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  3. Marine ecosystems certainly have less permanence than terrestrial ecosystems

Marine ecosystems certainly have less permanence than terrestrial ecosystems

游客2025-02-19  37
问题     Marine ecosystems certainly have less permanence than terrestrial ecosystems. Ashore, ecologists are not confronted with shifting ecological discontinuities, or with changes in the characteristic conditions of individual ecosystems, because, unless man intervenes, the tree line on a mountain or the passage between grassland and savannah remains approximately static over a human lifetime. It is only on the millennial scale that such boundaries migrate significantly, or that characteristic regional ecosystems disappear. Urban sprawl, deforestation, overgrazing, and intensive agriculture are accomplishing in a few decades what nature cannily do in centuries, but that sad fact does not alter the argument. Although the human population explosion can produce pressures that rapidly shift ecological boundaries and modify ecosystems ashore, it is paradoxically more difficult directly to modify the average locations of the ephemeral and shifting ecological boundaries of the sea. We can accomplish this only indirectly by atmospheric modification, resulting in a changed global climate and a shifted ocean circulation. [br] The primary purpose of the passage is to
选项 A、examine differences in the threats faced by marine and terrestrial ecosystems.
B、explain how human intervention has affected marine and terrestrial ecosystems.
C、discuss a distinguishing feature of marine ecosystems
D、present a debate regarding the causes of marine ecosystem impermanence
E、discuss certain consequences of marine ecosystem impermanence
答案 C
解析 本文的主旨就是讨论海洋生态系统的不持久性，所以选项C正确。A项本文确实可以看作将两种生态系统做对比，但是对比的是持久性而非threats。B项文章通过将人类如何影响了陆地和海洋生态系统来说明海洋生态系统不持久，因此选项B只是文章得出最终结论的过程，属于细节。D项文中未提及a debate，文中只有作者的观点，不存在辩论。E项文章没提海洋生态系统不持久所带来的结果是什么。
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