To save this palm-filled paradise, biologists must kill the trees | Science

Dana Edmunds

PALMYRA ATOLL IN THE CENTRAL PACIFIC OCEAN—Duncan Coles lops the head off a juvenile coconut tree with a practiced swing of his machete. He and nine other volunteers chop away at the thicket, stepping over piles of fallen coconuts and fist-size hermit crabs. Soon they return to their decapitated kills, dousing each stump with blue-dyed herbicide. Two other volunteers use power drills to set upon the mature coconut trees towering overhead, boring holes and filling them with shots of herbicide.

The slashing and poisoning is part of an unprecedented endeavor to rid this remote atoll of all but a few coconut palms (Cocos nucifera). The gangly tree is an icon of idyllic tropical islands, but also an aggressive invasive species that crowds out native plants and animals. By removing 99% of Palmyra’s millions of palms, biologists hope to create more room on the atoll’s three dozen islets for indigenous forests and seabirds, including the world’s second largest colony of red-footed boobies. If all goes as intended, the restoration effort could help make this coral-ringed atoll, which has an elevation of just 2 meters, more resilient to sea-level rise and other ravages of climate change.

The researchers are now nearly halfway to their goal, and are intently tracking whether their tactics are producing the predicted ecological effects. If the restoration methods work, they could ultimately be replicated on other islands with abandoned coconut plantations run amok. On Palmyra, however, the team aims to transform virtually the entire landscape and ecosystem, something that’s only possible because the atoll has no human inhabitants relying on coconut palms to produce food, water, fuel, or building materials, says ecologist Alex Wegmann of the Nature Conservancy (TNC), which co-owns the atoll with the U.S. Fish and Wildlife Service (USFWS). “If there is anywhere safe to test this,” Wegmann says, “Palmyra is that place.”

Palmyra’s transformation into a laboratory for ecological restoration marks yet another twist in its rich history, which has included serving as a U.S. Navy base during World War II and a coconut plantation. More recently it was eyed as the potential home of a tropical nudist colony, a nuclear waste dump, and a launch pad to shoot rockets into space. Located 1600 kilometers south of Honolulu, the atoll sits just shy of 6° north of the equator, on the edge of the intertropical convergence zone, a band of ocean known for slack winds and abundant rainfall. Palmyra receives 4.3 meters of rain per year.

Palmyra Atoll sits in isolation 1600 kilometers south of Honolulu, just north of the equator.

Dana Edmunds

The atoll got its name in 1802 after its namesake, a U.S. sailing ship, splintered on its reef, a mere 4 years after the atoll’s first reported sighting. Sixty years later, Hawaii’s King Kamehameha IV declared Palmyra part of his kingdom, and it became a U.S. territory in 1898 when the United States annexed Hawaii. Since then, it has been mostly in private hands.

In 2000, TNC purchased the atoll for $30 million and sold most of the outer islets and lands that emerge at low tide to the U.S. government for $9 million. On Cooper Island, which has an airstrip, TNC built a research station, which has helped the atoll become a prime destination for scientists seeking to study marine ecosystems largely free of human impacts.

Others have looked at the ecological connections between land and sea. Researchers led by marine ecologist Douglas McCauley and terrestrial ecologist Hillary Young, both of the University of California, Santa Barbara, have traced how seabirds that roost and nest in Palmyra’s native forests act as nutrient transporters. The birds catch fish and squid in the surrounding ocean, then rain nutrient-rich guano onto the soil, nurturing vegetation and the animals that feed on the plants. The nutrients also wash into the atoll’s lagoon, producing blooms of plankton that feed manta rays and other marine life.

But that virtuous circle is weaker in forests dominated by the coconut palms. Originally from Thailand and the Philippines, according to genetic evidence, the palms were brought to the central Pacific Ocean by Polynesian seafarers an estimated 1500 years ago, as a fast-growing source of food and other resources. There are no signs of ancient human settlements on Palmyra, so the trees may have arrived on the atoll when currents washed their nuts—which can survive for months at sea thanks to their thick buoyant husks and fat reserves—from some distant inhabited island.

By the early 1800s, a passing sailor wrote that some of Palmyra’s “islets seemed exclusively appropriated to the cocoa-nut tree.” In 1885, entrepreneurs began to plant commercial groves, which continued intermittently through the 1950s. In a 1913 survey of the atoll, botanist Joseph Rock counted some 25,000 palms, bearing nuts which were “the finest and the biggest the writer has ever seen.” He sent some to an expert in Italy, who was impressed by “their large size … [and] distinctly trigonal shape … the three edges almost take the shape of wings.”

