Passage Summary: Island plants are dying out much faster than mainland plants because they don't have ways to protect themselves from the big animals that humans eventually bring to the islands.

Conclusion: The high extinction rate of island plants is due to their lack of defenses against large land mammals.

Reasoning: Island plants did not evolve defenses against large mammals because those animals typically only arrive on islands after human colonization.

Analysis: The biologist provides a causal explanation: the lack of defenses against mammals is the reason for the extinction. To strengthen this, look for an answer that confirms this causal link, perhaps by showing that island plants survive well when large mammals are absent, or that mainland plants survive better in the same environment because they possess those defenses. We want to ensure that it is the mammals, and not some other factor like climate or human activity itself, causing the extinctions.