Could an Invasive Fungus Seal the Fate of Hawaii’s Endangered Forest Birds?
A fungal disease is currently plaguing an endemic tree that dominates native forests in the Hawaiian Islands—the same forests that a slew of critically endangered birds rely on.
When Julia Diegmann started working for the Kaua’i Forest Bird Recovery Project in 2010, she could reliably see two endemic forest birds, ‘Akikiki and ‘Akeke‘e, from the parking lot of the Pihea Trailhead on the island of Kaua‘i. Today, that is no longer the case thanks to a gauntlet of threats the birds face. ‘Akikiki, ‘Akeke‘e, and other native Hawaiian birds have declined rapidly due to habitat loss, introduced predators, invasive plants and insects, and avian malaria. But a lesser-known threat also looms: a fungal disease that kills ‘ōhi‘a, one of the primary trees on which the birds depend for food and shelter. “They are getting hit from all sides,” says Diegmann, now the outreach specialist for the project.
An umbrella term for five endemic tree species and a dozen varieties across the Hawaiian Islands —the most prominent being ‘ōhi‘a lehua—‘ōhi‘a play an important role in Hawaiian culture and the native forests. The trees grow widely from sea level up to 9,000 feet and survive in deserts and rainforests. An adaptable plant, ‘ōhi‘a is one of the first species to sprout from barren lava, but also thrives as tall canopy trees in mature forest. The tree comprises 80 percent of native Hawaiian forests, supporting endangered Hawaiian snails, mosses, ferns, butterflies, insects, and birds. And yet, in less than a decade, the new fungal disease has become a serious threat to ‘ōhi‘a and the flora and fauna they support.
“‘Ōhi‘a are the most important habitat species, so if we suddenly lost them all we’d be in big trouble,” says Bret Nainoa Mossman, an avian biologist with the Hawai‘i Division of Forestry and Wildlife.
Nearly every Hawaiian forest bird relies on ‘ōhi‘a to some extent. The tree’s vibrant yellow, orange, and red feathery blooms feed ‘Apapane, ‘I‘iwi, and other endemic honeycreepers with their nectar and, eventually, fruit. Other species, like Kiwikiu, creep along the tree’s trunk, peeling off bark to extract hidden larvae. And ‘ōhi‘a’s dense canopy provides shelter from predators, as well as critical nesting sites.
First detected in 2010 on Hawai‘i Island, the fungal disease swept through lower elevation forests of Puna, killing 98 percent of ‘ōhi‘a trees in the region within several years. “It was private landowners who started saying, ‘Hey, these trees are dying in a way we’ve never seen before’,” says J.B. Friday, a forester with the University of Hawai‘i at Mānoa. “It turns out they were quite right—it was a new thing.” In 2014, scientists identified two fungi, Ceratocystis huliohia and Ceratocystis lukiohia, as the cause of the mortality. Their origins still unknown, the fungi likely hitchhiked to Hawai‘i on a plant or mutated from a different fungus introduced to the island. The less deadly fungus, Ceratocystis huliohia, probably existed at low levels on Hawai‘i Island for decades, experts speculate, but was only discovered because of the large number of trees dying from the aggressive one.
Though Ceratocystis lukiohia is more virulent, the two fungi act similarly, clogging ‘ōhi‘a’s vascular system and depriving the canopy of water. “The water gets clogged, like your kitchen sink on Thanksgiving Day,” says Kim Rogers, outreach specialist for the Kaua‘i Invasive Species Committee and Audubon magazine contributor. Once showing signs of infection, the trees can die in weeks or even days, giving rise to the disease's common name: rapid ‘ōhi‘a death, or ROD. “You can see a perfectly healthy looking ‘ōhi‘a tree and the next month [you] go out, and all of a sudden the entire canopy is reddish-brown and the tree’s dead,” Rogers says.
The disease’s prevalence on Hawai‘i Island—more than a million ‘ōhi‘a have died from the disease, already—is largely the result of introduced beetles and ungulates, such as feral goats, pigs, and cattle. Ambrosia beetles burrow into a diseased tree, releasing excrement, tree shavings, and fungal spores, which become airborne or fall to the ground, hitchhiking on invasive predators or human boots. Ungulates damage the trees with their hooves, and those wounds provide easy access for a fungal spore to take root. Research by Ryan Perroy, a geographer at the University of Hawai‘i at Hilo revealed that in fenced, ungulate-free areas, fewer ‘ōhi‘a died from ROD.
To reduce the disease’s spread, officials prohibited transporting ‘ōhi‘a between islands in 2016. Still, the aggressive fungus was detected Kaua‘i in 2018. “I have suspicion that someone unknowingly brought some infected wood over,” Friday says. So far, the more lethal fungus hasn’t appeared outside Kaua‘i and Hawai‘i Island—surprising because Maui lies less than 30 miles from the latter—but accidentally introducing the fungus elsewhere is highly plausible. With no known cure, the possibility of the deadly disease spreading to other islands is a steady source of stress for ecologists working to save the trees.
