Naturalist: The decline of coral reefs has various causes. One contributing factor is predation on coral by organisms such as...
GMAT Two Part Analysis : (TPA) Questions
Naturalist: The decline of coral reefs has various causes. One contributing factor is predation on coral by organisms such as the crown-of-thorns sea star, whose preferred food source is coral polyps. Human fishing practices have decreased the sea star's predators, such as the harlequin shrimp. It is also possible that runoff containing nutrients for phytoplankton has resulted in larger phytoplankton blooms: the crown-of-thorns sea star gladly eats phytoplankton.
Indicate in the table which cause-and-effect sequence would most likely, according to the naturalist, result in coral reef decline. Make only two selections, one in each column.
Phase 1: Owning the Dataset
Argument Analysis Table
| Passage Statement | Analysis & Implications |
| "The decline of coral reefs has various causes" |
|
| "One contributing factor is predation on coral by organisms such as the crown-of-thorns sea star, whose preferred food source is coral polyps" |
|
| "Human fishing practices have decreased the sea star's predators, such as the harlequin shrimp" |
|
| "runoff containing nutrients for phytoplankton has resulted in larger phytoplankton blooms" |
|
| "the crown-of-thorns sea star gladly eats phytoplankton" |
|
Key Patterns Identified
- Established Facts: Crown-of-thorns eat coral; they also eat phytoplankton; their predators are decreasing; phytoplankton can increase
- Key Relationships: Two pathways to increased sea stars (fewer predators OR more food)
- Ultimate Impact: More crown-of-thorns sea stars → more coral predation → reef decline
Phase 2: Question Analysis & Prethinking
Understanding What Each Part Asks
- Part 1 (Cause): What initial change starts the sequence?
- Part 2 (Effect): What results from the cause that then leads to coral reef decline?
- Relationship: We need a two-step chain where Cause → Effect → Coral Reef Decline
Valid Inferences (Prethinking)
Based on our fact analysis:
- Primary Chain: Increased phytoplankton → Increased crown-of-thorns → Coral decline
- Alternative Chain: Decreased harlequin shrimp → Increased crown-of-thorns → Coral decline
- Key Insight: The crown-of-thorns increase is the critical intermediate step
Phase 3: Answer Choice Evaluation
Let's evaluate each option for both Cause and Effect roles:
Option Analysis
- "An increase in phytoplankton"
- As Cause: Supported by passage (runoff causes this)
- As Effect: Not supported as an effect that leads to coral decline
- "A decrease in phytoplankton"
- As Cause: Not mentioned in passage
- As Effect: Would actually help coral (fewer sea stars)
- "An increase in crown-of-thorns sea stars"
- As Cause: Could work, but what causes this increase?
- As Effect: Perfect! This directly leads to coral decline
- "A decrease in crown-of-thorns sea stars"
- As Cause/Effect: Would help coral, not harm it
- "An increase in harlequin shrimp"
- As Cause/Effect: Would decrease sea stars, helping coral
Answer Selection Process
For the Cause column: We need what initiates the harmful sequence. "An increase in phytoplankton" works perfectly because:
- The passage explicitly mentions this can happen (nutrient runoff)
- It provides more food for crown-of-thorns sea stars
For the Effect column: We need what results from the cause AND leads to coral decline. "An increase in crown-of-thorns sea stars" is ideal because:
- It logically follows from more phytoplankton (more food = larger population)
- It directly causes coral decline (they eat coral polyps)
Verification
The complete sequence: More phytoplankton → More crown-of-thorns sea stars → Coral reef decline ✓
This matches the naturalist's explanation perfectly without adding speculation.
Final Answer
- Cause: An increase in phytoplankton
- Effect: An increase in crown-of-thorns sea stars