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.
Consider the following assumptions:
With reference to the passage, which of the above assumptions is/are valid?
Assumption 1 is valid. The passage states that a keystone species influences "the flow of energy and materials through ecosystems." Food chains and food webs are the mechanisms through which energy and materials flow in an ecosystem. Therefore, the role of keystone species is integral to the structure and function of these food networks.
Assumption 3 is valid. The University of Washington study detailed in the passage demonstrates that the removal of the keystone species (sea star Pisaster ochraceus) led to significant ecological disruption. Specifically, 28 species of animals and algae disappeared, and the ecosystem eventually became dominated by a single competitor, Mytilus. This severe reduction in biodiversity and simplification of the ecosystem structure indicates a collapse or substantial degradation of the ecosystem.
Assumption 2 is not valid. While the passage uses an aquatic example (sea star in rocky intertidal ecosystems) to illustrate a keystone species, the initial definition provided is general: "A species that exerts an influence out of proportion to its abundance in an ecosystem." The passage does not state or imply that keystone species are exclusively found in aquatic ecosystems; it merely uses one such example. Therefore, restricting keystone species to aquatic ecosystems is an unsupported overgeneralization based on the passage.