Top 10 Invasive Species That Are Only Invasive Because of Us


Every entry on this list is a menace. They may be cute, or iconic (actually, a few of them are pretty gross) but  every one of them — animal, plant, insect or fish — single-handedly warp, subvert and destroy any environment they’re introduced to. Although we can’t blame them because in nearly every case they were perfectly happy in their own natural habitat until, for whatever reasons, humans took them from one place to another. So, in a way, every one of these entries is your fault.

Just like, for example …

10. Tumbleweed


There’s no image more  likely to invoke the American West than the lonely tumbleweed blowing across an empty landscape. But the tumbleweed isn’t an American plant  — it’s actually a weed called Russian thistle from, well, Russia. It was accidentally introduced by Russian immigrants in the 1800’s, and perhaps managed to grab a stranglehold on the landscape during the dustbowl, when it became the only viable feed for hungry cattle.

Tumbleweed is actually highly damaging to the environment for various reasons. Firstly, its ability to absorb a lot of water means it’s hard for other crops to compete. That same ability means it increases soil erosion and scrapes away layers of topsoil as it roams the landscape. It also helps cause and spread fires. Think about it — tumbleweeds tend to build up against walls, ravines and fences and, as they become dry, act like tinder. One spark and they can go up like a bonfire. They also spread fires. Just imagine trying to fight a fire on a landscape as big as the great basin when tumbleweeds continually blow across the fire line, catch light and spread it like rolling torches to other undamaged areas.

There’s also another danger from tumbleweeds. WORLD DOMINATION. Okay, maybe not world domination but just look at this video demonstrating what happens when they build up in huge numbers, as they’ve tended to do lately, and blow across highways.

9. The Northern Snakehead


Just like the creature from the black lagoon, the snakehead is a monstrous fish creature covered in slime with a set of sharp, nasty teeth. When they’re young, they feed on stuff like plankton and other small watery creatures, but as they get older they can take down mammals as big as rats. Like the walking catfish, they’re also able to breathe air and can survive on dry land for almost five days. And due to the fact they’re apex predators (the top of their food chain in their natural habitat,) when they’re introduced to other environments they kill pretty much everything. When you combine these two things, you’re left with a pretty terrifying invasive species that’s been establishing a foothold across the United States.

When they first appeared in US waters, fishermen were told to kill them on sight. However, the snakefish has proved itself relatively badass by surviving and even spreading and thriving — one was recently caught in Florida that weighed 17 pounds. So authorities in Virginia and Maryland have come up with another solution: eating them. That is, of course, if you’d actually want to eat one. As mentioned, they have a protective layer of slime so they probably won’t taste very nice.

8. The Grey Squirrel


In the UK, the native red squirrel only exists in specific habitats. It can’t compete against its American cousin, who was introduced to England by Victorians who thought that it looked exotic (it should be mentioned that it looks identical to the red squirrel, except grey.) Things have gotten so bad for the red squirrel that it only exists in Scottish forests and nature reserves. Over the last hundred years or so the gray squirrel population has grown to 5 million, meanwhile there are less than 14,000 red squirrels left.

The reason that the grey squirrel has been wiping out the red so effectively are two-fold. Firstly, the grey squirrel carries a virus called squirrelpox (and even though it sounds adorable, trust us, it’s not.) Squirrelpox doesn’t really affect the grey squirrel — they simply carry it with no ill effects. The red squirrel, on the other hand, can be killed by squirrelpox (see, not adorable.) The grey squirrel is also larger and more effective at ground foraging, meaning the red squirrel loses out in competition for food.

Things have gotten so bad that widespread grey squirrel culling seems like the only option, even being touted by liberal stalwart the Guardian, who it should be noted are usually quite against killing.

7. Japanese Knotweed


Knotweed was imported to England from Japan in the 19th century purely because it looked pretty (it doesn’t really — when flowered it looks like bamboo.) At the time it was believed that knotweed would brighten up ornamental displays. Unfortunately, 19th century gardeners and botanists had no idea how formidable knotweed was, and it soon made a jailbreak and started growing just about everywhere.

Unlike most invasive species, knotweed isn’t just destructive toward native species and plants it also destroys buildings, roads, walls – basically anything made from concrete. It slowly burrows its way upwards, using its roots to pry apart stone and then grows through the cracks. Knotweed is also pretty hard to kill as it keeps its stems deep underground, grows back quickly when cut, is fairly resistant to poison, and is able to grow 10cm in as little time as one day.

Here’s a timelapse video of knotweed growing straight through a road:

6. The Lionfish


One of the biggest causes of invasive species spreading throughout the world occurs when people misguidedly release their pets back into the wild. Unfortunately, despite their best intentions, more often than not they end up releasing them back into the wrong wild — hundreds or thousands of miles away from their own habitat.

That’s what happened with the lionfish. The lionfish’s habitat is the Indo-Pacific Ocean, yet pet owners released it into the Atlantic. It’s thought that every lionfish in the Atlantic Ocean are descended from just a handful released in Florida. Those same pet owners have caused what the president of the Ocean Support Foundation referred to as “The worst environmental disaster the Atlantic will ever face.” With no natural predators, and an ample supply of food, it’s been doing all right for itself and causing some hefty ecological devastation along the way.

