Comprehension : Comprehension

Q 444 / 466

UPSC CSE Prelims 2011

Read the following passage and answer the items that follow. Your answers to these items should be based on the passage only.
A species that exerts an influence out of proportion to its abundance in an ecosystem is called a keystone species. The keystone species may influence both the species richness of communities and the flow of energy and materials through ecosystems. The sea star Pisaster the flow of energy and materials through ecosystems. The sea star Pisaster ochraceus, which lives in rocky intertidal ecosystems on the Pacific coast of North America, is also an example of a keystone species. Its preferred prey is the mussel Mytilus Californians. In the absence of sea-stars, these mussels crowd out other competitors in a broad belt of the intertidal zone. By consuming mussels, sea star creates bare spaces that are taken over by a variety of other species. A study at the University of Washington demonstrated the influence of Pisaster on species richness by removing sea stars from selected parts of the intertidal zone repeatedly over a period of five years. Two major changes occurred in the areas from which sea stars were removed. First, the lower edge of the mussel bed extended farther down into the intertidal zone, showing that sea stars are able to eliminate mussels completely where they are covered with water most of the time. Second, and more dramatically, 28 species of animals and algae disappeared from the sea star removal zone. Eventually only Mytilus, the dominant competitor, occupied the entire substratum. Through its effect on competitive relationships, predation by Pisaster largely determines which species live in these rocky intertidal ecosystems.
Which of the following is/are implied by the passage?

  1. Mussels are always hard competitors for sea stars.
  2. Sea stars of the Pacific coast have reached the climax of their evolution.
  3. Sea stars constitute an important component in the energy flow in intertidal ecosystem.

Which of the statements given above is/are correct?

EXPLANATION

Correct Option (D)

Statement 3 is correct. The passage explicitly defines a keystone species as one that influences "the flow of energy and materials through ecosystems." It then identifies the sea star Pisaster ochraceus as an example of a keystone species. Therefore, it is directly implied that sea stars constitute an important component in the energy flow within the intertidal ecosystem.

Incorrect Options:

Statement 1 is incorrect. The passage states that mussels (Mytilus californianus) are dominant competitors that "crowd out other competitors" for space in the intertidal zone. Sea stars, however, are described as predators of mussels, consuming them to create bare spaces. Their relationship is one of predation, not competition between mussels and sea stars themselves.

Statement 2 is incorrect. The passage focuses on the ecological role and impact of sea stars as a keystone species within their ecosystem. It does not provide any information or discussion regarding the evolutionary history, developmental stages, or "climax of evolution" of sea stars. This statement is an unsupported generalization outside the scope of the provided text.