To understand how the loss of ‘ōhi‘a could affect native bird populations throughout the island chain, Rick Camp, a quantitative ecologist at the U.S. Geological Survey Pacific Island Ecosystems Research Center, took advantage of bird counts completed in the lower Puna region of Hawai‘i Island in 2003 and 2004, comparing them to follow-up surveys in the same area in 2016, after ROD wiped out a majority of the ‘ōhi‘a. His results provided a glimpse into the future if ROD spread rampant, finding striking changes in the bird community.
The most worrisome result, says Camp, was that ‘Amakihi, a native forest bird, declined by 70 percent. Before ROD, ‘Amakihi’s numbers there had increased between the 1990s and early 2000s, and the population seemed surprisingly resilient, surviving even when living side by side with mosquitoes and avian malaria. “Seventy-plus percent decline in ‘Amakihi was as much shocking as it was alarming,” Camp says.
Also concerning was that the surveys documented Saffron Finch and Yellow-fronted Canary, two non-natives, only in 2016; both species prefer shrubbier habitat over dense canopies, reflecting a shift in the forest composition. Another non-native bird, Warbling White-eye, dropped by 30 percent. Both the Warbling White-eye and ‘Amakihi are more generalist species—they don’t rely on ‘ōhi‘a completely—so Camp says these declines reveal the real danger when the disease spreads unchecked. “It’s actually hitting everything.”
These results and the devastating loss of ‘ōhi‘a in Hawai‘i Island’s lower elevation forests rang alarm bells. Fortunately, the fungi prefer warmer temperatures, so the cooler upper elevation forests on Hawai‘i Island, where more of the endemic forest birds live, have been cushioned from the full brunt of the disease. But it has spread into mid-elevation forests around Hawai‘i Volcanoes National Park, and similar to what has happened with the island's malaria-laden mosquitos, warming temperatures could tip the balance, making higher elevations hospitable for the fungi.
Currently, Kaua‘i’s endemics face more imminent danger from ROD. At least 300 ‘ōhi‘a on the island have died from the disease, including one tree close to the last refuge for endangered forest birds on the Alaka‘i Plateau. For ‘Akikiki and ‘Akeke‘e, two critically endangered honeycreepers on Kaua‘i, avian malaria and habitat loss have shrunk their available habitat to an area less than 16 square miles—a drastic reduction from their historical range across most of Kaua‘i. If ROD wipes out ‘ōhi‘a here, the birds have nowhere else to go. “It’s the only place where these birds persist on earth,” Diegmann says. “To lose even more habitat for them would be devastating, just devastating.”
Ecologically, experts say the loss of ‘ōhi‘a would have far-reaching consequences: ‘Ōhi‘a help maintain a healthy forest and watershed, critical for drinking water, with cascading effects on the entire island ecosystem. “If we have a healthy watershed, we have a healthy ocean and a healthy coral reef,” Diegmann says. “It’s not just ‘save the birds,’ it’s ‘save the whole ecosystem’.”
Losing ‘ōhi‘a would also be a blow to Hawaii’s culture. Associated with guardian spirits and Hawaiian gods, respectively, Hawaii’s honeycreepers and ‘ōhi‘a feature prominently in traditional stories, songs, chants, and hula. Native Hawaiians used ‘ōhi‘a wood to build houses and canoes, flowers to make leis, and leaves to drink as tea. And vibrant bird feathers, particularly from the scarlet ‘I‘iwi, adorned helmets, capes, and long, flowing cloaks of the ruling class. “To lose any of those ancestors or individuals, be it ‘ōhi‘a or the birds, would be a huge loss for this place and culture and what makes Hawai‘i , Hawai‘i ,” says Rachel Kingsley, an outreach associate with the Maui Forest Bird Recovery Project.
With so much at risk, Hawaii’s scientists, volunteers, and residents have responded quickly to protect ‘ōhi‘a. Signs at trailheads, equipped with boot-brush cleaning stations and disinfectant spray, inform hikers about ROD’s prevalence. Biologists take extra caution to avoid transporting the microscopic fungus spores accidentally. Before and after field work, they clean vehicles and gear, inside and out, and sanitize with 70 percent alcohol. “I like to say leave the mud where you find it,” Rogers says. In 2018, the governor declared April 25th as ‘Ōhi‘a Lehua Day. Organizations like the Kaua’i Invasive Species Committee give away free sanitation kits and ‘ōhi‘a trees to raise awareness and encourage action to protect the trees.
Meanwhile, scientists continue to brainstorm new ways to save ‘ōhi‘a and the forest birds. Currently, they are investigating if particular ‘ōhi‘a varieties can survive once infected, and then breeding those survivors to create ‘ōhi‘a fully resistant to ROD. These resilient seedlings would be good candidates for forest restoration. Unlikely to fully eradicate ROD from the islands, the biologists believe such efforts to reduce the spread and develop ROD-resistant trees could work, and are enough to keep hope alive amid a daunting predicament. “We want those forests to still be there," Mossman says, "because the birds will undoubtedly be there again if we give them the opportunity."