The Sea Star That Reshaped a Shore
Written by Studio AM.
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On a rocky Pacific shore, mussels can crowd together so tightly that little bare stone remains. They are excellent competitors for space. Yet a familiar predator can keep that strength from turning into total control: a sea star that pulls open mussel shells and feeds on the animals inside.
Ecologist Robert Paine tested this relationship on a section of coast. He repeatedly removed the predatory sea stars from one area while leaving another area alone for comparison. Without the predator, mussels spread. Several other species lost places to attach, feed, or shelter, and the community became less diverse. The sea star was not the most numerous animal on the rocks. Its influence came from which animal it ate.
This experiment helped shape the idea of a keystone species. In an arch, a keystone is one stone, but its position helps support the whole structure. In an ecosystem, a keystone species has an effect much larger than its abundance might suggest. Removing it can reorganize relationships among many other species.
The label does not mean every predator is a keystone in every location. Food webs change across habitats and seasons, and later studies may reveal additional causes. Paine's result mattered because it joined a clear intervention to a broad ecological change. Instead of merely noticing that sea stars and diversity occurred together, he changed one factor and watched the shore respond.
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Questions
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Question 1 of 4
What is the main idea of the passage?
The answer is C: Removing a predatory sea star let mussels dominate, showing how one keystone species can strongly shape an entire shore community.
The experiment connects removal, mussel expansion, and declining diversity to explain influence beyond abundance.
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Question 2 of 4
Why was the comparison area important?
The answer is D: It helped identify changes caused by removing the sea stars.
A comparison makes the intervention easier to distinguish from ordinary changes on the shore.
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Question 3 of 4
What does “intervention” mean here?
The answer is A: a deliberate change made to test what happens
Paine deliberately removed one predator and observed the response, so the intervention was the tested change.
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Question 4 of 4
What happened after sea stars were removed?
The answer is B: Mussels spread and several other species lost space.
The second paragraph describes mussel expansion followed by fewer places for other species.
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