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  3. One basic weakness in a conservation system based wholly on economic motives is that most members of the land community have no

One basic weakness in a conservation system based wholly on economic motives is that most members of the land community have no

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问题     One basic weakness in a conservation system based wholly on economic motives is that most members of the land community have no economic value. Yet these creatures are members of the biotic community and, if its stability depends on its integrity, they are entitled to continuance.
    When one of these noneconomic categories is threatened and, if we happen to love it, we invent excuses to give it economic importance. At the beginning of the century songbirds were supposed to be disappearing. (46) Scientists jumped to the rescue with some distinctly shaky evidence to the effect that insects would eat us up if birds failed to control them. The evidence had to be economic in order to be valid.
    It is painful to read these roundabout accounts today. We have no land ethic yet, (47) but we have at least drawn nearer the point of admitting that birds should continue as a matter of intrinsic right, regardless of the presence or absence of economic advantage to us.
    A parallel situation exists in respect of predatory mammals and fish-eating birds. (48) Time was when biologists somewhat overworked the evidence that these creatures preserve the health of game by killing the physically weak, or that they prey only on "worthless" species. Here again, the evidence had to be economic in order to be valid. It is only in recent years that we hear the more honest argument that predators are members of the community, and that no special interest has the right to exterminate them for the sake of a benefit, real or fancied, to itself.
    Some species of trees have been "read out of the party" by economics-minded foresters because they grow too slowly, or have too low a sale value to pay as timber crops. (49) In Europe, where forestry is ecologically more advanced, the noncommercial tree species are recognized as members of the native forest community, to be preserved as such, within reason. Moreover, some have been found to have a valuable function in building up soil fertility. The interdependence of the forest and its constituent tree species, ground flora, and fauna is taken for granted.
    To sum up; a system of conservation based solely on economic self-interest is hopelessly lopsided. (50) It tends to ignore, and thus eventually to eliminate, many elements in the land community that lack commercial value, but that are essential to its healthy functioning. It assumes, falsely, that the economic parts of the biotic clock will function without the uneconomic parts.
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答案这种保护系统往往忽视陆地群落中诸多缺乏商业价值、但对其健康运行至关重要的物种，而最终导致它们的灭绝。
解析 主语It指代上句中的a system of conservation，谓语是tend to do结构，但一共有两个to do放在tend后面做宾语，第二个to do是to eliminate，其后面的宾语是many elements，而这个词后面又有两个由that引导用but并列的定语从句对其进行修饰。
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