From our vantage point, roughly a mile above sea level, the land below us undulates, covered in a verdant carpet of impenetrable green. Our small yellow dragonfly of a helicopter, a four-seater Hughes 500D, bucks and bounces through turbulence, as we climb along a deep gash in the eastern side of Haleakala, the 3,055-meter-high (10,023-foot) shield volcano that dominates Maui’s eastern lobe. Below us unfurls the remote Kipahulu Biological Reserve, part of Haleakala National Park and one of the United States’ most pristine yet threatened biological sanctuaries. Through occasional breaks in the vegetation, we can see boulder-strewn streambeds and towering cliff faces in the rugged and unforgiving landscape.

Few members of the public have ever been allowed to visit this remote sanctuary. But after years of inquiries, I was invited to join a scientific expedition into the valley. Leading the trip is 42-year-old Seth Judge, a wildlife biologist from the University of Hawaii at Hilo. He sits beside me, snapping photos and taking videos of the forbidding terrain, where he has been conducting bird surveys for the past two years. “Look at this place,” Judge says through radio static and the percussive thwack of the blades. “It never gets old coming up here.”

It’s no accident that the Kipahulu Valley is one of Hawaii’s last bastions of more or less pristine biodiversity. This massive erosive scar running down Haleakala’s eastern flank is more than 750 meters (2,500 feet) deep in places. That challenging topography makes it an almost perfect natural fortress, protected by near-vertical cliffs and dense vegetation. The terrain has isolated the valley, keeping its ecosystems mostly intact while driving a stunning adaptive radiation of unique organisms. “A lot of species are endemic here because they were able to evolve without interference from species from the rest of the island,” says Woody Mallinson, Haleakala’s natural resources program manager. Today, as many as 13 threatened and endangered species of animals and more than 40 species of plants inhabit its roughly 3,400 hectares (8,500 acres). Dozens are found nowhere else on Earth; dozens more have disappeared. “For some of the listed species, they are historic and haven’t been seen in decades,” Mallinson says.

The oft-told narrative of Hawaii’s ecosystems is one of continual assault by foreign invaders, including escaped pests like rats; domestic animals, such as pigs and goats; and crops like sugarcane, coconut palm, and taro. Even the supremely isolated Kipahulu Valley is not entirely immune. To keep its assemblage of rare species protected, the National Park Service has erected a defensive perimeter of fences around it.

Perhaps the park’s most important management strategy, though, is the fact that the Kipahulu is kept strictly off-limits to the million-plus visitors the park receives annually. There are no trails or roads into the reserve. The few park managers and scientists allowed in are met with quarantine-like protocols to ensure they’re not carrying in any invasive pests. Even scientific ventures are carefully weighed against the potential harm done by each footstep, as seeds can be carried into remote regions on the soles of boots or the skids of helicopters.

Fortunately, these efforts seem to be paying off. With rigorous conservation and restoration work being carried out inside and outside the park’s boundaries, the decline of some native species has slowed. Others are showing signs of recovery. But of course, fences, chemicals, and traps can’t beat back climate change, which is radically transforming the valley’s ecosystems and fueling the spread of disease, including avian malaria—one of the greatest threats to the Kipahulu’s critically endangered songbirds. Nature seems to be making some unexpected adjustments, as species adapt to the new paradigm, giving us a glimpse of the rapid evolution that gave rise to the Kipahulu’s tremendous biodiversity in the first place. The question is whether science and nature can keep pace with, or perhaps even get out in front of, the massive changes being wrought by global climate change.

As the helicopter approaches the alpine zone, the forest below thins noticeably. The radio voice of our pilot, Wayne Temple, chimes in through our headsets: “There’s the landing site,” he says, pointing to a small grassy zone around a pond that appears roughly the size of a nickel. This tiny body of water is significant. It is one of only a small handful of lakes on all of the Hawaiian Islands. Though Haleakala’s windward slopes experience some of the highest rainfall totals of any place on Earth, that precipitation has little chance to collect on its steep slopes before running off or percolating through the porous volcanic soil.

