India, Sept. 29 -- Nobody invited the rats to Palmyra Atoll. They became unwanted visitors on this group of 25 small islands in the central Pacific, home to seabirds and the broadleaf tree Pisonia grandis. A 2018 PLOS ONE study found that black rats likely arrived in the 1940s, during World War II, and stayed for decades. The same study notes that Pisonia, a tree where many seabirds nest, rarely grew from seed when rats were present. So, conservationists took things into their own hands and responded with an eradication campaign. In June 2011, crews spread bait across the atoll, and by June 2012, monitoring confirmed the rats were gone.

Palmyra sits roughly 1,000 miles southwest of Hawaii. Both rats and land crabs fed on Pisonia seeds. T...