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Difficulty: HardCommand of Evidence: Textual

In a study of the sympatric sea stars *Asterias forbesi* and *Asterias vulgaris*, marine biologist Dr. Angela Chen investigated how the two species coexist along the rocky intertidal zones of the North Atlantic. Traditional ecological models suggest that niche partitioning between these species is based on prey size selection: the larger *A. vulgaris* typically preys on larger blue mussels (*Mytilus edulis*), while the smaller *A. forbesi* consumes smaller ones. Chen, however, hypothesizes that this apparent size-based dietary partitioning is merely a byproduct of habitat partitioning by depth, where *A. vulgaris* feeds in deeper subtidal zones where only larger mussels survive, and *A. forbesi* feeds in shallower intertidal zones dominated by smaller mussels. Which finding from the study, if true, would most directly support Chen's hypothesis?

  1. When *A. forbesi* and *A. vulgaris* are found at the same depth, the average size of the mussels consumed by both species is nearly identical.Answer
  2. B
    In laboratory trials where water depth is held constant, *A. vulgaris* actively selects larger mussels than *A. forbesi* does when presented with a range of sizes.
  3. C
    Blue mussels located in the subtidal zone grow larger than those in the intertidal zone because they are submerged longer and have more opportunities to filter feed.
  4. D
    Both sea star species feed on other marine organisms, such as barnacles and snails, when blue mussels are scarce in their respective habitats.

Answer

The correct answer states that when both sea star species are found at the same depth, the average size of the mussels they consume is nearly identical.
The correct answer provides evidence that when both sea star species are present at the same depth, they consume similarly sized mussels. This supports the hypothesis that the apparent size-based diet differences observed in the wild are a byproduct of depth-based habitat partitioning, as the size differences disappear when depth is controlled.

Step-by-Step Solution

1
Identify the researcher's main claim.
Dr. Chen's hypothesis is that the difference in the size of mussels eaten by the two sea star species is caused by their depth-based habitat partitioning, not by a direct preference for different prey sizes.
This establishes the target claim that the correct evidence must support.
2
Determine what evidence would support this hypothesis.
Evidence showing that when the depth variable is controlled or removed, the sea stars no longer partition their diets by prey size.
If depth is the sole driver of the prey size difference, eliminating depth differences should eliminate the prey size differences.
3
Evaluate the choices to find this evidence.
The finding that the two species eat the same size of mussels when at the same depth directly supports Chen's hypothesis.
It isolates depth as the causal factor for the observed dietary differences.

Key Concept

Command of Evidence: Textual
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Command of Evidence: Textual

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