakes:
- Thinking that since the passage discusses predation, it must mention multiple predators?
→ Focus only on what's explicitly stated about P. ochraceus specifically - Assuming that ecosystem complexity must involve predator competition?
→ The passage's complexity comes from environmental factors, not interspecies competition
Why It's Wrong:
• The passage actually assumes mussels ARE dominant or potentially dominant in P. ochraceus habitats
• The text states mussels "can be a dominant species" in the absence of the starfish
• This contradicts what the passage tells us about mussel populations
Common Student Mistakes:
- Misreading the relationship between mussels and dominance?
→ The passage says mussels become dominant WITHOUT the starfish, not that they're non-dominant in some habitats - Confusing the cause-effect relationship between predation and dominance?
→ P. ochraceus prevents mussel dominance; it doesn't depend on mussels being non-dominant
Why It's Wrong:
• The passage never mentions the size of P. ochraceus populations as a varying factor
• The concept of keystone species specifically involves species with large effects despite small abundance
• Population size isn't discussed as something that changes between habitats
Common Student Mistakes:
- Thinking that since keystone species are defined by abundance effects, population size must be the key factor?
→ The definition establishes that keystone species have small populations; the variable is their IMPACT, not their numbers - Confusing the keystone definition with habitat-specific factors?
→ The passage shows the same small population has different impacts in different places
Why It's Wrong:
• While the passage mentions community diversity as a general factor affecting keystone status, it doesn't specifically connect this to P. ochraceus's varying roles
• The specific example of P. ochraceus focuses on wave exposure, not species diversity
• This factor is mentioned in the general conclusion but not as part of the P. ochraceus case study
Common Student Mistakes:
- Seeing diversity mentioned in the passage and assuming it applies to all examples?
→ The P. ochraceus example specifically focuses on physical environment (waves), while diversity is mentioned as a separate general factor - Confusing the specific example with the broader conclusion?
→ The question asks specifically about P. ochraceus, not about all factors that can affect keystone status