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National parks are being overrun by invasive species

National Geographic logo National Geographic 6/24/2020 Rene Ebersole
smoke coming out of the water: Hawaii Volcanoes National Park, with its vulnerable endemic species, is a hot spot in the National Park System’s battle against invasive animals. © Photograph by J.Wei, The New York Times/Redux

Hawaii Volcanoes National Park, with its vulnerable endemic species, is a hot spot in the National Park System’s battle against invasive animals.

The coquistodores hunt at night.

Wearing headlamps and muck boots, the band of volunteer conservationists trudges into dark forests in Hawaii Volcanoes National Park and surrounding communities, turning over leaves and shining lights on tree trunks. Their quarry is a tiny frog called the coqui. No bigger than a quarter, the coqui makes an ear-splitting call as loud as a lawn mower: Ko-kee! Ko-kee! It takes special know-how and fortitude to home in on a frog in a blackened forest ringing with frog calls. But the coquistodores are efficient cutthroats. When they find a coqui, they catch it, and drench it in citric acid, killing it.

a green frog: Native to Puerto Rico, the coqui is a tiny frog that has disrupted Hawaii’s ecosystems. Volunteer conservationists work with Hawaii Volcanoes National Park to track and kill the invasive animals. © Photograph by Florida Images, Alamy Stock Photo

Native to Puerto Rico, the coqui is a tiny frog that has disrupted Hawaii’s ecosystems. Volunteer conservationists work with Hawaii Volcanoes National Park to track and kill the invasive animals.

The coqui is an invasive species in Hawaii, where it has no predators and plenty of insects to eat. The small brown treefrog likely hitchhiked in plants or other goods shipped from its native Puerto Rico, where the animal is beloved, inspiring songs and poetry. Beyond their homeland, however, coquis are widely considered a persistent plague.

The Hawaiian government estimates that in areas of the islands where coquis have gotten a firm toehold, the population can be as dense as 20,000 frogs to an acre, more than double their average densities in Puerto Rico. As a result, the animals are transforming the landscape, angering homeowners, and damaging the ecosystem.

“Hawaiian forests are characterized by their quiet,” perhaps because they developed in such isolation from the rest of the world, says park ecologist David Benitez. “The coquis shatter that. They also prey on native insects in the forest, and that may be affecting the food source for endemic birds.”

a group of people walking down a dirt road: Receiving an average of 1.6 million visitors a year, Hawaii Volcanoes National Park is actively reshaped by lava flows and explosions. But volcanic activity isn’t the only force changing the landscape—invasive species also materially alter the ecosystem. © Photograph by Thomas Linkel, Laif/Redux

Receiving an average of 1.6 million visitors a year, Hawaii Volcanoes National Park is actively reshaped by lava flows and explosions. But volcanic activity isn’t the only force changing the landscape—invasive species also materially alter the ecosystem.

With a manifest of unique wildlife vulnerable to outside influences, Hawaii Volcanoes National Park is on the front lines of the invasive species battle. Nationwide, more than 6,500 foreign species have moved into the U.S., collectively causing more damage to the environment, economy, and human health than all natural disasters combined, reports the U.S. Geological Society. The coqui is among hundreds of those species that have invaded the national parks—which were created for the central purpose of protecting and showcasing America’s natural heritage.

“Half the parks have reported problems with invasive species,” says Jennifer Sieracki, the Park Service’s invasive animals program coordinator. “But we suspect the vast majority of parks are affected.”

(Related: Learn more about how invasive species work.)

Introduced rats, quagga mussels, gypsy moths, lion fish, as well as feral hogs, goats, and cats are among the more than 300 animals on the parks’ most-wanted list of species to eradicate. But managing invasive species on 85 million acres of wilderness—from Hawaii’s lava-caked calderas and Alaska’s fjords to New Mexico’s Carlsbad Caverns and Florida’s Dry Tortugas—is a monumental task.

Because the spread of invasive species often extends beyond park borders, the only way to beat them will be if the parks join forces with other agencies and communities—as well as more than 320 million annual park visitors, Sieracki says. “We need everyone to work together to deal with this issue.”

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Volcanoes ranger Benitez remembers enjoying quiet nights before the coquis arrived in his backyard, about four miles north of the park entrance. “At first I was going out and hand capturing them, like the coquistodores,” he says, “but then it got to the point where there were too many to keep up with.”

