San Nicolas Island fox
The San Nicolas Island fox qualified for downlisting to a lower category of extinction risk after invasive mammal eradication from islands in the Channel Island Archipelago. Rory Stansbury/Island Conservation
Damage to booby egg from invasive rat on Isla de la Plata, Ecuador

Invasive rats commonly raid bird nests

Damage to booby egg from invasive rat on Isla de la Plata, Ecuador.

Smack in the middle of the South Atlantic, situated between the tips of South America and Africa, lies tiny Gough Island. This 25-square-mile hunk of rock is home to roughly 10 million seabirds representing 20 different species, and is the only known breeding spot for some. But something bloody is afoot. Invasive house mice, emboldened by an absence of natural predators and plentiful food sources, have turned into killers. Feverishly nibbling and gnawing in groups, the mice are eating nesting bird chicks, such as the Tristan albatross, which even as a nestling weighs 300 times more than any house mouse. The chicks evolved in the absence of predators and as a result, are consumed alive over the course of days while they sit defenseless in their nests. Google “Gough Island mice” and you will see heinous images of downy chicks being feasted upon.

All over the globe, vulnerable ecosystems like Gough Island are in a fight against invaders. Non-native plants and animals that have somehow entered these environments tip the natural balance and wreak havoc on native species, causing extinctions of local flora and fauna and putting human health and economies at risk. Invasive species in the United States inflict billions of dollars in damages, annually. And although all ecosystems suffer greatly from invasives, islands are particularly vulnerable. Many of them are home to species that exist only on those islands.

For these embattled environments, a tactic of eradication–killing all of the invasives—has proven to be the most effective course of action. Some people view this negatively. Killing potentially large numbers of animals seems counterintuitive to conservation. But more and more evidence has shown that removal of invasive species from threatened ecosystems is not only effective at restoring endangered habitats and species, but necessary.

Invasive species

Asian Longhorned Beetle

Asian Longhorned Beetle

This invasive beetle native to Eastern Asia was accidentally introduced into the Eastern United States and Canada by hitching rides in solid wood packaging material. It bores into hardwood trees to lay eggs where its larvae then hatch and kill the tree from the inside out. 13 different species of hardwood trees are victims of this incredibly destructive invader.

Any organism can conceivably become an invasive species, if it finds its way into an ecosystem where it doesn’t belong. But there are some characteristics that make some species more harmful than others. If it spreads aggressively, grows and reproduces quickly, and feasts mercilessly, it will make a particularly deadly invader. Given this loose set of traits, invasives come in all shapes and sizes, from the kudzu vine that’s choking out the Southern U.S., to Asian carp filling up the Mississippi, pythons smothering the Florida Everglades.

On islands, where invasive species sow serious destruction, mammals tend to be some of the worst offenders. Islands make up just 5.5 percent of Earth’s land area, yet house 15 percent of all its terrestrial species. They are hotspots of biodiversity, with many plants and animals that exist nowhere except on their respective islands.

Mammals are not native to oceanic islands. “Birds took over the ecological niches that we’re more used to here on the mainland,” says Nick Holmes, director of science at Island Conservation, a non-profit organization that specializes in the removal of invasives from islands. “Once invasive mammals were introduced to those islands, accidentally or on purpose, they would turn around and quickly dominate.” As a result, 61 percent of the global extinctions since 1500 have occurred on islands. Largely as a result of invasive mammals who prey upon and compete with native fauna for habitat and food.

Brown rats

Brown rats (Rattus norvegicus)

Rats are one of the most devastating invasive species to island ecosystems.

The most common invasive mammals include livestock like goats and pigs, but if you had to pick the worst of the bunch it would probably be rodents and cats. (If you’re not counting humans.) Conservation biologist Holly Jones of Northern Illinois University says “Rats are completely widespread. They’ve been introduced to over 90 percent of the world’s archipelagoes.” She adds that “they’re also bad because they’re omnivores; they eat everything.” Jones is the lead author of a comprehensive paper detailing the restorative properties of invasive mammal eradications that was published recently in the Proceedings of the National Academy of Sciences. Holmes was also a coauthor of the paper.

Cats are not as widespread as rats, but are more adept at totally annihilating native populations. Jones remarks that cats are known as “super predators” in the island restoration world, because “They don’t only hunt what they need to eat. They hunt for fun.” Picture one particularly devilish cat sitting next to a mound of dead seabirds and you’re not far off the mark.