Craning his neck to scan the terrain below, Temple eases the aircraft down with a soft thud, blades still whirling. Judge, a wiry figure with intense eyes and an angular nose, makes a chopping motion with his hand, and we quickly hop out. After we exit with our gear, the helicopter rises from the ground within seconds, the downdraft sending up droplets from the surface of the pond. Amidst the swaying native grasses are familiar yellow flecks: dandelions. At some point these common weeds might have hitched a ride here, most likely via seeds attached unwittingly to the shoes or clothing of outsiders like ourselves.

Under the towering rock face of the ridge above us, which rises steeply, some 450 meters (1,500 feet) from where we stand, Jenna Fish, a 29-year-old field technician with the National Park Service and the other member of our three-person party, revels for a moment in the silence. A native of Wailuku, a city on Maui that lies west of Haleakala National Park, she knows the mountain well. “Hard to believe that just over that ridge there are a bunch of hikers,” she says, pointing up the precipitous slope toward the cabins at Paliku camp, a popular backpacking destination in Haleakala’s crater. Just then, something high on the ridge catches her eye—a single pine tree. She takes out her binoculars and quickly identifies it as a Monterey pine (Pinus radiata), a species endangered in its home range along California’s coast, but which has become an invasive blight in large swaths of the park since it was introduced to Hosmer’s Grove, near the park entrance, in the early 1900s. Fish takes her GPS coordinates and a photo of the tree so she can relay the information to the park’s invasive vegetation removal crew. Then we hoist our packs onto our backs and set out toward West Camp, the highest camp in Kipahulu Valley, which will serve as our base of operations.

For the next three days, Fish, Judge, and I will traverse a rugged section of the upper valley on foot, scouring it for hard-to-find plants and listening for a host of rare forest birds, including the Maui parrotbill, or Kiwikiu (Pseudonestor xanthophrys)—one of the most imperiled bird species on the planet. Our journey will follow the nearly invisible traces of another expedition that took place 50 years earlier.

In July 1967, an expedition set out to conduct the first comprehensive scientific survey of the Kipahulu Valley. The team of approximately 28 comprised botanists, ornithologists, evolutionary geneticists, and plant geographers from The Nature Conservancy, Hawaii Volcanoes and Haleakala National Parks, the Bureau of Sport Fisheries and Wildlife (today the U.S. Fish and Wildlife Service), the University of Hawaii, and several other organizations. Their trip would not be an easy one. Shortly after the team arrived, they endured a storm that blew for 12 hours and dropped nearly 23 centimeters (9 inches) of rain.

When the skies finally cleared, the scientists found themselves amid an astonishing array of native plants and animals. In this “lost” Hawaii, they encountered a menagerie of exotic birds, including the rare Maui parrotbill and a species of nectar-loving “honeycreeper,” the Maui Nukupu’u (Hemignathus affinis), previously thought to be extinct. Other strange species, too, revealed their dramatic co-evolution. The curved beak of the scarlet-colored ‘I’iwi (Drepanis coccinea), for example, is a perfect match for the trumpet-shaped flowers of the blue opelu (Lobelia grayana)—a species of lobelia found only in the rainforests of eastern Maui.

After four weeks in the field, the team had recorded 228 species, 90 percent of which were deemed native. Just two years after the expedition, in 1969—with lobbying from the likes of aviator Charles Lindbergh and philanthropist Laurance Rockefeller—Kipahulu received federal protection.

Judge’s work is a continuation of those first survey efforts. The key difference is that his main goal is not to search for undiscovered species but to take stock of what remains.

From the landing zone, we pick our way through dense stands of Ohi’a (Metrosideros polymorpha) and spiny tufts of pukiawe (Styphelia tameiameiae), or “pokey-owie,” a mnemonic Fish uses to remember the name of this prickly plant that she’s spent countless hours crawling through. We slip through mud, banging shins and knees. We slide down steep slopes overgrown with vegetation, and inch over mossy rocks and rotten logs.

After 45 minutes, we’ve traveled less than half a kilometer. Judge examines his GPS intently. “We’ve got to be getting close,” he says as he shoulders through a woody thicket. Suddenly we emerge into a clearing. The small opening is framed by a stand of Ohi’a, their tentacular limbs thrown outward. The perfectly placed trees appear to be painted upon the massive canvas of blue sky plunging to the Pacific Ocean, more than a mile below. We’ve arrived at West Camp. No more than 145 kilometers (90 miles) to the southeast, the great volcanoes of Mauna Loa, Mauna Kea, and Kohala tower above the deep azure of the sea.