In his 20 years at Volcanoes National Park, Benitez has done battle with an army of invasive species: rats, feral cats, weeds, mysterious fungi. Islands like Hawaii are particularly vulnerable to harmful intruders, he says, because their unique flora and fauna have evolved in an extremely isolated setting, in absence of the pressures that harden mainland wildlife. Two thousand miles from the nearest landmass, Hawaii is home to roughly 8,800 endemic species, including over 500 federally endangered species—more than any other state.

(Related: Here’s how these threatened animals could bounce back.)

When an aggressive interloper reaches new land, native animals can be caught defenseless. Benitez points to snakes as a prime example. “There are no snakes native to Hawaii, though occasionally snakes are intercepted near ports of entry and on cargo,” he says “The consequences of an established snake population could be catastrophic and irreversible.”

Consider what happened on the Pacific island of Guam, Benitez says. Brown snakes exploded in numbers after arriving on Guam in the 1950s, most likely transported on military equipment. Today scientists estimate there are as many as 15,000 snakes per square mile on the 212-square-mile island. The snakes outnumber humans 10 to 1, and frequently short out electrical systems. They’ve wiped out 10 of Guam’s 12 endemic bird species, including the Guam kingfisher, last seen in 1986. The U.S. Department of Agriculture has spent millions trying to exterminate the snakes, including carpet bombing the island with dead mice laced with acetaminophen, which is toxic to brown snakes and some other animals.

Guam’s first brown snakes are believed to have stowed away in the landing gear of a cargo plane, or on a ship, a common mode of transport for invading animals. Rats are especially notorious for jumping ships and conquering islands.

“They are so ubiquitous,” Benitez says. “You find them almost anywhere there are humans. And they can cause tremendous damage, gnawing on plants, destroying seeds, spreading disease, and eating birds.”

Running free on islands, rats have devoured whole seabird colonies—even whole species. In fact, rats have caused between 40 to 60 percent of all recorded bird and reptile extinctions in the last four centuries. The threat is so serious that the state of Alaska created a “rat spill response program” to work with the U.S. Coast Guard to prevent infested ships from running aground on vulnerable islands. Much like the teams of experts trained to deploy when there’s an oil spill, rat response crews rapidly assess the dangers posed by shipwrecks and take steps to prevent the rodents from scurrying ashore, including setting bait traps on beaches.

Plague of animals

The arrival of invasive species hasn’t always been accidental. Some were brought to America by early explorers who wanted livestock for food, including cattle, sheep, goats, and hogs.

Today many of those animals’ descendants are running rampant. Feral swine are a particularly destructive and growing problem. More than 6 million feral pigs roam at least 35 states, from Florida to California and as far as Hawaii, reports the U.S. Department of Agriculture. They cost as much as $2.5 billion in damage to wilderness, properties, and farms each year.

Wild pigs don’t look much like their farm cousins. Standing up to three feet tall and weighing as much as 400 pounds, they are often black or spotted, and hairy, with long whiskers. They root up forests and streambeds, and carry brucellosis, swine fever, and a chickenpox-like illness fatal to wild coyotes and foxes. The hogs’ muddy wallows become mosquito breeding grounds, which can further spread disease, sickening people and wildlife.

In Hawaii, where mosquitoes were absent until the early 1800s, avian malaria transmitted by the insects is now driving many endemic birds—including the islands’ colorful honeycreepers—toward extinction. Infections are highest at low elevations, but global warming is magnifying the threat, making it possible for mosquitoes to survive at higher and higher elevations, killing more and more birds. “Our birds didn’t evolve with the pathogens that cause malaria,” Benitez says. “Mosquitoes may be irritating and annoying, but they’re contributing to extinctions.”

(Related: What will make us care enough to save endangered species?)

Some parks have management programs to exterminate invaders, but killing animals isn’t always popular. For example, controlling feral cats can be extremely controversial. Yet invasive feral cats, which hunt even when they’re not hungry, pose one of the biggest threats to wildlife. A study published in January 2020 examined how cats, rats, and pigs—invasive to the island of Kauai—affected the Newell’s shearwater and Hawaiian petrel, both endangered species. Roughly 50 percent of depredations were by rats; 36 percent by cats; and 10 percent by pigs. But the cats were most destructive, according to the researchers, because they killed more breeding adults than chicks, having longer lasting repercussions for the population.

