Palmyra rat eradication leads to dramatic ecosystem restoration

Biologist Coral Wolf remembers returning to remote Palmyra Atoll in 2011 for the first time after the rats were eradicated there. Amid the tropical forest of native pisonia trees was a carpet of tiny seedlings.

“It was very exciting,” said Wolf, a Kaaawa resident and scientist with Island Conservation, a nonprofit that works to prevent extinctions by removing invasive species from islands. “The pisonia seedlings were at a density never seen before.”

The restoration of Palmyra’s native forest following the eradication of introduced predatory rats from the atoll’s 25 islets was chronicled in research published this week in the science journal PLOS ONE.

The study, which showed a 5,000 percent increase in native trees, adds to the body of evidence regarding the transformative impacts of removing invasive species.

Rats are known to be voracious predators of seabird eggs, chicks and the seeds and seedlings of native plants that seabirds rely on for nesting.

In Hawaii, Island Conservation has worked with the state to remove rats and protect the seabirds and plants of Lehua, the little island off Kauai’s west side. Additionally, scores of other conservation groups are working to control rats and other invasives in natural areas across the main islands.

In June 2011 the U.S. Fish and Wildlife Service, the Nature Conservancy and Island Conservation removed non-native rats from Palmyra Atoll following more than seven years of planning. It was the first step in a longer-term effort to restore the atoll’s ecological balance.

A team of researchers conducted pre-eradication monitoring of trees at Palmyra Atoll in 2004 and 2007, and returned for monitoring after the rat removal in 2011, 2012, 2014 and 2016, Wolf said.

Monitoring included the counting of seedling abundance within 55 strip transects and some 50 locally rare tree plots across the atoll. While the team counted 140 native, locally rare tree seedlings before the eradication, there were 7,756 seedlings afterward in 2016.

There were other benefits to the ecosystem, according to scientists. Removing the rats allowed seabirds to flourish as the reinvigorated native forest provided new key nesting and roosting habitat.

When seabirds perch in the trees, their guano droppings add nutrients to the soil, which finds its way into the ocean to benefit reefs and give fish populations a better chance of overcoming changing temperatures and other climate change impacts.

Alex Wegmann, Palmyra program director for The Nature Conservancy of Hawaii, said sea level rise and changing ocean temperature and chemistry will continue to stress Palmyra’s ecosystems.

“Restoring Palmyra’s native tropical rainforest allows greater whole-ecosystem resilience to climate change impacts,” Wegmann said in a news release.

Scientists said two land crab species were observed for the first time on the atoll’s islets following eradication, bringing the number of land crab species at Palmyra to nine and further bolstering one of the world’s most diverse land crab communities.

Moreover, rat removal led to the eradication of the Asian tiger mosquito. Officials said the unexpected outcome might even lead to new ways of controlling mosquitoes and mosquito-borne diseases elsewhere.

The island remains rat-free because of strict biosecurity rules, Nature Conservancy officials said — though it helps that the remote patch of land is 3,400 miles from the nearest continent.