Far less awe-inspiring is West Camp’s physical condition. The plastic tarps lashed to the roof are badly tattered and covered in leaf litter and dirt. Most concerning are the copious piles of rat feces that cover the floor. “Not good,” Judge says as he starts sweeping. The droppings confirm one of his biggest concerns. Invasive rats (and the Indian mongooses Herpestes javanicus introduced to eliminate them) have decimated bird populations elsewhere on the islands. Judge wasn’t sure if we’d find them up this high, but they’re here.

Over the past 120 years, as invasive predators have solidified their foothold across the archipelago, there have been at least four presumed extinctions of honeycreepers on Maui alone, says Judge. The most recent was the Poouli, or black-faced creeper (Melamprosops phaeosoma), last seen in captivity in 2004. “The Maui Akepa (Loxops ochraceus) hasn’t been seen since 1994, and the last confirmed sighting of the Maui Nukupu’u was in 1995. The Bishop’s o o (Moho bishopi), was last spotted in 1904,” he says, listing the vanished species as a sports fan might rattle off the names of retired athletes. “Those birds were pollinators in this valley, and so when you lose them, you lose a lot of ecosystem function,” he says. “That can explain the rarity of certain plants as well.”

Before dinner, Judge produces a map and quickly revisits the survey plans for the days ahead. Our plan is to walk Transect 17—one of 10 scientific baselines established by federal and state agencies since the 1980s—to count forest birds in order to estimate their population size and density. At designated listening stations along the 2.5-kilometer route, Judge will conduct 8-minute counts, listening for bird calls. He warns us not to be fooled by the seemingly modest distance. “The transect isn’t a trail,” he explains, but a line through the forest, defined only by strips of plastic flagging hung from the trees by previous research teams. “Be ready for the hardest hiking you’ve done in your life.”

We awake an hour before sunrise to clouds and mist. The humidity-laden air is startlingly cold, the sort of chill one doesn’t expect in a tropical paradise like Hawaii. We gear up and plod into the forest by headlamp. Travel is made more difficult by the fact that Judge doesn’t use a machete in these pristine sections of the upper valley. “I just can’t,” he says. “There’s too much here that is found nowhere else.” There is also a scientific rationale: Hacking out a trail increases disturbance, which, in turn, increases the possibility of aggressive invasive plants gaining a foothold.

After an hour, we arrive at our first listening post, which is marked with a wild array of multi-colored flagging. Judge drops his pack and retrieves a small notebook and pencil. He looks down at his watch and notes the time. Then he tells us to be silent before going into an almost trance-like state, listening intently for individual calls. The air remains chilly and the forest is surprisingly still. Only a few birds can be heard in the distance over the sounds of our breathing and water droplets falling from the leaves.

“It’s awfully quiet,” Judge says after the count. He enumerates the scant array of songs he heard: There were no ‘Akohekohe (Palmeria dolei), or ‘Amakahi (Chlorodrepanis virens). The birds Judge did detect were mostly natives, along with some introduced birds such as red-billed leiothrix (Leiothrix lutea) and Japanese white-eye (Zosterops japonicas) that now thrive in the valley. More disconcerting, he detected no chips or the telltale cher-wee song of the Maui parrotbill.

Ornithologists that came to Hawaii in the early 1900s found parrotbills across the island, in a mix of dry and wet forests and at high and low elevations. The birds, which have been on the endangered species list since 1967, are insectivores, using their hook-shaped beaks to shred tree bark and extract hidden grubs and other insects. But the lower elevation habitats have since been eliminated or marginalized, driving the birds farther up the mountain. Judge looks around and points emphatically at the ground on which we are standing. “Right now, this is their spot,” he says. “This is where they should be.”

Of course, lack of detection during a survey doesn’t mean that parrotbills aren’t present. Judge describes the Kiwikiu as notoriously “sneaky” and “cryptic.” Estimates based on dozens of surveys over the years paint a bleak picture, however. “We think there are fewer than 500 parrotbills left in the wild,” Judge says during the expedition. “That population level is sort of the tipping point. If it drops below that, it’s very difficult to bring a species back.”