In case we haven’t highlighted just how damaging this one fish species can be, it’s able to kill an enormous 90% of all life in one reef. Plus, unlike most of the other animals or plants on the list, getting rid of the lionfish is not a simple case of catching or culling. There’s no way to effectively kill large numbers of lionfish in the Atlantic without killing other animals. Add to that the fact that the lionfish is hard to spot, because of its effective use of camouflage, and has eighteen venomous barbed spikes on its body.

5. The Yellow Crazy Ant


The yellow crazy ant is thought to have originated somewhere in West Africa (or possibly India or China.) But that’s only a best guess. The truth is that no one really knows where they came from originally. Their number one trait is that they spread like a virus across the world through a series of behaviors that are overly aggressive to all other ant species, quickly making them the dominant ant wherever they go and are very hard to kill. So far, they have super-colonies in various places such as India and Australia, and are deemed a pest pretty much everywhere. One of their favorite types of food is crabs. Which is why they are especially damaging to the ecology of Christmas Island, where the ecosystem is completely dependent on the various species of crabs there.

The yellow crazy ant has been systematically wiping out the Christmas Island crab by overwhelming them in their thousands, spraying them with formic acid and then devouring their bodies. Since the 90’s it’s believed that they’ve killed more than 10 million red crabs. This has caused devastation as the weeds that the Christmas Island crab has long since kept in check have began to take over the whole island.

4. The Asian Mongoose


The Asian Mongoose was introduced to Hawaii and various Caribbean islands in the 1800’s because, at the time, the islands were under siege from another invasive species: the rat. It later turned out that using an invasive species to fight another invasive species wasn’t the brightest of ideas. At the time, it was believed that the introduction of the mongoose would be beneficial to sugar cane farming, because they would kill off the rats that were slowing down production.

The mongoose did make short work of the rats, however after the rats were gone — and without any predators to keep the mongoose in check — they had nothing left to eat but the rare animals that remained. So, the mongoose made short work of them too. So far, the mongoose has driven at least seven species of rare animals to extinction in Puerto Rico and has been linked to five more in Jamaica. Ironically, the mongoose is facing extinction in its own natural environment.

3. The Walking Cat Fish


The walking cat fish was imported to Florida from Asia in the sixties. And there wouldn’t have been a problem if it weren’t for the fact that the walking cat fish can live up to its name, literally walking out of its body of water to go off in search of a new one. Okay, so it doesn’t literally walk (its got no legs, for a start.) It sort of waddles, dragging itself along by its fins. Most fish can’t survive out of water for very long, due to the fact their fins aren’t unable to extract oxygen from air (they’re specially adapted to do that from water), but the walking cat fish can. It’s this skill, and the fact that they can survive for long periods in stagnant water, that made them useful in Southeast Asia where they originated.

However, when they were imported to Florida, they subsequently walked away and have been systematically colonizing random puddles, lakes and ponds ever since. And having had time to get to grips with Florida and its lack of predators, it’s become somewhat of a menace. Don’t believe us? Just type “walking cat fish” into YouTube, and you’ll be bombarded with videos just like this one:

2. Brown Tree Snake


Invasive species can cause an ecosystem to go out of whack for multiple reasons. They often have no natural predator and cause the massive decline of lynchpin species that prevent other species from becoming too widespread. So it is with the brown tree snake on the island of Guam. It was introduced in the 1940’s and quickly began preying on the island’s native birds. In a few decades, it had eliminated ten of Guam’s native species. With no predators left to keep them in check, the island’s population of spiders exploded. This has made Guam a hellish spider-infested nightmare land that even the least arachnophobic person would think twice about visiting.

Things have gotten so bad with the current overpopulation of spiders, a new plan has been brought into effect where millions of dead, poisonous mice will be dropped on the island. Brown tree snakes are also scavengers, so it’s hoped that they’ll eat the poisoned mice and that will be the end of them. But seeing as there are 20 snakes for every acre of land, they’ll have to drop an awful lot of mice.

1. The European Catfish


The European Catfish is the second species of catfish on our list. Like the walking catfish, the European catfish is also able to breathe air for a short time, and have been using that skill to their terrifying advantage. They started out in Germany, but have somehow found their ways into rivers in Italy, Spain and France. What gives them the ability to survive and thrive is that they’ve learned to hunt on land by snatching birds from the shore. Here’s a video of one hunting pigeons on the banks of the Tarn:

The catfish have somehow learned to beach themselves momentarily, catching birds by opening their mouths and creating a vortex which sucks the pigeons in. That is adaptive behavior, the beginning of evolution, in action. The fact that the European catfish can add pigeon to the menu might not seem like such a big deal, as pigeons are essentially a pest. But the upper hand that it now has is bad news for the native species of wherever it chooses to invade next.

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1 Comment

  1. Jennifer Jordan on

    This article is in sore need of editing.

    “That same ability means it increases soil erosion and scrapes away lawyers of topsoil as it roams the landscape.”

    And the one on squirrels can’t keep gray and red straight.