Palmyra Atoll, a remote, uninhabited speck of land, coral and sea halfway between Hawaiʻi and American Samoa, is one of the healthiest, intact atolls on the planet—so ecologically sensitive that visiting researchers freeze their clothes at night to kill invasive species. Recently, conservationists have been removing Palmyra's biggest unwelcome invaders, non-native coconut palm plantations, so Pisonia grandis, an important indigenous rainforest tree, can grow back instead.
But new research published in the journal Current Biology shows that ridding Earth's most remote atoll of introduced coconut palms may not be enough to re-establish native forests; the right soil fungi need to be there too. The symbiotic relationship between Pisonia and its fungal partners is so strong that every tree root sampled across Palmyra's thin soils contained critical underground fungi.
Since Pisonia is the backbone of Palmyra and many of the world's other 598 atolls, the findings suggest that unique soil fungi are central to life on these extraordinary, precarious island ecosystems.
"It reveals a hidden dependency at the heart of an entire ecosystem," said lead author Charlie Cornwallis, an ecology professor with Lund University in Sweden, who worked on the study in partnership with the Society for the Protection of Underground Networks (SPUN), University of Oxford, The Nature Conservancy and other institutions. "The health of Palmyra's coral reefs ultimately depends on seabirds, which depend on Pisonia trees for nesting, which depend on fungi. Remove any link in that chain and the whole system could unravel."
Around the world, mycorrhizal fungi form complex hidden networks that partner with 80 percent of vegetation, providing water, phosphorus and nitrogen to plant roots in exchange for carbon. But no one had ever documented the ways fungi on Palmyra Atoll might help nutrients move between the forest and the sea. So a team of experts, led by evolutionary biologist and SPUN co-founder Toby Kiers, traveled to the middle of the Pacific Ocean to map the atoll's underground networks.
Palmyra is not undisturbed. Native forests were cleared and replanted with coconut palms as far back as the 1850s. The U.S. military during World War II re-engineered the landscape even more—and likely introduced invasive black rats. The rats wiped out seedlings and bird hatchlings and decimated crab populations. After TNC and partners eradicated the rodents in 2011, Pisonia seedlings and crabs blanketed the ground across the atoll. By the time fungal researchers arrived in 2022, workers had removed 1.5 million coconut palms. The atoll was experiencing a rebirth.
The scientists hiked beneath skies awash in flapping terns and waded through lagoons thick with fish. They bypassed crabs by the thousands, including coconut crabs with leg spans as long as baseball bats. They even got bumped by the occasional shark. In every sample they took below Pisonia trees across 27 of Palmyra's islands, they found rare fungi, several species of which had not been documented anywhere else on Earth. The further scientists got from Pisonia trees, the fewer of its fungal partners they mapped. They even found fungi on tree roots suspended several feet off the ground, suggesting that soil is not essential for the fungi to associate with their hosts.
The research suggests that Pisonia might need help to reclaim areas cleared of coconut palms. So scientists identified fungal "hot spots" where soil could be transplanted to help seedlings thrive.
"Until now, restoration has almost exclusively focused on native plants. That is changing," said Kiers, whose groundbreaking work documenting fungal networks recently earned her the Tyler Prize , known as the "Nobel Prize for the Environment," and a prestigious MacArthur Fellowship . "Research is showing how successful restoration involves introducing native plants together with native fungi."
Those fungi help more than just trees. Pisonia produces dense canopies that offer nesting spots for millions of seabirds, from red-footed boobies to great frigatebirds. The birds' falling guano flushes through the sandy soil into the surrounding ocean, feeding plankton that nourishes spectacular coral reefs. Those support abundant marine life, including enormous concentrations of reef sharks. Guano droppings on land also feed vegetation eaten by land crabs, which dig burrows and aerate and mix the soil in ways that appear to increase the density of Pisonia's fungal partners, the scientists found. By comparing fungal diversity inside and outside of crab burrows, the team found links between burrowing activity and higher fungal diversity.
"It's not that other ecosystems aren't interconnected," co-author Stuart West, a professor of evolutionary biology at the University of Oxford, said. But on Palmyra "it's rammed in your face and amplified because it's all squashed onto a small, simple island with fewer things."
Alex Wegmann, lead scientist for The Nature Conservancy's Island Resilience Strategy called it "one of the few places you can literally say, with science backing you up, 'That tree right there is important to that coral out there.'"
The study is a reminder that conservation isn't just about species you can see. On remote island systems like atolls, if symbiotic partners are lost, replacements can be scarce. Documenting hidden underground fungi may be crucial to preventing ecosystem collapse.
TNC's Palmyra Preserve is encompassed by the U.S. Fish and Wildlife Service's Palmyra Atoll National Wildlife Refuge (out to 12 nautical miles) and by the Pacific Islands Heritage Marine National Monument (out to 50 nautical miles), one of the largest swaths of ocean and islands protected in the world. This research was conducted under a permit from FWS.