The behavior and reproductive strategies of the parrotbills may be partly to blame. They tend to lay a single egg per season, but the heavy rainfall in the park—one of the rainiest places on the planet—often causes the nests to fail. Incubating an egg in the cold with an average of more than 1,000 centimeters (400 inches) of rain per year is no easy task. Approximately half of the pairs of parrotbills succeed in rearing a chick in any given year. “And that’s before you factor in the non-native predators,” Judge says. With the added pressure of rats and mongooses as well as climate change, this reproductive strategy may prove to be the parrotbill’s undoing.

At listening stations 2 through 5 we fail to detect a single parrotbill call. The route continues downhill and briefly the slope relents. For a short time, we are able to walk upright and unharried in a stand of gorgeous, mature Ohi’a. The trees are widely spaced and the understory is scattered with an assortment of flowers, ferns, and other native plants. Fish extends a hand to gently halt me in my tracks. She reaches down to touch a tiny plant at my feet. “Phyllostegia bracteata,” she says. Though it is endangered and found only in Maui’s wettest, most remote forests, its rough-veined leaves and square stem are familiar. Fish says the plant is commonly referred to as “mintless mint.”

Fish explains that the minty flavor of mint is derived from compounds known as carvones. While pleasant to humans, these compounds are unpalatable to grazing animals. The imperiled “mintless” mints endemic to the Kipahulu Valley, in contrast, contain none of these natural repellants. “The Kipahulu mint evolved without pressure of being eaten,” she says, “So it didn’t need a chemical defense.”

Though nearly every plant species we encounter in this section is native, the forest is far from complete, Judge notes. “There is a lot that is missing.” Take, for example, the bird-pollinated blue lobelias. Several species of these now-rare plants grow only on high volcanic mountainsides of Maui, Kauai, Oahu, and the Big Island. Before fences were erected, pigs and goats decimated the plants, which served as a key food source for various insects and birds in the local ecosystem. Today, many of the dozen species of lobelioids found in abundance during the 1967 expedition are critically endangered—none more so than blue Apelu, the species to which the sickle-like beak of the ‘I’iwi is perfectly matched.

The disappearance of these foundational plant species, in turn, may have triggered a population crash of highly adapted insects and spiders that augmented the birds’ nectar-rich diets with much-needed protein. “It’s all connected,” Judge says. “When you lose one species, it’s like losing a link in the chain, and you can have a cascading effect that results in other species disappearing.”

Preserving the Kipahulu in a time of biological siege means aggressive management. The job became vastly more complex in 2003, when the Kipahulu Valley was hit with perhaps the single most transformative event in its recent ecological history. A native moth ravaged the koa forests in the lower valley, stripping the trees bare of their leaves. The loss of canopy cover allowed more sunlight onto the valley floor, which, in turn, gave Clidemia hirta, a highly invasive plant species from Central and South America, the opening it needed to take hold and take over. Now, it’s a tough battle between the unrestrained growth of this perennial shrub (along with dozens of other invasive plants such as Kahili ginger (Hedychium gardnerianum) and Australian tree fern (Cyathea australis) and the small vegetation crew tasked with removing them.

While fences can’t stop the spread of invasive plants, they are highly effective against invasive animals. (During my two weeks in Haleakala, I was told repeatedly that without them there simply would be no park.) “The forest is rather resilient,” Judge says. “If you can keep pigs out and remove that disturbance, the forest can respond.” Haleakala’s 65-mile network of perimeter fences, which were built starting as early as 1976, are largely the product of the vision of one man, a former chief of resource management named Ron Nagata.

As luck would have it, days after my trip to the Kipahulu Reserve, I met Nagata on a backpacking trip I took to the nearby PalikU cabins. In his early 70s, Nagata is a sinewy, modest man who speaks in clipped sentences. He looks up at a high ridgeline above the cabins and notes that these barriers are only as good as their maintenance—no small task in this ragged terrain. “Just look at that up there,” Nagata says, waving a hand at the nearly vertical mountainside covered in thick vegetation. “The entire mountain is in the process of eroding away, but the vegetation is helping prop the fences up.”