Eradication as a tool

21.5 billion dollars is spent annually on biodiversity conservation, yet, in many locales, it can be hard to see the impact. With invasive species eradication, however, the positive effect on native flora and fauna restoration is apparent. Jones’s paper on the matter, which compiled info from 30 other scientists around the world, found that 236 different native island species have benefitted from eradication on some 181 islands. In the world of conservation, it’s rare to find a strategy that very clearly works so well. Especially one as clear-cut and defined as eradication. “It isn’t simple—don’t get me wrong—but it’s relatively simple compared to other conservation interventions,” says Jones.

As obvious as the end goal of eradication is, carrying it out can still be a complex challenge. General rules of thumb are followed to some extent: hunting and trapping is used for large vertebrates like goats, and typically rodenticide is used for rats and mice, explains Holmes. But at the end of the day, each one is different.

Conservation biologist Peter Haverson, who has taken part in numerous successful bird eradications worldwide, reflects that “each one [eradication] is island-specific, country-specific. Sometimes traps will work on one island, yet an island 30 miles away with exactly the same species might have individuals that aren’t interested in that type of trap.” Haverson recently completed removing the Indian red-whiskered bulbul from an island in the Seychelles after its competitive presence put the native bulbul at risk. They used nets to trap most of the birds on the island and rifles to get the last remaining few. “It’s always the last two that are the most important,” says Haverson.

Seychelles magpie robin

Seychelles magpie robin

Seychelles magpie robins moved to a lower extinction risk category following conservation efforts including reintroduction to five islands following a mammal removal.

In the Galapagos islands, conservation rangers carried out the largest goat eradication ever, removing 55,000 goats from one island alone. The feral goats had spread across the islands like a wildfire, bulldozing forests down to grassy plains and endangering native birds, invertebrates, plants, and the famed Galapagos tortoises. The rangers hunted the goats from the ground and the air.

With the goats gone, the archipelago’s forests underwent an astonishing recovery. Without removing the feral goats, where would the Galapagos Islands be? Would we be willing to wave goodbye to the iconic tortoises in order to protect invasive goats? These are the questions that conservationists ask themselves when considering an eradication. The answer is usually quite plain.

Eradication on islands vs. mainlands

Anacapa Island, CA

Anacapa Island, CA

Anacapa Island is the site of a successful rat eradication that has benefitted local fauna enormously. Despite the challenges involved in eradication, it is much simpler to carry out on islands than mainlands, where invasives can travel vast distances.

Most, if not all, eradications of invasive creatures have occurred on islands. There is a reason for this. Islands are, quite obviously, isolated areas—they have the natural barrier of water that can keep living things out once they’re removed. Therefore, once an injurious critter is plucked off of an island environment, as long as proper biosecurity measures are in place, you can prevent reinvasion. This is not the case on mainlands, where everything is connected, and invasives can generally move freely about the land (some mainland environments like mountains are virtual islands and can be treated with the same ecological concepts). “It’s not ‘if,’ but ‘when’ they’re going to get to a place,” says Jones, and as such, eradications are exceedingly rare on mainlands. “It’s more about constant control.”

Haverson acknowledges that “You can eradicate anything you want to,” but “it all comes down to time and money, because you can put millions of pounds into something and hundreds of people and you will get there–but can you justify that amount of money?” Maybe you can. Invasives don’t just damage native species, but also present a threat to human health, agriculture and infrastructure, dishing out billions of dollars in damages per year as they spread disease, chew up crops and the like.

Burmese pythons
Burmese pythons can grow upwards of 20 feet in length; 18-footers have been captured in Florida. Once they reach 8 feet, they become a top predator. Liz Barraco

Although total eradication may be a pipe dream on mainlands, the way of dealing with invasives is the same: get as many of them out of the environment as you can. Using that guiding principle, people have come up with creative ways of doing so. In the Florida Everglades, for example, where Burmese pythons have taken command of the swamps, they’ve started hosting python hunting contests with cash prizes for the winners.

In New York City, the legendary Explorers Club served up invasive species at its annual dinner this year. Diners could eat Asian carp sashimi, lionfish filets, and even get a cut of boar. Why deplete native stocks of fish when the Mississippi River is practically overflowing with perfectly edible Asian carp, a fish that we don’t want in the river to begin with?