Red-footed boobies roost in a native Pisonia tree. Palmyra’s seabirds tend to shun introduced palms in favor of native species.

Dana Edmunds

But although the palms have flourished, the forests they dominate have far fewer birds and fewer manta rays foraging along their shores. Seabirds probably avoid nesting in coconut palms, Young says, because of their branchless trunks, relatively small canopies, and swaying fronds. They prefer a more stable architecture—the forks, nooks, and crannies of native trees.

As a result, coconut-dominated forests tend to have nutrient-poor soils, less biological diversity, and a different suite of insects (many of them invasive species) compared with native forests. To better understand such differences, ecologist John McLaughlin, a postdoctoral researcher in Young’s lab, has fogged palms and native trees with pesticides, using giant funnels to catch the insects that fall out. It’s part of an ambitious food web project to reveal the abundance, diets, and interactions of 300 species. On a bad day in the palm forest, McLaughlin says, the funnels will thwack with the sound of palm-size cane spiders and invasive giant cockroaches hitting the Teflon-coated plastic cones. That’s when, he says, “You know it’s time to get out of there.”

Research has shown that fast-growing coconut palms can suck up moisture so fast that they increase water stress in nearby native trees. The palms are more drought and salt tolerant than native trees—advantages that help them establish dominance and spread as a monoculture. Then, in a coup de grâce, they crush competitors with falling nuts and fronds.

Ripping out the palms has long been on the list of restoration projects on Palmyra. First, however, managers decided to attack another invader, black rats (Rattus rattus), which likely arrived on ships during World War II. With no predators, rats multiplied into the tens of thousands. They ate the seeds and gnawed the saplings of native trees and attacked seabird colonies, including those of sooty terns, which nest on the ground. Rats are the key suspects behind the absence on Palmyra of eight other species of ground or burrow-nesting birds, including shearwaters and petrels, all found on central Pacific islands that have remained rat-free.

The first attempt to eradicate the rats in 2002 failed, partly because Palmyra’s abundant land crabs outcompeted the rodents for the poisonous bait. The crabs’ physiology allowed them to eat the poison—the anticoagulant brodifacoum—without ill effect.

Coconut crabs, which can reach 4 kilograms, scavenge what they like.

Dana Edmunds

The second effort was successful only after Wegmann and colleagues radio-collared rats and discovered that the rodents liked to hang out in the crowns of coconut palms. The crowns became a convenient platform for stashing cotton gauze sacks of poison bait, delivered by workers firing slingshots or dangling from helicopters. Crabs do not reach the palm tops.

Once rats were exterminated in 2011, researchers watched with delight as native tree saplings began to spring from the forest floor. There were also happy surprises. Scientists discovered two additional species of land crabs that had likely gone undetected because voracious rats suppressed their numbers. And researchers realized they were no longer being bitten by Asian tiger mosquitoes, a pest that attacks during the day and can carry dengue and yellow fever. It appears the mosquitoes depended on rats rather than humans or birds for blood meals, says parasitologist Kevin Lafferty of the U.S. Geological Survey.

The eradication of the rodents, however, only made the palm problem more apparent. Without rats devouring palm flowers, nuts, and sprouts, mounds of coconuts built up on the forest floor, and lagoons became lumpy with floating nuts, each a tree in waiting. Falling coconuts and fronds posed such a threat that managers, volunteers, and others were required to wear hardhats while in the forests, or to enter with a buddy, so they wouldn’t get knocked out and potentially picked apart by crabs scavenging anything that falls to the ground. The largest scavenger, the coconut or robber crab, can weigh up to 4 kilograms, hoisted by legs spanning 1 meter.

With such obstacles in mind, managers began to plan a $2.2 million campaign to obliterate most of the palms, its funding largely split between USFWS and TNC, with a $250,000 grant from the nonprofit Wildlife Conservation Society. The nonprofit Island Conservation promises $100,000.

Managers knew it would be a monumental task: Mapping revealed that 42% of Palmyra’s 248-hectare landmass was covered with palm forests, holding some 29,000 adult trees and 2.1 million juveniles. “The numbers seemed daunting,” recalls Wegmann, the program’s science director. “But at least, unlike the rats, we knew that the palms couldn’t hide or run away.”

Nick Holmes (left) and Alex Wegmann (right), scientists with the Nature Conservancy, look for seabirds after volunteers have cut down and poisoned palms.