Still, despite the overriding sense of ecological doom there is hope, says Mallinson, the natural resources manager, as scientists continue to stumble upon unknown species. For example, a rare species of fern, Athyrium haleakalae, was discovered in 2013, along steep cliffs in streambeds and plunge pools of East Maui. Mallinson explains that he and another plant biologist recently found a plant called ‘Ahinahina, or silversword, along a remote cliff near the Kipahulu Reserve boundary that may prove to belong to a previously unknown endemic species.

The National Park Service and other organizations are also actively planting endangered native plants and re-introducing native animals in various sections of the park. A captive breeding program undertaken by the San Diego Zoo’s Maui Bird Conservation Center, for example, has produced more than a dozen Maui parrotbills in the past 25 years. The park crew is also exploring “assisted migration”—anticipating how climate change will alter habitats and planning where to reintroduce endangered species based not on where they were in the past but where they can viably be in the future. “We’re asking ourselves, ‘If a species can’t make it here anymore, where would they be going?’ ” Mallinson says. “If they’re not going to make it fast enough because the climate is changing, we need to be a step ahead.”

Reestablishing East Maui’s native forests could be the key to bolstering its native bird populations. Though the number of Maui parrotbills is low, explains Chris Warren, an avian researcher with the Maui Forest Bird Recovery Project, it’s historically been thought that their numbers seem to be relatively stable. “The theory is that we have as many parrotbills as can fit in the areas where they exist. The push now is to rebuild more forest so the birds’ overall population can grow.” To date, Warren’s group has planted tens of thousands of seedlings in wet habitats outside the national park in order to stabilize the soil and restore the forest that blanketed Haleakala’s southern slopes prior to the past century of logging and grazing.

This effort is but one piece of a much larger collaboration between state and federal environmental agencies and various organizations. The vision—known informally as the “Haleakala Lei”—is to restore a continuous tract of forest around the volcano, providing corridors between populations that are currently isolated from one another. “If we can make connections between these core habitats, I think the parrotbill numbers could increase,” Warren says.

Back in the upper Kipahulu Valley, the terrain underfoot abruptly transforms from idyllic Ohi’a forest to wild jungle choked with vines as thick as forearms and Akala (Rubus hawaiensis) brambles covered with thousands of tiny, needle-like barbs (dozens of which remain embedded in my palms weeks after our visit). We continue along the meandering line of tattered flagging through dark forest, sometimes staring into the vegetation for minutes at a time to find the next marker, which, inevitably, is never more than a few feet away.

Our mode of locomotion transforms from upright walking to a primal grope. Tangles of ferns conceal deep holes and slick chutes plunging into rocky ravines littered with moss-covered boulders. In spite of the seemingly indomitable nature of this section of forest, Ohi’a trees here and across Hawaii are reeling from a fungus (Ceratocystis fimbriata) that causes a condition called Rapid Ohi’a Death; some trees succumb to the disease within days of being infected. Fallen trees are everywhere, some of them massive, easily 2 meters (6.5 feet) in diameter.

After another hour of negotiating overgrown terrain, we emerge in a magnificent clearing. Lacy runnels of mist run down the ramparts of a horn-shaped peak called Kuiki, which towers above our already-high vantage, piercing the clouds. Suddenly, a metallic trill—a tropical riff on the robotic tittering of R2-D2—emanates from the treetops. We look up and see a group of three or four ‘I’iwi flitting nervously from branch to branch. Their red feathers and matching scarlet beaks are luminous, almost otherworldly, against the flat, gray morning light. I raise my camera to my eye, hoping to capture a photo, but they refuse to sit still; I capture nothing but a series of iridescent red blurs. “They’re high on nectar,” Judge notes, “Like little kids hopped up on sugar.”

These birds, like Kipahulu’s other native honeycreepers and honeyeaters, rely on the red blooms of the Ohi’a trees that surround us. The specialization of Hawaii’s forest birds, says Judge, is on par with the co-adaptation Charles Darwin described in the finches of the Galápagos archipelago. But given the planet’s rapidly changing climate, such exquisite co-adaptation can become untenable, especially when paired with avian malaria, a disease that has ravaged bird populations throughout the tropics. “The earliest blooms tend to appear at lower elevations,” Judge says. And if the birds are chasing resources down into those lower elevations, they can be exposed to the malaria pathogen, he adds.

