On a tiny atoll in the middle of the Pacific Ocean, a remarkable rehabilitation project is playing out. Palmyra Atoll—used as a U.S. military base during World War II—has no permanent human residents, so its exceptionally fertile environment has become an ideal laboratory for understanding how tropical ecosystems are impacted by invasive species: in this case, black rats and coconut palm trees. It’s also a prime example of how tricky it can be to restore native flora and fauna following the eradication of an invasive species. On Palmyra—several dozen coral…