Consider the case of Thomas Austin.

Upon resettling in Australia from England in the late 1850s, Austin found himself missing one of his favorite leisure time activities back home: rabbit hunting.

So, to settle this bout of homesickness, he wrote to his brother back in England and asked him to send along 12 pairs of rabbits. In doing so, he reasoned, “the introduction of a few rabbits could do little harm and might provide a touch of home, in addition to a spot of hunting.”

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As if things would ever be that simple regarding an animal notorious for the speed at which it reproduces. A decade later, two million rabbits could be removed from Australia on a yearly basis across the country with little to no effect on the overall population.

Yes, they were cute. And yes, they were everywhere, eating all the plant life in sight and lending a paw in topsoil erosion of a massive scale.

The European rabbit population in Australia is just one notable example of an invasive species. Simply put, it’s a non-native organism that gets brought into a new environment and, because a series of factors stacked in its favor, thrives to the detriment of the native species that already inhabit that environment.

In America, it’s how we’ve gotten kudzu vines covering up miles of the countryside and gigantic Asian carp devastating aquatic ecosystems and leaping out of the water to sock unsuspecting boaters upside the head.

In the great majority of cases, we humans are the ones causing these ecological disasters, either intentionally or unintentionally. But the good news is we can also do our part to slow the pace of invasive species or turn back the clock on habitats that have already been damaged.

“I don’t ever like to see somebody give up and say there’s nothing we can do,” says Karan Rawlins, invasive species coordinator at the Center for Invasive Species and Ecosystem Health at the University of Georgia. “There’s always something we can do.”

Intentional Introductions, Unintentional Consequences

The rabbits, kudzu, and carp all fall under the umbrella of “intentional introductions.” These are species that humans deliberately brought into a new habitat for a specific purpose. The kudzu was meant to help control soil erosion. The carp were brought over for aquaculture, for controlled breeding and harvesting as food.

The problem, though, is it’s hard to keep these populations under control. That’s how you get kudzu soaking up every sunray in the countryside and carp eating so much that they’re decimating the food chain in the Mississippi River—with the fear that they’ll someday find their way into the Great Lakes.

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The problem is so severe, according to Bruce Stein, the National Wildlife Federation’s chief scientist and associate vice president, that invasive species are the second-leading threat to native species in the United States, behind only habitat loss and degradation.

“There is a lot of effort that goes into the development of risk assessment techniques to try and more accurately predict which species might pose a threat and which would not,” Stein says. “Not all non-native species become invasive and destructive. In general, what you can say is the species that have become very invasive and successful are those that have escaped their natural limits on population growth.”

In this way, intentionally introduced species tend to have unforeseen consequences.

The cane toad has been brought to multiple countries to help planters deal with insects devouring their crops, only for the toads to reproduce at a prodigious pace, eat everything in sight, and secrete toxins from their skin that kill other animals, including pets. Speaking of pets, the Burmese python first came to South Florida in the possession of exotic animal enthusiasts, only to be released into the wild and start consuming the vast majority of the small-mammal population in Everglades National Park.

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Japanese chestnuts first started shipping over to the U.S. in the early 20th century to diversify and beautify landscapes, only to bring with them a blight that wiped out large swaths of the American chestnut population.

Rawlins says pathogens are a special concern with plants that are brought into the country. You never know what’s in the soil that comes with them.

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“Those plants, even when they bring in a disease with them, through evolution, are already resistant to that, have a natural immunity to it. Our native plants are not and are quite often killed instead of just knocked back,” Rawlins says. “This is a problem that all of us had a hand in creating. We’ve brought these things in because they were beautiful, useful, and because we loved plants. That’s how they got here.”

Except when it’s not.

Unticketed Passengers

Imagine, for a moment, that you’re a bird, a Laysan albatross. You’ve nested on Midway Atoll in the North Pacific Ocean for generations. You’ve never had to worry about terrestrial predators. Maybe you saw a spider or beetle every now and then, but they never bothered you.

One day, though, little furry things started eating your eggs and your chicks. Then, when that didn’t sate their hunger, they started moving on to you. If you’re evolutionarily hard-wired to protect your nest at all costs and you’ve never encountered a voracious mouse before, what would your response be if one started nibbling on you?

The answer, unfortunately, seems to be “keep incubating those eggs.”

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“They’re so predator naïve,” says Holly Jones, an assistant professor of biological sciences at Northern Illinois University. “They just don’t do anything. They just sit there and let the mice eat them. It’s pretty gruesome. This is the world’s biggest breeding population of albatross, which are already not doing well.”

Nobody intentionally brought the mice to Midway Atoll. And, with the nearest population center, Hawaii, about 1,300 miles of water away, it’s not like the mice swam there.

