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Study Shows 5000% Increase in Native Trees on Rat-free Palmyra Atoll

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New research demonstrates a strong positive benefit to native trees following invasive species removal.

Media Release

New research published in PLOS ONE this week demonstrates dramatic positive benefits for native trees following rat removal at Palmyra Atoll, a magnificent National Wildlife Refuge and natural research laboratory located about 1000 miles south of Hawaii.

In one of only a few studies of its kind, scientists measured the effects of rat removal on the tropical Pisonia grandis forest at Palmyra Atoll, which provides critical seabird nesting habitat. Before removal, no seedlings of native Pisonia grandis trees were found in research plots. Immediately following removal of invasive rats, seedlings proliferated and plots had an average of 8 seedlings per square meter. For five native tree species, including Pisonia grandis, fewer than 150 seedlings were counted in the presence of rats, and more than 7700 seedlings were counted five years after rats were removed.

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The progress of Pisonia growth on Palmyra Atoll from 2011 to 2016 featuring lead scientist, Coral Wolf. Credit: Island Conservation

Lead scientist Coral Wolf from Island Conservation said:

Once rats were gone, changes became immediately apparent. We were so excited to walk into a forest stand of towering Pisonia trees and find a mat of tiny seedlings carpeting the forest floor — something that hadn’t been observed at Palmyra in recent decades as far as we know.”

Palmyra’s tropical rainforest also provides important habitat for a native gecko, insects, crabs and other rare species. Pisonia grandis forests are reported to be in decline globally.

Save the trees, save the seabirds

Prior to removal, invasive rats devoured native seeds and seedlings, as well as seabird eggs and chicks, at Palmyra Atoll. Removing the rats allowed seabirds to flourish, and the rebounding native forest provides the birds with important nesting and roosting habitat. When seabirds perch in the trees, they provide nutrients to the soil below through guano droppings. These nutrients are then taken up by native plants and also wash into the ocean where they dramatically benefit surrounding reefs. Restoring the native forest at Palmyra Atoll thus helps keep coral reefs and thriving fish populations healthy and improves their chances of overcoming changing temperatures and other climate change impacts.

Sea level rise and changing ocean temperature and chemistry will continue to stress Palmyra’s ecosystems,” said Alex Wegmann, Palmyra Program Director for The Nature Conservancy of Hawaii. “Restoring Palmyra’s native tropical rainforest allows greater whole-ecosystem resilience to climate change impacts.”

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An aerial view of Palmyra Atoll. Credit: Island Conservation

Measuring the positive effects on native plant communities like the Pisonia forest at Palmyra Atoll will help bolster the case for island management and invasive species eradication in other places around the globe.

It is extremely rewarding to walk through the forested islands of the atoll and see the physical evidence of our management actions literally taking root,” said Stefan Kropidlowski, Palmyra Atoll Refuge Manager for the U.S. Fish and Wildlife Service. “Partnerships made the rat eradication successful and will be essential in the future as we use the best available science and research to inform the next challenges we face together.”

An unplanned benefit of the rat removal was the eradication of the Asian tiger mosquito, or Aedes albopictus. This outcome could open new avenues for controlling mosquitoes and mosquito-borne diseases elsewhere.

Two land crab species were observed for the first time on the atoll’s islets after rat removal, bringing the number of land crab species at Palmyra to nine. Land crabs are the indigenous engineers of low-lying tropical oceanic island ecosystems, and now Palmyra has one of the world’s most diverse and best protected land crab communities.

In an age of well-founded concern about the environment, Palmyra Atoll’s response to the eradication of introduced rats reminds us of the resilience of intact ecosystems and gives us hope for a better tomorrow,” Wegmann said.

Long Days in Paradise

The team of researchers conducted pre-eradication monitoring of trees at Palmyra Atoll in 2004 and 2007, and repeated the monitoring post-eradication in 2011, 2012, 2014, and 2016. Monitoring included teams of two conducting counts of seedling abundance within 55 strip transects and approximately 50 locally rare tree plots distributed across the atoll. The team counted 140 native, locally rare tree seedlings pre-eradication and 7,756 seedlings post-eradication (2016).

The 2016 monitoring team of five was led by biologist Coral Wolf, who was five months pregnant at the time. Fieldwork consisted of trudging through waist-high water and high-kicking through dense tropical rainforest. The team stayed well-hydrated by harvesting coconut water as needed.

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Coral leading the team through the shallow waters on Palmyra Atoll. Credit: Abram Fleishman/Island Conservation

Why Islands?

Forty-one percent of the world’s most highly threatened vertebrates are found on islands, with invasive species introduced to islands being a leading cause of extinction. Removing invasive species from islands is an effective and proven way to save our world’s most vulnerable species.

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To date, there have been more than 500 successful projects to remove invasive rodents from islands. The pace, scale, and complexity of these efforts are increasing in recognition of the threat invasive species pose to biodiversity.

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Resources available: interviews, high-resolution photos, b-roll

For more information, please contact:
Sally Esposito, Director of Marketing and Communications, Island Conservation, +1 706-969-2783, sally.esposito@islandconservation.org
Evelyn Wight, Senior Communications Manager, The Nature Conservancy of Hawaii, +1 808-587-6277, ewight@tnc.org
Megan Nagel, Public Affairs Officer, U.S. Fish and Wildlife Service, +1 808-792-9530, Megan_Nagel@fws.gov

Notes to editors:

  • Palmyra Atoll, approximately 1,000 miles south of Honolulu, Hawai‘i, is a National Wildlife Refuge managed by the U.S. Fish and Wildlife Service and a scientific research station managed by The Nature Conservancy. The area includes 25 islets covering 580 acres of land, and thousands of acres of healthy coral reefs. In 2009, the refuge and waters surrounding it were included in the Pacific Remote Islands Marine National Monument.
  • Palmyra supports ten nesting seabird species, including one of the largest Red-footed Booby colonies in the world, and the largest Black Noddy colony in the Central Pacific. Many nationally and internationally threatened, endangered, and depleted species depend on Palmyra Atoll, including sea turtles, pearl oysters, giant clams, reef sharks, coconut crabs, a large diversity of fish (at least 418 species), and marine mammals.
  • 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 ate eggs and chicks of ground and tree-nesting birds, particularly sooty and white terns. Rats also ate land crabs and the seeds and seedlings of native tree species.
  • In June 2011, the U.S. Fish and Wildlife Service, The Nature Conservancy, and Island Conservation removed destructive, non-native rats from Palmyra Atoll. The Palmyra rat removal project was the result of more than seven years of planning and was the first step in a longer-term effort to restore the atoll’s ecological balance. The partners confirmed the island free of invasive rats in 2013 and, thanks to strict biosecurity rules, it remains rat-free today.
  • Although Palmyra Atoll appears to be relatively unscathed, plantation-era coconuts have become invasive and an ongoing restoration project to remove invasive species is underway. The U.S. Fish and Wildlife Service, The Nature Conservancy, Island Conservation and other partners are collaborating on this multistage project to restore the natural ecosystem, with each action building on the next. The first phase focused on the removal of black rats. Restoring the native forest community, including reducing and controlling invasive coconut palms, is an important next step.  Future plans also include considering the translocation of two endangered, insular bird species that are at risk in their current ranges and active restoration of Palmyra’s extirpated seabird colonies.

About Island Conservation

Island Conservation prevents extinctions by removing invasive species from islands. To date, we have successfully restored 63 islands worldwide, benefiting 1173 populations of 468 species and subspecies. Working together with local communities, government management agencies, and conservation organizations, we select islands that have the greatest potential for preventing the extinction of globally threatened species.

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