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.
What is the crux of the passage?
The passage introduces the concept of a keystone species, defined by its disproportionate influence on an ecosystem's species richness and material flow. It then elaborates on this through the example of the sea star Pisaster ochraceus. The core argument is that by preying on the dominant competitor, the mussel Mytilus californianus, the sea star creates open spaces, allowing a variety of other species to coexist. The study cited explicitly demonstrates that the removal of sea stars led to a significant loss of species diversity, with Mytilus eventually monopolizing the substratum. This evidence directly supports the conclusion that keystone species are fundamental in ensuring species diversity by regulating competitive interactions and preventing competitive exclusion. Thus, the central message or 'crux' of the passage is the vital role of keystone species in maintaining biodiversity.
This statement is factually accurate as per the passage, which identifies Mytilus californianus as the sea star's preferred prey. However, this detail serves as a specific mechanism within the broader explanation of how a keystone species operates. The passage's primary objective is to illustrate the ecological impact of a keystone species on overall biodiversity, not merely to describe its dietary habits. Therefore, while true, this option represents a narrow aspect rather than the overarching theme or 'crux' of the entire passage.
The passage focuses on the ecological effects of a keystone species, specifically how its presence influences species diversity within an ecosystem. It details the impact of the sea star's predation on other species. However, the passage does not discuss the factors that determine the survival or viability of the keystone species itself, nor does it suggest that the survival of a keystone species is contingent upon the availability of a preferred prey. This option misinterprets the direction of influence described in the text.
The passage states that the sea star Pisaster ochraceus is "an example of a keystone species" found on the Pacific coast of North America. The use of the indefinite article "an" indicates that it is one among potentially many. The passage does not provide any information or make any claim, either explicitly or implicitly, that the sea star is the sole keystone species in that particular geographical region. Therefore, this statement introduces an assertion not supported by the provided text.