RAS question
In ecology, 'Keystone species' refers to:
Correct answer: (D) A species that has a disproportionately large impact on its ecosystem relative to its abundance.
In ecology, a keystone species is a species whose impact on its ecosystem is disproportionately large relative to its abundance.
Explanation
A keystone species is not defined by being the most numerous organism in an ecosystem. The point is ecological influence: EPA describes such species as having effects much larger than would be predicted from their abundance, with a disproportionate effect on community composition and ecosystem function. That is why option D is the only precise definition. The examples in the question's explanation fit the same idea: sea otters affect kelp forests, elephants aid seed dispersal, wolves can trigger trophic cascades in Yellowstone, and fig trees support ecosystems through year-round fruiting. In each case, the species matters because its presence shapes ecological structure or function beyond what its numbers alone would suggest.
Why the other options are wrong
- (A) A type of pollution is a stressor or contaminant category, not a species whose presence shapes community composition or ecosystem function.
- (B) A weather phenomenon is an atmospheric event, whereas a keystone species is a living ecological component with effects disproportionate to its abundance.
- (C) A conservation technique is a method of management, while a keystone species is an organism whose ecological role is unusually influential.
Concept
This tests the Environment and Ecology concept of species roles within ecosystems, especially how ecological importance may differ from abundance. It recurs in RAS because biodiversity, conservation and ecosystem-function questions often turn on recognising such precise ecological terms.