Passage

A species that exerts. an influence out of proportion to its abundance in an ecosystem is called a ke

Question

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 occurred 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.

Consider the following assumptions:

1. The food chains/food web in an ecosystem are influenced by 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?

Accepted Answer

The correct answer is A because only assumption 1 is supported by the passage.

Analysis of Assumptions:

1.  The food chains/food web in an ecosystem are influenced by keystone species.
    VALID. The passage states, "The keystone species may influence both the species richness of communities and the flow of energy and materials through ecosystems." It then provides an example where the sea star (a keystone species) preys on mussels, creating space for other species. This directly demonstrates an influence on the food web (predation) and species composition, which are integral to food chains/webs.

2.  The presence of keystone species is a specific characteristic of aquatic ecosystems.
    INVALID. The passage defines a keystone species generally and then provides an example from an aquatic ecosystem (sea star). However, it does not state or imply that keystone species are *only* found in aquatic ecosystems. The definition is broad, and the passage only offers one specific example, which happens to be aquatic.

3.  If the keystone species is completely removed from an ecosystem, it will lead to the collapse of the ecosystem.
    INVALID. The passage describes significant changes upon removal of the sea star: "28 species of animals and algae disappeared... Eventually only Mytilus, the dominant competitor, occupied the entire substratum." While this shows a drastic reduction in biodiversity and a major alteration of the ecosystem, it does not explicitly state or necessarily imply a "collapse" of the ecosystem. The ecosystem is severely degraded and simplified, but it still exists, albeit dominated by a single species. "Collapse" is a strong term not directly supported by the passage's description of the outcome.

Answer

1 only

Answer

2 and 3 only

Answer

1 and 3 only

Answer

1, 2 and 3

Explanation

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