They probably either came as stowaways on ships or swam to the island from nearby shipwrecks. It’s a common source of unintentional rodent introductions into island habitats. Jones says invasive mammals have been the cause of two-thirds of the extinctions on islands in the past 400 years.

Remember the Stephens Island wren?

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John Gerrard Keulemans (via Wikimedia Commons)

“The thing that makes islands so vulnerable to different invasive animals is that most islands don’t have ground mammals on them at all,” Jones says. “They’re these relatively super simplistic food webs. When you get a new predator in this environment, that species have not evolved with, they just have no other behavioral or other mechanisms to protect themselves.”

Islands are far from the only destination for unintentional introductions. The state of international commerce is quite adept at spreading species around the world.

The Asian tiger mosquito breeds in pools of stagnant water that build up in tires tagged for shipping to new countries. With it comes debilitating diseases for humans, including dengue, West Nile, and Japanese encephalitis. The Asian longhorned beetle both feasts on—and nests in—wood, which is how it gets a lift around the world on wood packing crates.

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You can see why that’s a problem, right? Conservationists estimate that it threatens 30 to 35 percent of the trees in the developed areas of the eastern U.S.

“The rate of human-mediated transport has so drastically tipped the balance,” Stein says. “In essence, the rate of moving species around the globe due to improved travel, increased global trade is leading to a homogenization of our biota. Not just us, but worldwide.”

How to Stop the Invasions

Stein says there’s a three-tiered method for dealing with invasive species. The most ideal course of action is prevention: Keep them out. Failing that, the focus should be on early detection and rapid response. Failing that, it’s time for long-term control and management.

Most of the species in the U.S., unfortunately, are in stage three. Stein says the U.S. has a “blacklist” of species that aren’t allowed to be transported into the country because they’re already a problem. He advocates for an approach that’s more like the one taken in New Zealand: a “white list” approach that requires people to document a low risk of invasion if they want to bring in a species.

If you can’t, then your pet ferret is not getting in.

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“Only after great documentation and effort do we put things on the list of injurious species and ban their import. That’s kind of like closing the proverbial barn door after the horse is out,” Stein says. “Those are things that are already here, damaging and invasive. Making that change would be one of the biggest things, and just in general improving the risk assessment as part of our efforts.”

There are even steps governments can take in the prevention sphere for unintentional introductions, such as regulations on disposal of ballast water—which empty ships take on at their home port and unload at their destination…along with whatever happened to be in it—or more stringent inspections of shipping materials. Hawaiian officials, seeing the devastation the brown tree snake has caused on Guam, are extremely vigilant about checking the cargo holds and wheel wells of planes coming from the island.

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New Zealand, for its part, has declared an all-out war on rodents. The Kiwi government hopes to completely clear the country of rats, possums, and stoats—all of which were intentionally introduced—by 2050.

We can take steps on a personal level, as well. If you’re an avid angler, make sure your boat is clean before going to a new lake, and don’t dump non-native bait into the ecosystem. Check the treads on your tires if you’ve been off-roading. Clean the grooves in your boots before and after you go on a hike.

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If you’re walking along a trail and you see an abundance of leaves that aren’t dotted by little insect bites, it’s probably because that’s an invasive species for which native insects haven’t evolved a taste. Use your smartphone and report that to the EDDMapS system. Rawlins says her organization has more than 50 free apps for people across the country to collect and submit occurrences of invasive species into national and regional databases.

“Pick up the challenge and help with that. It’s quick and easy to do,” Rawlins said. “You can do that whether you’re fishing, hunting, hiking, camping, walking around the neighborhood, visiting your city or county park, anywhere.”

Never a Lost Cause

For more than 100 years, scientists thought the New Zealand storm petrel was extinct. But a funny thing happened after the eradication of cats and rats from Little Barrier Island, which is about 50 miles northeast of the urban center of Auckland.

The petrels reappeared. A small colony had been maintaining itself on steep cliffs, away from the predators. Jones says seabird populations tend to show remarkable resiliency once their agents of endangerment have been removed.

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“There’s this huge change on these islands in a really short time,” Jones says. “If you think about never having evolved with a predator, then getting hammered by it, then that predator is gone, the ecological impact is really big.”

In the plant world, Rawlins says the Georgia Forestry Commission recently has had great success spraying for privet—an intentionally introduced hedge plant—in the winter, when it’s still growing but native species are resting and not affected by the chemicals. Privet is a greater concern in Georgia than kudzu, Rawlins says, because it is 17 times more prevalent and grows into the canopy of forests rather than just in the sunny areas.

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The point is, even if beating back an invasive species and reclaiming a habitat seems like an insurmountable task, the fight is never really over.

“I would never want to look at an area and say, ‘That’s just lost. There’s no point in doing that,’” Rawlins says. “That’s an area you would look at it and go, ‘Oh my goodness, how are we ever going to do anything with that?’ But they found a way.”

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