A Rat-Free Palmyra Atoll

Originally published in 2013, this article is part of an ongoing series of stories on the restoration of Palmyra Atoll. More than 10 years later, Palmyra Atoll remains rat free, the rainforest continues to recover, and work here informs and influences island conservation across the Pacific.

Located 1,000 miles south of Hawai’i,Palmyra Atoll is one of the most spectacular marine wilderness areas on Earth. The area includes 25 islets covering 580 acres of land, and 15,000 acres of some of the most diverse and spectacular coral reef systems in the world.

Palmyra Atoll is co-managed by the U.S. Fish and Wildlife Service and The Nature Conservancy. These two partners plus Island Conservation are working together on the Palmyra Atoll Restoration Project, which aims to protect ten nesting seabird species, migratory shorebirds, the rare coconut crab and one of the largest remaining native Pisonia forests in the Pacific Islands.

The first step in this restoration was a big one: removing non-native rats. That project recently completed and appears to be successful.

Alex Wegmann, then-program director for Island Conservation, shared his thoughts on the effort to restore a rat-free island.

Q:  While most people don’t exactly love rats, they don’t think of them as a conservation threat. Could you explain what happens when non-native rats come to an island?

Alex Wegmann:  Basically, rats can eat an island into a state of ecological crisis.  Rats (members of the Rattus genus) are highly adaptable omnivores, eating birds, reptiles, invertebrates, insects, and plants. They reproduce very fast, and can eat almost anything in such quantity that they can alter large-scale natural processes, such as seed dispersal.  They can overtake native species and effectively push them out by eating all the food.  They are also excellent swimmers and even eat marine creatures in shallow waters like small crabs and clams.

Q:  What particular effects were they having on Palmyra?

Alex Wegmann:  At Palmyra, non-native rats (black rats or Rattus rattus) were affecting seabirds, crabs, native plants, and insects. Rats were like “ecosystem engineers” affecting many of the complex interactions between terrestrial and marine species at Palmyra Atoll. Rats ate eggs and chicks of several species of seabirds, particularly the White Tern and Sooty Tern. There are at least six species of ground-nesting seabirds that are missing from Palmyra, and rats are the likely cause of their absence.  Rats were eating land crabs, insects, and geckos. Also, Palmyra is home to one of the last remaining stands of the native tree Pisonia grandis in the Central Pacific.  Rats limit the recruitment of Palmyra’s native tree species by chewing their way through their thick husks and seed-coats and consuming the seeds and seedlings. Finally, rats contributed to a high mosquito population, both by providing them with a steady supply of their own blood, and by creating ideal breeding habitat by chewing holes into coconuts that were quickly filled by the near-constant rainfall.

Q:  It appears that the effort has been a success, but there’s always a chance of a survivor or two lurking. How can you ensure that the team has eradicated every last rat?

Alex Wegmann:  Our monitoring results show there are no rats after one year. The standard wait period for confirming rat eradication projects is the time it takes for two new generations of rats to emerge. In tropical environments like Palmyra, rats reproduce about every three to four months, which allows for three to four generations per year.

After one year, we conducted rigorous monitoring and found no rats.  The monitoring included a network of 286 rat detection stations that covered the entire atoll. Each station was checked and refreshed four times, with a three-day detection period between each check. Team members also spent hundreds of person-hours scoured the atoll for “natural” indicators of rat presence such as chew marks. Rat sign was not detected at all.

Q:  Is there a chance rats could find their way to Palmyra again and start the process anew? Are there preventative measures conservationists are taking?

Alex Wegmann:  As long as humans continue to visit Palmyra, the risk of reintroduction is ever-present. Rats are adept at stowing away on boats or even airplanes. There is a biosecurity plan in place and both The Nature Conservancy and the U.S. Fish and Wildlife Service will need to ensure that all foreseeable rat introduction pathways are kept closed. The plan requires that all researchers, all staff involved with provisioning the field station, and anyone else who visits the Atoll for any reason follow strict biosecurity measures. The on-site staff managing activities at Palmyra Atoll during the research season and during other visits will also conduct ongoing rat monitoring and are prepared to respond to a rat incursion.

Q:  Obviously eradications efforts result in the deaths of animals. How do you respond to those who believe that we shouldn’t be killing rats?

Alex Wegmann:  I agree with them. No one in their right mind takes pleasure in killing for killing’s sake. We shouldn’t be killing rats, but we also shouldn’t be transporting rats to places (especially islands) that are beyond their natural range. Invasive alien rats are detrimental to island species and ecosystems.  The number of native organisms killed by introduced rats on an annual basis eclipses the number of rats that would be killed during an eradication project.

Killing rats is not the goal. This is why we design eradication projects so that they are acute, successful actions, happening only once and spanning the least amount of time possible. Restoring balance to protect native species on islands is our objective, and removing rats is a necessary step towards this goal.

Q: William Stolzenburg, in his recent book Rat Island, called the eradication of non-native predators from islands “the greatest wildlife rescue in history.” Do these eradications offer a chance to reclaim islands for native wildlife? Are there opportunities to take projects like this to other islands?

Alex Wegmann:  Yes. Removing invasive alien predators like non-native rats from islands is a prerequisite to the protection and restoration of populations of native plants and animals.  Most of the world’s islands host populations of introduced rats that threaten native species.  Conservationists have been eradicating invasive species from islands for over a century, though most conservation-based eradications have occurred within the last 30 years.  This project applied methods developed on temperate and subarctic islands to methods that fit Palmyra’s unique tropical environment. Armed with lessons and tools from projects like Palmyra Atoll and the hundreds of other projects before it, conservationists will undoubtedly relieve other islands, and their native species, from the threat posed by introduced rats.

Q:  What is Palmyra like without rats? What do you expect to see in the future?

Alex Wegmann:  Following the removal of rats from Palmyra, we expect many positive changes for native species. Some will be rapid, like an increase in seabirds, more native tree seedlings, and more invertebrates such as dragonflies or katydids. Other changes will be gradual, like the growth of native trees to re-create the native forest canopy, or the reestablishment of populations of seabird species that have been missing since non-native rats took over.  These changes will be measured scientifically, but now, one year after rats were removed, we already can see lots of positive changes in native species populations.

There are more fiddler crabs, ghost crabs, geckos and cockroaches. There are more seedlings of many different native plants and trees. There are fewer mosquitoes but more native insects and geckos. And perhaps best of all, there are much larger colonies and great fledgling success for the ground-nesting Sooty Terns and tree-nesting White Terns and Black Noddies.