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Set 3: Central Ideas (Intermediate)

Explanation

Answer: C

PASSAGE

The following text describes the concept of a keystone species. A keystone species is a species on which other species in an ecosystem largely depend, such that if it were removed the ecosystem would change drastically. The concept was coined by Robert Paine in 1969. An example is the sea otter; without otters to eat them, sea urchin populations explode and consume the kelp forests, destroying the habitat for many other marine animals. Thus, a keystone species has a disproportionately large effect on its environment relative to its abundance.

What defines a species as a 'keystone species'?

A. It is the most abundant species in the ecosystem
B. It creates the physical landscape of the environment
C. It has a critical impact on the ecosystem despite often low numbers✓ Correct
D. It is the only predator at the top of the food chain

Detailed Explanation

Choice C is correct. The text states they have a 'disproportionately large effect... relative to its abundance.'

Key Evidence:

• "disproportionately large effect"

• "relative to its abundance"

Why others are wrong: A (Opposite (relative to its abundance implies it's not simply the most abundant)), B (Too specific (describes ecosystem engineers, a different concept)), D (Too specific (not all keystone species are top predators)).

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