問題詳情

Section BThe word “biodiversity” is a term heard with increasing frequency in the ongoing debateover how best to protect the world’s environment, and more specifically, how to preserve itsrapidly dwindling numbers of plant and animal species. In very general terms, “biodiversity”refers to the number of plant and animal species that can be found in a particular habitat orecosystem. This is apparently a very simple concept, but the simplicity of it belies itssignificance. A better popular understanding of the real meaning of biodiversity and of itsimportance to the circumstances in which we presently find ourselves is critical to the successof efforts to protect the environment, and therefore essential to our own survival.Global biodiversity, which is the total number of plant and animal species existing on theplanet at any one time, can only be estimated; and only very roughly estimated at that.Undoubtedly, many species remain to be discovered. Some of these, such as insects andmicroscopic life, are small enough to have escaped our notice, while others dwell in areas wehave only begun to explore; the species inhabiting the deepest ocean depths, for example. Itmust also be remembered that we are in the midst of a mass extinction event. Species are nowdisappearing at a rate estimated to be between 1,000 and 10,000 times faster than the averagefor the history of life on earth. Taken together, these two uncertain elements prevent the globalbiodiversity estimate from becoming much more than an educated guess. The current bestestimate is somewhere in the neighborhood of 100 to 200 million species.This number, while it may be of some interest to nonscientists, isn’t of much real valueto researchers. They are generally more concerned with local biodiversity. Despite theapparent enormity of the number of species that inhabit the earth as a whole, localbiodiversity－the number of species found in any one habitat－fluctuates greatly as we crossthe boundaries separating the ecosystems that make up the Earth’s biosphere. It reaches itshighest levels on the coral reefs and in the tropical rain forests where there may be thousandsof species per acre.Of course, it is this value－the measure of local biodiversity－that is most useful foranyone concerned with assessing the health of an ecosystem or protecting it from destruction.In measuring the biodiversity of a particular ecosystem, biologists are usually quite impartialwhen weighing the relative significance of each species. Most are assigned a value of one, thetotal number of species then representing the target value. However, there are two conditionsunder which one species may be weighted more heavily than others. This would certainly bethe case for any species that by virtue of its genetic uniqueness would constitute a special lossto the global gene pool in the event of its extinction. The tuatara is a good example of justsuch a species. As the only surviving member of a family of reptiles that, except for it, diedout 60 million years ago, the tuatara qualifies on grounds of genetic uniqueness to beweighted more heavily in calculating the biodiversity of its habitat.A species may be accorded bonus points in the biodiversity equation for another reasonas well; it may be deemed more significant by virtue of the role it plays in the ecosystem. Anexample of this would be the California sea otter. The preferred food of this species of marinemammal is the sea urchin; a marine invertebrate, which feeds on a certain type of sea grassknown as kelp. Were it not for the otter’s contribution in controlling the sea urchin population,the undersea kelp forests would surely be decimated by a dramatic increase in the populationof sea urchins. This would remove a key supporting element－the kelp－from the habitat ofmany fish and invertebrate species that depend on it. The effect of this would be a dramaticloss of local biodiversity. This, in turn, would fundamentally alter an entire undersea habitatand undoubtedly put at risk a number of different species that have adapted to its peculiarcharacteristics. Therefore, the behavior of the otter in actively maintaining the local diversityof species dictates that it should be assigned a higher biodiversity value.
46. What was the author’s main purpose in writing the passage?
(A) To illustrate why biodiversity is essential to our survival.
(B) To explain the concept of biodiversity.
(C) To show why local biodiversity is more important than global biodiversity.
(D) To clarify the dangers that result from declining biodiversity.

參考答案

答案：B
難度：適中0.5
統計：A(0),B(0),C(0),D(0),E(0)

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