Wildlife boom expected after eradication of 30,000 rats on Pacific island
Unexpected positive results already being recorded
Aerial view of Palmyra Atoll. Copyright Island Conservation. Photo Erik Oberg
January 2013. Wildlife numbers are expected to rebound at Palmyra Atoll, a 580-acre collection of islets located about 1,000 miles south of Hawaii, that has been given a rat-free bill of health one year after about 30,000 rats were eradicated as part of a major effort to remove these invasive predators, the U.S. Fish and Wildlife Service (FWS), The Nature Conservancy (TNC), and Island Conservation (IC) announced.
Removing non-native rats was the top priority for the Palmyra Atoll Restoration Project, a multi-year effort to protect 10 nesting seabird species, migratory shorebirds, coconut crabs, and one of the largest, last remaining native Pisonia grandis forests (a rare flowering tree in the Bougainvillea family) in the tropical Pacific.
"The collaborators did an outstanding job. The science on these efforts has been evolving, and while there have been some learning experiences along the way, the Palmyra effort stands out as a great example of how to do it right and get rid of destructive invasive species while still protecting the native wildlife," said Dr. George Wallace, Vice President for Oceans and Islands at American Bird Conservancy.
Palmyra Atoll is cooperatively managed by US Federal Wildlife Society and The Nature Conservancy as a National Wildlife Refuge and a scientific research station. In 2009, the refuge and waters surrounding it, which include thousands of acres of healthy coral reefs, were designated as a part of the Pacific Remote Islands Marine National Monument.
Black rats arrived during WWII
Non-native black rats were likely introduced to the atoll during World War II, and the population grew to as many 30,000 rats. The invasive rodents eat eggs and chicks of ground and tree-nesting birds, particularly sooty and white terns. Rats also eat land crabs and the seeds and seedlings of native tree species.
No change in the Bristle-thighed curlew population found at Palmyra; special care was taken to ensure this imperilled species was not negatively impacted by the rat removal project. Bristle-thighed Curlew by Michael Walther
To reverse this trend, in June 2011, FWS, TNC and IC carefully and strategically implemented the removal of the destructive, non-native rats from Palmyra Atoll, using brodificoum, a rodenticide that has been successfully used in similar projects on other islands. The Palmyra project was the result of more than seven years of planning and research to ensure that native species were not harmed during the removal, and was the first step in a longer-term effort to restore the atoll's ecological balance.
Crab population explosion
"This wonderful atoll is again able to thrive the way nature intended-without rats. Palmyra has been infested with rats for so long, there will be benefits to wildlife we didn't even fully anticipate-such as the explosion of the fiddler crab population that we're seeing," said Susan White, Monument Superintendent/Refuge Project Leader, Pacific Reefs National Wildlife Refuge and Monuments Complex, U.S. Fish and Wildlife Service. "Palmyra's crucial role in sustaining the Pacific oceanscape is solidified because of this remarkable team of exceptionally talented people."
Using the same proven methods that were used years before to detect the extent of the rat problem on Palmyra, scientists conducted surveys over a month-long period this summer and confirmed that the entire atoll is currently rat-free. In the tropical climate at Palmyra, rats reproduce approximately once every 3-4 months, so conducting surveys one year after the removal effort is sufficient time to detect rats remaining on the atoll. During the summer, the project partners established a network of 286 rat monitoring stations that covered the entire atoll. Each station was checked four times during the course of one month. Aside from the detection stations, team members spent hundreds of hours scouring the atoll for indicators of rat presence. In accordance with observations of the recovery of native species over the past year that suggested that the project was successful, the recent monitoring found no rats after one year.
"Millions of seabirds, trees, crabs and other native species can now thrive in their home without the threat of being eaten by rats. Staff and visitors to the atoll have seen a large increase in the numbers of crabs, insects, seedlings and seabirds. Our collective efforts to bring balance back to Palmyra are working. The scientific rigor, attention to detail, and collaboration is a testament to the integrity and cooperative nature of our partnership," said Suzanne Case, Executive Director of The Nature Conservancy's Hawai'i program.
Dramatic increases already observed
The University of California Santa Cruz Coastal Conservation Action Lab (UCSC-CCAL) is monitoring the response of Palmyra's terrestrial ecosystem by comparing measures of seabird, shorebird, and plant populations taken before and after rat removal. In the summer of 2012 they found dramatic increases, including:
- Over 130% increase in native tree seedlings (Palmyra has ten locally rare native tree species), and the first record of Pisonia seedlings (no seedlings were observed in 2007 prior to rat removal);
- A 367% increase in arthropods (such as insects, spiders, and crabs); and
- No change in Bristle-thighed Curlews found at Palmyra (special care was taken to ensure this imperilled species was not negatively impacted by the rat removal project)
"With the atoll free of rats, we are already seeing a dramatic increase in many things that rats preyed upon: nesting seabirds, migratory shorebirds, native tree seedlings, and small invertebrates like fiddler crabs. The island is truly rebounding," said Gregg Howald, North America Regional Director, Island Conservation.
Although Palmyra is rat-free today, the threat of re-introducing rats or other invasive species is present anytime a boat or airplane travels to the atoll. A detailed biosecurity plan is in place to minimize the threat of non-native species being introduced to the atoll.
The removal of introduced species, such as black rats, is a proven, effective conservation tool that has been successful on numerous islands across the globe, including the Galapagos archipelago, a multitude of islands in New Zealand, the Channel Islands off the coast of California, and Hawadax Island (formerly ‘Rat Island') of the Aleutian Island chain in the Alaska Maritime National Wildlife Refuge.