PASSAGE 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 californianus . 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 occured in the areas from which sea stars were removed. First, the lower edge of the 46. 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.

Q.1 What is the crux of the passage ?
a.Sea star has a preferred prey.
b.A preferred prey determines the survival of a keystone species.
c.Keystone species ensures species diversity.
d.Sea star is the only keystone species on the Pacific coast of North America.

Q2. With reference to the passage, consider the following statements :
1. Mussels-are generally the dominant species in intertidal ecosystems.
2. The survival of sea stars is generally determined by the abundance of mussels.
Which of the statements given above is /are correct?
(a) 1 only
(b) 2 ony
(c) Both 1 and 2
(d) Neither 1 nor 2

Q3. 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?
(a) 1 and 2
(b) 2 only
(c) 1 and 3
(d) 3 only

Q4. Consider the following assumptions:
1.The food chains/food web in an influenced ecosystem are keystone species.
2.The presence of keystone species is a specific characteristic of aquatic ecosystems.
3.If the keystone species is completely removed from an ecosystem, it will lead to the collapse of the ecosystem.
With reference to the passage, which of the above assumptions is/are valid?
(a) 1 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3







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