单选题 . ①Ecologists 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. ②Even within those limits, they are frustrated by porous frontiers. ③As 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. ④On them, ecologists have figured out how the size of a given habitat determine how many species it can support. ⑤They have then applied that knowledge to the mainland, showing how fragmented ecosystems become like archipelagoes, where extinctions can strike.

38. 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

【正确答案】 C

【答案解析】 C：根据文章可知，多孔边界和后文的isolated相反。isolated在文中指的是独立的、只被占领过几次的岛屿环境，因此那些多孔边界指的便是被多次占领并且不独立的环境，因此选项C正确。