Dana Edmunds

Felling Palmyra’s dense palm groves with chainsaws was a no-go: The toppling trunks would crush native vegetation and wildlife, and release a potentially harmful pulse of carbon and other nutrients into the lagoon and coral reef as the vegetation decomposed. Instead, the researchers settled on cutting down the saplings and poisoning the adult palms with the herbicide glyphosate. The dead trees, still standing, would decompose more slowly, reducing nutrient pulses—at the cost of making some islets look like sand ashtrays filled with giant half-burned match sticks.

Project managers had hoped the palm cull, which began in 2019, would be done by the end of this year. But the coronavirus pandemic has put the program on pause; the most recent team of volunteers was airlifted off the atoll at the end of May. The crews had made progress by then, poisoning 54% of the adult palms and felling 40% of the sprouts. They hope to be able to come back and finish the job next year.

“We are not trying to eradicate coconuts,” notes Stefan Kropidlowski, manager of the USFWS refuge on Palmyra. “We are trying to remove the coconut monoculture and allow the native forests to get a foothold.” Managers expect to retain 1% of the palms, mainly on islet edges for erosion control. They’ll also preserve some trees that produce the unusual trigonal nuts, and leave plots of palms that will serve as controls in planned studies, including examinations of changes in soil carbon storage and food web diversity.

Once the palms are mostly gone, crews will plant native trees, especially the giant Pisonia and the tree heliotrope. The hope is that the native plants will shade out invasive palms and become the dominant species, Wegmann says. Yet some scientists who do fieldwork on Palmyra fear managers may end up on a conservation treadmill, endlessly cutting back invasive palms. Responding to such criticism, project managers say they have found it’s easy to kill palm sprouts with a machete before they start to produce nuts. “It takes coconuts 10 years to reach reproduction age,” Wegmann says. “If there is maintenance, it will be minimal.”

Whether the palm cull will boost seabird populations remains to be seen, says Young, whose students and colleagues are helping monitor the effects. She suspects seabird numbers are limited more by the availability of food than nesting sites. “It may benefit the birds, but there are a lot of unknowns.”

Coconut palms line the shores of Palmyra Atoll. Although the restoration plan calls for killing millions of the island’s palms, some will be left along shorelines to help protect the island from wind and waves.

FARLOW/OLSON/National Geographic

Wegmann is optimistic. “We are realigning the atoll to be as seabird friendly as possible,” he says. Palmyra currently hosts 11 seabird species, including a colony of some 25,000 red-footed boobies (only the Galápagos have a bigger colony). There are also nesting colonies of roughly 100,000 sooty terns, smaller groups of great frigatebirds, black and brown noddies, white terns, and tropicbirds. Biologists believe perhaps eight other seabird species once used the atoll, including wedge-tailed shearwaters, tropical shearwaters, Phoenix petrels, and white-throated storm petrels. In hopes of attracting them back, researchers have set up a sound system that broadcasts their calls.

They are also considering relocating some very rare species to Palmyra. Its remoteness and protected status could make it a safe haven for endangered birds such as the Kiritimati (Christmas Island) warbler and the Guam kingfisher, which is now kept alive through a captive breeding program.

Larger bird populations spread across the atoll, and their output of guano, could help Palmyra’s surrounding coral reefs become more resilient to climate change. Case studies from the Indian Ocean, focusing on rat-free islands with larger seabird populations, have found that the extra guano produces more plankton and algae, as well as faster growing fish that keep coral reefs clean and more productive. A thriving reef can recover more quickly after coral bleaching events caused by marine heat waves, researchers note, and coral growth may be better at keeping pace with rising seas.

Here on Palmyra Atoll, the coral reefs are already in good shape. Earlier restoration work has removed rusting shipwrecks that were leaching iron onto the reefs, fueling the growth of an invasive anemonelike animal called a corallimorph, which smothers corals. Someday, managers hope to remove the last of the causeways built by the U.S. Navy. They impede the flushing of ocean water, leaving parts of lagoons with hot, stagnant water that can kill corals.

Palmyra is in a better position than many of the world’s 339 low-lying atolls to cope with sea-level rise. Even under a worst case scenario of a 1-meter rise in the 21st century, models suggest nearly half of its land would remain above water. Still, managers of other atolls overrun by coconut palms and rats—including one in French Polynesia and another in American Samoa—are keeping a close eye on Palmyra’s restoration experiment. Its success, they say, could provide clues to how to navigate an uncertain future.


Next Post

How Italy’s ‘father of the swabs’ fought the coronavirus | Science

Thu Aug 27 , 2020
“Lock down the village, test everybody, and isolate the positives. It really works,” says Andrea Crisanti. KATTY HUERTAS By Douglas StarrAug. 27, 2020 , 12:00 PM Science‘s COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation. Andrea Crisanti was on a 30-hour flight from Italy to Australia for […]