GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - QGPQ95ZV2227S7NQJ

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