Rats ruled Palmyra Atoll for 70 years; after a sling-and-helicopter bait drop, its native tree seedlings went over 7,700

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.

Rats invaded Palmyra Atoll during World War II, harming local seabird-nesting trees. (Representational/Unsplash)Why the forest wasn’t developingPalmyra sits roughly 1,000 miles southwest of Hawaii. Both rats and land crabs fed on Pisonia seeds. The authors also cite an earlier experiment in which 99 percent of planted Pisonia seeds did not survive one month. Before the rats departed, the researchers found no Pisonia seedlings in any of their 55 survey strips.

However, as the 2012 conference paper, ‘Pushing the Envelope in Paradise: A Novel Approach to Rat Eradication at Palmyra Atoll,’ reports, the first attempt at Palmyra failed in 2001 and 2002. One trouble was rats hanging around in coconut palm crowns. Most of the bait that was dropped from the air landed on the ground instead. Another problem was that the palms leaned over the shoreline so that aerial bait there would have fallen into the sea. So the crew made “bolas,” two small cotton sacks of bait tied together with twine. They were slung by hand, shot out of slingshots, or dropped in by a crew member from a helicopter. In June 2011, 41 people from five countries spread 38,561 kilograms of bait across 235 hectares during a 28-day operation. Searchers found 13 dead shorebirds, likely from bait exposure, while 14 shorebirds were captured beforehand for protection, the authors add.

The PLOS ONE paper says more than 99 percent of the rats were gone by July 2011, and monitoring confirmed the atoll rat-free by June 2012. Then came the seedlings. A month after eradication, the researchers tallied as many as 688 Pisonia seedlings per 100 square meters. Five of the six native tree species studied showed increased recruitment, although not all gains were statistically significant. Two rare native trees, Cordia subcordata and Guettarda speciosa, had no seedlings before the eradication and some afterward, though the difference was not statistically significant. Before the eradication, in 2004, the team counted 140 seedlings among the five locally rare native trees it tracked, and 7,756 in 2016, according to Island Conservation's media release on the study. Pisonia numbers declined in later years but stayed above zero.

Rats also preyed upon coconut seeds and seedlings, which the authors say is probably why counts of coconut seedlings were 13 times higher in 2016 than before the eradication. These palms are not indigenous to Palmyra. That same year, researchers found no Pisonia seedlings in coconut palm-dominated forest. The team offers several possible explanations for Pisonia's later decline, including being outcompeted by coconut seedlings and more land crabs. They wrote that the palms would likely need active management.

Five years is not a long time. The authors warn that the results may only reflect short-term trends and that monitoring must continue. To the best of their knowledge, theirs is the first study to look at seedlings after a rat eradication on a tropical island. So nature did not simply reset itself. Removing one big pressure just opened the door, and what came through still needs watching.

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