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Trouble in Paradise
INVASIVES WEEK: How have invasive species managed to throw Kauai’s five-million-year-old ecosystem into jeopardy in just 200 years? Simple: They’ve worked together.

This story is a part of OnEarth's Invasive Species Week.

Aloft in a helicopter, I look down on the Alakai Wilderness Preserve, a dark, billowing blanket punctuated by smatterings of bright green. From where I’m sitting, it’s hard to see anything but a seemingly endless expanse of tree cover. This 9,000-acre forest on the Hawaiian island of Kauai is a sanctuary for native trees like olapa and ohia, which combine on nearly every visible acre to create an overhead canopy as lush and dense as the understory of ferns and mosses on the forest floor. Together, these plants absorb mist and rain, depositing both into the creeks, streams, and aquifers that form the island’s watershed and provide islanders with almost all their fresh water.

Over the radio, a voice crackles: “Are those goats?” Our pilot circles, and suddenly, out of the right side of the helicopter, I see three fat, wild goats flattening themselves against a narrow trail in fearful response to the noise of our rotor before bounding out of sight. The chopper crests Mount Kawaikini, the highest point on the island, and heads toward Mount Waialeale, one of the wettest spots on earth and a key source for Kauai’s aquifer system. Through my headphones I hear the voice of Trae Menard, coordinator of the Kauai Watershed Alliance and statewide director of forest conservation for The Nature Conservancy. “There’s our fence,” he says.

Looking down, I can see the stretch of hogwire and hogpanel running the length of a plateau next to Waialeale. Finished three years ago at a cost of $900,000, the fence is the first of four barriers that will eventually enclose more than 5,000 acres Menard and other conservationists have identified as most vital to the health of the watershed. Like all watersheds, Kauai’s provides a variety of essential ecosystem services, from recharging local freshwater supplies (through the collection and absorption of rain), to controlling erosion and providing habitat for countless forms of wildlife. But on this and other Hawaiian islands, the watershed is also the repository for many of the sacred and storytelling elements that help define a unique culture: the plants and flowers used in making leis or performing the hula, for example, or the sturdy koa wood from which canoes have been crafted for many centuries.

Fences are designed, of course, to keep things in—but they’re also meant to keep things out. In this instance, what Menard and his colleagues are trying to keep out are animals: specifically populations of feral ungulates that wreak havoc on the landscape and jeopardize the aquifer. Under the plan that the Kauai Watershed Alliance has formulated and is in charge of enforcing, any such animals discovered inside the area will be hunted or otherwise removed. Only through a vigilant program of maintenance and monitoring, Menard’s team believes, can they be kept away.

Over the last 4.5 years, 69 pigs, 47 goats, and 1 deer have been cleared from the first enclosure. Now the area is believed to be largely free of invasive interlopers—the three nervous goats we saw earlier were on the other side of the fence—but as we fly close to the enclosure’s cliffs, we see a stand of trees shaking wildly. As they bend back and forth, they reveal a young feral pig caught in a snare. The animal likely snuck around the fencing by ascending a rocky path along the ridge. Snares like this one are controversial; conservationists try to use them sparingly. But for now, Menard says, they’re the best option for securing steep and uneven ground. Eventually, he tells me, the snares will be replaced by more fencing. “The goal isn’t to kill animals,” he says. “It’s to protect the watershed.”

After the helicopter briefly lands in a valley so we can check another pig trap—a large circular enclosure with motion-sensor gates and cameras—we climb back aboard, and soon we’ve exited the boundaries of the Alakai. The thick native forest gives way to bare streaks of iron-red mud. Below us now are thickets of strawberry guava, an allelopathic tree that destroys its arboreal rivals by emitting a chemical that can keep other plants from growing nearby. Occasionally we spot an Australian tree fern, a frond-topped behemoth that’s capable of growing to 30 feet in height.

Both these plants, just like the feral ungulates I saw in the Alakai, are nonnative to Hawaii. All are major players in an ongoing ecological battle between native and invasive species that has been playing out here for hundreds of years, and that now feels as though it is reaching its climactic turning point. And at stake in this conflict are not just watersheds but also the entire balance of biodiversity on which an ecosystem, and a culture, depend.

* * *

Hawaii’s absorbent forests took about five million years to evolve, during which time only a single native land mammal—the Hawaiian hoary bat—emerged. Sometime around the fourth century, Polynesian settlers began bringing small domestic pigs and dogs over with them in their boats. Then, starting in the 18th century, the first European visitors and the settlers who followed introduced to Hawaii a larger species of pig (which eventually interbred with the pigs brought over by the Polynesians), along with cattle, sheep, and goats. Soon these ungulates were ripping with their sharp hooves at the sensitive forest floor, tearing out the ground cover of hairless nettles, thornless raspberries, and other plants that had never bothered to evolve defense mechanisms against herbivorous predators—because, up until then, they had never needed to.

