Correct Option (c)
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.
Incorrect Options:
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.