On the U.S. mainland, cats kill one to four billion birds each year, according to Smithsonian scientists, as well as 12.3 billion small mammals and hundreds of millions of reptiles and amphibians. Some parks trap cats and give them to animal shelters in hopes that they can be adopted. Others trap, neuter, and release cats, but experts say such programs have not been proven to successfully reduce cat predation on wildlife. Faced with the likelihood that cats were going to wipe out endangered Hawaiian petrels nesting in lava rock burrows on the slopes of the Mauna Loa, Volcanoes National Park built a five-mile-long cat fence.

“It’s the longest predator-proof exclosure in the U.S.,” Benitez says. “And it’s working really well to directly address the threat to the colony.”

Playing defense

Recently, Benitez has witnessed a new invasion. About ten years ago, the island’s ʻŌhiʻa trees mysteriously started dying. The ʻŌhiʻa, a flowering endemic in the myrtle family producing flowers that range in color from fiery red to yellow, is an iconic Hawaiian species that can grow a hundred feet tall and live for centuries. ʻŌhiʻa are quick to colonize new lava flows and are therefore a foundation of the Hawaiian forest system. Hundreds of thousands of ʻŌhiʻa trees have died from an introduced fungus that clogs the trees’ natural plumbing, preventing water from flowing to the canopy.

“Quickly the trees turn brown and red, and then they die, which is why the illness was named Rapid ʻŌhiʻa Death,” says Benitez. “Once a tree becomes infected, there’s no cure. It’s really concerning.”

Scientists are scrambling to figure out how to halt the spread of Rapid ʻŌhiʻa Death. “The mechanism for the disease’s transport is still not well understood,” says Ryan Perroy, a geography professor at the University of Hawaii at Hilo who is helping to monitor the spread. “We think the pathogen enters through a wound, which could be created by a windstorm, people using heavy machinery, or feral ungulates, like sheep, goats, cattle, and pigs.” Perroy is using high-resolution cameras and other sensors to improve early detection of the disease and map the likely progression.

The use of such predictive mapping is helping the park service identify locations where invasive species might be expected to spread. Other useful technologies include trail cameras, employed to track animals’ movements and estimate population sizes, and fecal DNA analysis to confirm invaders’ identity and determine what they’re eating.

Still, critics say the parks have barely begun to address the problem of invasive species on any significant scale.

“Only 23 percent of national parks even have a plan for what they’re going to do about invasive species,” says Ashley Dayer, an expert in conservation and social science at Virginia Tech University who lead a 2019 report examining the national parks’ management of invasive species. “And the issue is only going to get worse, partly because of climate change expanding the ranges of invasive species, and also because of increasing park visitation, creating the opportunity for more species introductions.”

(Related: Here’s how the exotic pet trade spreads invasive species.)

Experts say one of the keys to protecting the parks from invasive species will be recruiting the public to help.

“These are America’s parks,” Dayer says. “The people are the owners, and parks should be thinking about how to engage them to be part of the solution.” Some parks enlist volunteers to weed invasive plants or work with experienced local community groups, like the coquistodores. Others hold BioBlitz events where any visitor can become a citizen scientist for a day, helping document species to create a snapshot of biodiversity.

A few parks even pay authorized hunters to locate and remove invaders. In Florida, where Burmese pythons—most likely pet snakes released into the wild—have multiplied and inundated Everglades National Park, the state hires python removal agents. These hunters search for up to 10 hours a day, earning $8.46 an hour; $50 for pythons measuring up to four feet long; and an additional $25 for every foot beyond that. Dayer says the situation with Burmese pythons in the Everglades is an important reminder that when people purchase a new pet, they should be committed to caring for it, or they should find it a new home—not release it into the wild—if they no longer want it.

As the parks reopen after pandemic closures, there are some simple actions that everyday park visitors can take to prevent the spread of harmful species.

“Before you hit the trails, we ask that you inspect your boots and gear,” says Benitez. “Make sure you’re not inadvertently carrying invasive plant seeds that can spread.” That one small gesture alone may prevent the arrival of a new species that could forever change a treasured landscape.


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