Meanwhile, avian malaria is creeping up the mountains. Like the variant that affects humans, avian malaria is limited by elevation; above a certain line, the average annual temperature is too low for the mosquitos that carry the disease to survive. But as average temperatures in the region have increased, the elevation at which mosquitos can survive and the malaria pathogen can complete its life cycle has also shifted upward.

A recent USGS study on the island of Kauai determined that the prevalence of malaria had increased more than nine-fold at an elevation of 1,350 meters (4,430 feet)—from 2 percent to 19.3 percent—between 1994 and 2013. Another 2013 study by researchers from the University of Hawaii at Manoa found that avian malaria could be spread at elevations as high as 1,900 meters (6,233 feet), prior to warming temperatures.

Here in the Kipahulu Valley, Judge explains, there is still some wiggle room, as viable forest habitat extends up to just above 2,130 meters (about 7,000 feet). But on Kauai, where the highest peak tops out at just above 1,560 meters (about 5,000 feet) the threat is far more immediate. Judge says the spread of infection in Kauai has biologists fearing the extinction of several species of honeycreepers including the ‘I’iwi (which, coincidentally, would receive federal threatened status four months after this expedition). “There is no more mountain for those birds to escape to.”

In the face of this upward march of maladies, one vibrant yellow honeycreeper, known as the ‘Amakahi, possesses a suite of adaptations that seem to have given it a leg up. Like many honeycreepers, the ‘Amakahi has a hooked beak that conceals a tube-like tongue ideally suited for siphoning nectar from various blooms. But unlike other species, Judge explains, the ‘Amakahi’s beak is versatile enough to consume fruits and other food sources as the environment changes. The linchpin of their success, though, is their tolerance for, or perhaps resistance to, the malaria pathogen. Scientists are still trying to understand the underlying mechanism. “It’s just one bird,” Judge says. “But it might tell us something about how different species adapt under these multiple pressures.”

The next morning, we rise before the sun and make the arduous trek to the final listening station, Number 17. The misty weather of the previous day has given way to full sun, but the survey yields nearly the same results. Of the natives, Judge only counts eight ‘Apapane (Himatione sanguinea) and two Maui creepers, or ‘Alauahio (Paroreomyza montana). In our two days in the backcountry, he has not heard a single Maui parrotbill.

Tired and hungry, we begin the arduous 450-meter (1,500-foot) ascent back to West Camp. Steam rises from the forest floor as the previous day’s rain has turned to fearsome humidity. Along the way, Judge retrieves a box-like contraption called a songmeter, which he had strapped to a tree several days earlier. The recording device’s sensitive microphones pick up sounds deep in the forest. Judge hopes the dozens of hours’ worth of recordings will reveal a more robust sampling of species than he gathered at the listening stations, but he does not seem overly confident. The forest, he notes, is too quiet, eerily devoid of the chatter of its old denizens.

We arrive back in camp in the early evening and watch the stars flicker into place in the vertical cut of the valley. The ghostly calls of ‘Ua’u, Hawaiian petrels (Pterodroma sandwichensis), echo from the valley walls as they glide through the darkness toward their nests atop Haleakala’s volcanic summit.

Tomorrow the helicopter will come to extract us, and Judge will begin the painstaking task of compiling and analyzing the data he’s collected. For the moment, however, we relax, resting our tired muscles and contemplating the Kipahulu Valley’s sublime beauty, which is inextricably tied to its vulnerability. Just how long, in this era of climate change, can the Kipahulu be preserved as a redoubt of biodiversity?

“That’s the big question,” Judge says. The work we’re doing in the park now will provide an immediate benefit to the birds, he says, but slowing the rate of climate change will be what benefits them long term. Judge points instinctively into the darkness, beyond the thousands of kilometers of ocean that isolates these islands, and says, “All the work we’re doing up here amounts to pretty much nothing if we can’t figure out how to change our ways out there.”