Himalayan ginger

As settlement and development of the islands continued, nonnative plants joined nonnative animals to form an alliance of invasives that would, in just 200 years’ time, disrupt the balance of Hawaii’s ecosystem and put conservationists on the defensive. During the five million years that Hawaii’s forests were evolving, new species of native plants would establish themselves on the order of once every 20,000 to 90,000 years, depending on the island. But it’s taken only two centuries for half of that native forest to disappear; what remains must compete with 20,000 nonnative plant species, at least 200 of which pose an existential threat, among them, the Australian tree fern, the strawberry guava (originally from Brazil), African tulip trees and Himalayan ginger. Collectively, these invaders—many of which are beautiful to look at and contribute visually to the tropical feeling that makes Hawaii such a popular tourist destination—continue to choke out native species, overtaking ever-expanding pockets of land across all of the major Hawaiian islands.

Exacerbating the problem is the dynamic that has arisen between invasive plants and the invasive animals that eat them. Feral pigs, for example, love the sweet red fruit of the strawberry guava, which they devour in mass quantities before defecating the seeds, which are left on the ground to germinate inside their own perfect patches of fertilizer. Increasingly, the soil into which these seeds are deposited has already been denuded of native vegetation—also courtesy of pigs and other feral ungulates. “The combination of ungulates and invasive plants species—it’s really a one-two punch,” says Chipper Wichman, director and CEO of the National Tropical Botanical Garden and chair of the Kauai Watershed Alliance. “If native systems are intact, there’s a relatively high level of immunity, and it’s harder for invasive species to get in there. But if the forest is disturbed by ungulates or hurricanes, it creates openings for the invasive species to establish themselves.”

Most Hawaiians may not sense, directly, just how deleterious an impact this one-two punch has on their home or their health. But if current trend lines continue, they’ll sense it soon enough. As a result of climate change, average rainfall on the islands has decreased by 15 percent over the past 20 years. Now climate models are forecasting another 5 percent to 10 percent decrease in rainfall during the rainy season, and a 5 percent increase during the dry season. Combined with a landscape already so altered by nonnative plants and animals, this disruption of ancient rainfall rhythms will almost certainly mean more flooding and erosion.

* * *

To counter these immediate threats, some conservation groups are working on the ground to resculpt the islands’ forests—uprooting unwanted plant species and replacing them with native ones. So what are the weapons that make up an invasive-killer’s arsenal? I find out on a steamy summer morning as I accompany staff members from the Kokee Resource Conservation Program, and a group of about a dozen mostly college-aged volunteers and interns working a small patch of forest in Kokee State Park in northwestern Kauai. Several are planting seedlings of loulu, a native palm; some are planting young maile, a green vine that’s commonly used to make leis. I join a pack of others who pull up strawberry guava and kahili ginger by the fistful. We’re all wearing green nylon belts with red pockets holding machetes, squirt bottles of herbicide, pliers, and tally counters. The more lethal team members prowl the woods pulling up the smaller plants by their roots, which are then treated with a restrained squirt of herbicide. Larger specimens receive a machete hack or a deep cut near the base of the plant before getting sprayed. As we dispatch one plant after another, we click handheld counters, tallying our victims for reports to be filed later.

At stake in this conflict are not just watersheds, but the entire balance of biodiversity on which an ecosystem, and a culture, depend.

On a typical weeding day, says Katie Cassel, the program’s coordinator, workers remove around 3,100 weeds over three acres. If those figures sound small, keep in mind that one of the more nefarious evolutionary tactics these invasives have developed—one that helps define them as invasives—is their remarkable reproductive prowess. The flower-filled head of a mature ginger plant yields around 250 seeds, for example, and the fruit of a strawberry guava can contain anywhere from 25 to 70. A single Australian tree fern generates millions of spores on the underside of its fronds—then releases them to the Hawaiian trade winds, on which they can travel for more than seven miles before landing, germinating, and taking root.

At one point during the weeding excursion, an intern grows excited. “It’s an ATF!” she shouts, pointing to an Australian tree fern. “Take it out!” another intern encourages. The first intern unsheathes her machete and puts it to use. Once the tall weed has fallen, both interns slice off two curled fronds—each the size of a fist—before disposing of the unwanted plant. That evening, Cassel sautés the fronds for a dinner that also includes pork from a wild pig hunted on private land. (The ATF tastes a bit like asparagus, in case you were wondering.)

The traditions of subsistence farming and “living off of the land,” integral to Hawaiian culture, also play a role in the fight against invasive species. Many hunters sell wild pig and deer meat at roadside stands, or trade with fishermen for creamy opah (moonfish) or flaky opakapaka (pink snapper). A week after my visit to Kauai, in a quiet and pastoral “upcountry” neighborhood in Makawao on the island of Maui, I would see a group of young shirtless men standing around a 300-pound boar they had caught in a hunt. Feral pig hunts typically involve big knives and packs of dogs: The dogs take the pig down, then hunters aim their knives at the pig’s heart. Strung to the garage and bleeding out into the driveway, the boar had incisors as long as carrots.

