单选题 .  ①Widespread climate change challenges traditional notions that preserving specific chunks of land is an adequate way to protect endangered species. ②Commitment to particular places has taken conservation a long way, but it works only when the climate is relatively stable. ③When climate change rather than degraded habitat threatens a species' survival in a particular location, moving the species to new locales might become one way of preserving it. ④Some ecologists argue that such assisted migration is simply a way to mimic the natural process of dispersal: its adherents intend to transport species from places that have become uninhabitable through places that humans have made impassable. ⑤Although it has its risks, assisted migration may be a necessary step in the evolution of conservation.18.  The author of the passage implies that some endangered species are unlikely to migrate naturally to habitats better suited to their survival because ______