And in Australia, researchers are even trying to force natural selection’s hand by putting hundreds of small endangered endemic marsupials in a penned in area with a couple of feral cats. The hope is that in these evolutionary hyper chambers, the cats will pick off the “dumber” individuals, and allow the “smarter” ones who learned to evade the cats to breed. Their offspring might be fearful of and able to run and hide from cats. If they can’t get rid of all the feral cats in Australia, the researchers posit, then maybe they can teach the local fauna of fend for themselves. It’s a lofty goal, however, and one that could take 100 years or more—and that’s with natural selection running on hyperdrive.

Is it right?

Saved from hungry rats

Saved from the rats

The Scripps’s murrelet population on Anacapa Island, Channel Islands, California had a 3-fold increase in hatching success and a 14% increase in the number of nests per year following the removal of invasive rats.

Not surprisingly, any endeavor that involves the killing of animals, and in potentially large numbers, is going to draw some criticism and controversy, no matter how good the intentions of it are. Most critics point to the ethics of the matter. Killing animals whether they are invasive or not is wrong, they argue, and uncompassionate. Killing wildlife for conservation (which even has the word ‘conserve’ built into it) seems counterintuitive. Isn’t conservation supposed to be about conserving wildlife?

These are important questions, and well intentioned for sure, but unfortunately none of these criticisms solve the problem of invasive species destroying native ecosystems and driving endemic flora and fauna into extinction. “The act of not doing anything to prevent these extinctions is, in and of itself, an action—which is not compassionate to native species,” says Jones. “We can sit there and watch animals go extinct, or we can do something about it.”

It’s also worth noting that many (but not all) critics of invasive species eradication heavily disparage removals of mammals, birds and other large vertebrates, but are noticeably silent when the discussion moves toward eradicating invasive insects or plants from an environment. They wind up doing what they condemn conservationists of, which is putting more value on one form of life over another.

The reality of the situation is that there are many environments in the world that contain native species that exist only in that one ecosystem and no place else on Earth, whereas invasive species are often very numerous in other parts of the planet. “We can lose one species forever, or we can get rid of a species that’s doing fine elsewhere,” says Jones.

San Nicolas Island fox

San Nicolas Island Fox, Channel Islands, CA

The San Nicolas Island fox qualified for downlisting to a lower category of extinction risk after invasive mammal eradication from islands in the Channel Island Archipelago.

Instead of killing animals, the other logical option to rid an ecosystem of invaders would seem to be live trapping. It sounds like a logical alternative, and conservationists have trapped and transported other animals before for restoration purposes. In an ideal world, yes, this would be preferred, however, in an ideal world invasive species also wouldn’t be a problem. The issue with live trapping is that it actually doesn’t solve the issue at hand, it just ships the problem someplace else. Once trapped, where will you put that animal? Its native environment could be a world away, meaning significant sums of money would have to be dropped to get it there. And it certainly can’t go into any other habitat, or you’re literally just unleashing an invasive into a fresh environment to terrorize. Do you put it in captivity? You are now responsible for taking care of it. Throw a couple more wads of cash onto the pile. Finally, what if the number of individuals that need to be removed numbers in the thousands, or tens of thousands, or even more? At this point, the cost of the whole operation would probably be more than some countries’ entire GDP. No government would fund such an intervention.

In many ways, you can argue that humans are an invasive species, too. From Africa, we’ve spread out onto every continent on Earth settling into jungles, plains, forests, deserts, mountains and more. All environments we touch experience extinctions and suffer from varying degrees of degradation. Many scientists even believe we are currently causing a mass extinction event of global wildlife, like the one that ultimately claimed the dinosaurs.

But although Homo sapiens might be lord of the invasives—”We’re always going to be the worst ones,” remarks Haverson—we do have the ability to fix our damages. Which is why many conservationists believe we have an obligation to right the wrong when it comes to invasives since humans are, more often than not, solely responsible for introducing species into places they shouldn’t be, whether intentionally or unintentionally.

No other invasive species has this capability, unfortunately. “Killing things sucks,” says Jones “But when you realize the gravity of not acting, which in many cases equates to watching extinction happen in front of your eyes, I think there is no other choice.”