According to geological and evolutionary scales, pigs represent a recent addition to Hawaii’s forests. But if you’re keeping time with a civilizational clock, then they’ve always been a part of island life. Because wild pigs have long played, and continue to play, such an important role in the Hawaiian diet, any calls for massively reducing their numbers or fencing in prime hunting areas in order to save the watershed can be culturally tricky. “The forest is so important to us,” one of the Makawao hunters would tell me. “So is the water. But hunting—hunting is also life.”

Tension between hunters and conservationists started in the early 1990s, when the Department of Forestry and Wildlife first began building pig-proof enclosures in the Kohala Mountains on the “Big Island” of Hawaii. Vandals cut through the fencing in protest: They worried the project would limit access to their traditional hunting grounds, which had been free and open for generations. To varying degrees, the argument continues today. In 2012, the Pele Defense Fund, which advocates for traditional culture and customs, tried to block the erection of new fencing in the Kau Forest Reserve on the Big Island by suing the Department of Land and Natural Resources, which put the department in the awkward position of juggling conflicting mandates: to protect the watershed and to promote re-creation. And on Kauai, local hunters recently pressed for a bill that would require an acre of hunting land to be added for each acre fenced off by conservationists—a plan that conservationists say simply isn’t feasible.

Billy DeCosta, an environmental educator on Kauai who also describes himself as a fourth-generation hunter, says he isn’t entirely against fences. But he does worry that fencing, once installed, won’t be properly maintained—particularly large stretches of it in wet and rugged locations like the Alakai. Left unattended, he says, the structures will simply be swallowed up by the forest. “We try to fix problems that we think we can fix,” he says, waxing philosophical. “Sometimes we do more good. And sometimes we do more harm.”

Feral pigs on Kauai, HI

The Nature Conservancy’s Trae Menard counters that the Alakai fence, at least, will remain very well kept. He and his colleagues are currently experimenting with new technology that will help them monitor the structure, including fiber-optic cables that would run the full length of the fence, and drones outfitted with cameras. He adds that the fence features built-in gates and step-overs designed to grant hunters access to the land. On top of that, he says, The Nature Conservancy plans to schedule structured hunts—organized by locals—to help them in their goal of keeping the enclosures pig-free, should any pigs (like the one I saw) manage to sneak in.

Still, as the Division of Forestry and Wildlife’s Lisa Hadway seems to acknowledge, the competing concerns of conservationists and hunters must be thoughtfully balanced. “We can’t make all places be everything for everyone,” she says. “There have to be areas set aside for conservation and watershed protection, and there have to be areas set aside for game management and hunting.”

At the end of the long day on Kauai spent pulling up invasive plants by their roots, I drive with Katie Cassel of the Kokee Resource Conservation Program across the Kuia Natural Area Reserve, on the west side of Kauai, until we come to a narrow lane called Makaha Ridge Road. Her rickety old truck rounds a corner, and there lies a massive thicket of strawberry guava where—just a few seconds earlier—there was a diverse, fecund forest. She taps on the brakes, and we glide by it slowly. It’s dark in there: The plant growth is so dense, it’s impossible to see through it for more than a foot or two. Nothing else can grow on this particular plot of land, where erosion has made the soil smooth, flat, and hard-packed. It’s easy to see how rainfall here would simply roll off the ground’s surface, sluice its way to the nearest inland waterway, and quickly wash out to the ocean.

Cassel stops the truck and we get out to take a walk—but we cross over to the opposite side of the road, away from the guava thicket. Over here, the forest has yet to be overtaken completely. Even so, Cassel is able to identify a mix of native and nonnative plants. She singles out some blackberry plants, the vines of which have sharp thorns, and a series of flowering shrubs.

We head back to the truck. Before we get back inside it, as we stand together on a narrow strip of road dividing a thoroughly conquered acre of forest from one that could soon be fighting for its native life, I’m made aware of just how thin the line is between these two states of being. “If we stop our efforts,” Cassel says, “then this”— she points across the street, to the occupied territories—“is the future."

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image of Brooke Borel
Brooke Borel is a science writer and journalist. She is a contributing editor at Popular Science, and also writes for Slate, Modern Farmer, and other publications. Her first book, Infested: How the Bed Bug Infiltrated Our Bedrooms and Took Over the World, will be published in May 2015. MORE STORIES ➔
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Why are they using herbicides? Doesn't this create problems as well? What is in the herbicides being used?
Herbicides work well and do not travel far when used properly (unlike many of the big farmers) and usually are a necessity to ensure that large trees and ferns do not simply resprout after being cut down. The best method would be to pull the invasive plants out (not requiring herb licenses and special storage or equipment) but with tree roots that reach two feet or more into the soil, coupled with the infrequency many of these places are visited, many trees end up being too big to pull by the time they are found.