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　　Ecologists study how millions of species share the world, rather than take on the whole planet at once; they generally focus on a single ecosystem, be it a prairie, a tidal flat, or a sand dune. Even within those limits, they are frustrated by porous frontiers. As a result, ecologists have done some of their most important work on islands, nature’s own isolated laboratories, which may be colonized only a few times over the course of millions of years. On them, ecologists have figured out how the size of a given habitat determine how many species it can support. They have then applied that knowledge to the mainland, showing how fragmented ecosystems become like archipelagoes, where extinctions can strike.

　　1. It can be inferred that the term “porous frontiers” is used to refer to

　　A：a tendency for the area of an ecosystem to shrink

　　B：human-caused processes that alter the character of an ecosystem

　　C：movement of species into an ecosystem from outside

　　D：variation in what the term “ecosystem” signifies

　　E：lack of protection for endangered habitats

　　答案：C

　　2. In the context in which it appears, “isolated” most nearly means

　　A：completely uncontaminated

　　B：somewhat unusual

　　C：extremely rare

　　D：relatively inaccessible

　　E：strictly confined

　　答案：D

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