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Introduced Species Calamity

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Spock: Surely you must have realized what would happen if you removed the tribbles from their predator-filled environment into an environment where their natural multiplicative proclivities would have no restraining factors.
Cyrano: Well, of cour— what did you say?
Spock: By removing the tribbles from their natural environment, you have, so to speak, removed the cork from the bottle and allowed the genie to escape.
Star Trek: The Original Series, "The Trouble with Tribbles"

Natural ecosystems exhibit a very distinct balance of populations: predators, competing species, disease and other natural causes all play their part in ensuring that animals don't end up breeding to the point of overwhelming the local environment, especially in the case of Explosive Breeders. True, it's not unknown for animals to migrate to areas without the environmental elements that keep them suppressed and become an ecological threat to their new habitat, but this usually only happens under very special circumstances...

Unless pesky human beings get involved, of course.

In this case, humans (or some other form of intelligent life) introduce a new species to an environment without the usual elements that keep it in check. The reasons for this vary: this can be a deliberate attempt to alter the environment in their favour, an ill-advised effort to bring pets and livestock to a new land, an effort to create a new species, or it can have occurred entirely by accident - a spore stuck in the treads of a shoe, for example, or an animal stowing away aboard a ship.


With no predators culling their numbers before they can reproduce and no competitors fighting them for territory, the introduced species will quickly breed out of control, devastating the local environment, wreaking havoc on native species, and in some cases, even damaging human concerns as well.

In the real world, animals of this nature are known as invasive species: often notoriously difficult to manage, they are Very, Very Bad for all parties concerned, and the introduced varieties commonly remain a problem for generations after they were first imported.

Compare Fantastic Vermin, which may result from this; compare Outside-Context Problem and Normal Fish in a Tiny Pond. See also Alien Kudzu, which explores botanical examples of this trope.



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    Comic Books 
  • The DCU:
    • Superman: Among Lex Luthor's many reasons for hating Superman is that he views Superman as an invasive species that, if left unchecked, will eventually supplant normal humans. Depending on the author, this perspective is either utterly unfounded, technically accurate in the case of Kryptonian villains with Hostile Terraforming in mind, or bang on the money.
    • Wonder Woman Vol 1: The Rykorn manages to overlap with Alien Kudzu as their rapidly growing birthing stalks are of questionable sentience and the Rykorn themselves are Plant Aliens with a Hive Mind and connected to their stalks. In the course of a single night they overtake acres of land and require military and superpowered intervention in order to destroy the stalks before they overtake Texas and beyond.

    Fan Works 
  • Triptych Continuum: Played with in Goosed!: after spending a millennium trapped in the Empire, Crystal Geese are reintroduced to Equestria — which doesn't know how to deal with them any more, especially as all of the ones which were trapped outside went "suspiciously extinct". Fluttershy quickly figures out that the geese could do more damage through returning, because the local ecosystem has adapted to go on without them. However, once the hostile migration stops fouling Ponyville, it starts heading towards Protocera and its griffon omnivore citizenry — implying the species may be about to go extinct again.

  • In Godzilla: King of the Monsters (2019), King Ghidorah is at first thought to be a rival Alpha Predator to Godzilla. But upon further researching legends and myths, Monarch soon learns that Ghidorah is not from Earth, but is actually an invasive entity from another planet. After Godzilla is nearly killed by the Oxygen Destroyer, Ghidorah takes over as Earth’s Alpha and commands the Titans across the Earth to awaken and wreak havoc upon humanity.
  • Wreck-It Ralph: When Ralph accidentally brings a cy-bug from the war game Hero's Duty to the much friendlier racing game Sugar Rush, the cy-bug, with plentiful food and no-one to control its numbers, is quick to reproduce and infest the game.

  • Artemis Fowl: In Artemis Fowl: The Time Paradox, the introduction of rainbow trout into Twin Lakes, and the resulting extinction of the native yellowfin cutthroat trout, is used as a minor plot point. A flash freeze leaves a small number of yellowfin cutthroats frozen inside a block of ice, which is later acquired by the Extinctionists, a club for people with a perverse obsession with killing endangered animals, so they can serve the yellowfins to the guests at a banquet they're hosting in a display of Conspicuous Consumption. The Extinctionists consider this a tragedy not because of the extinction itself, but because the fish has a unique flavor which no-one will ever be able to enjoy again.
  • The Day of the Triffids: Triffids were created by Soviet Superscience for their superior vegetable oil which can be used as an alternate fuel source, but an attempt to steal their seeds and sell them in the West goes wrong and leads to the seeds being scattered around the world when the airplane carrying them is shot down over the ocean. Even then mankind is able to keep control of the situation by hunting down wild triffids and confining them in farms, until social collapse from mass blindness allows them to break out. With humanity no longer the apex predator keeping them in check, triffids soon reproduce out of control.
  • Invasive Species: A scientist discusses the possibility of using introduced species against an alien race called the Hauch'k, reasoning that certain Earthly plants and animals could irreversibly damage their fluorine-based atmosphere. Unfortunately, the Department of Defence turns him down, believing that they'll be able to win the ten-year-long war through conventional methods. The ending implies that the Hauch'k have successfully embarked on a similar program, for the air outside is starting to smell of fluorine...
  • Jurassic Park eventually reveals that several smaller dinosaurs have already managed to escape from the park thanks to the incompetent management, some even making their way to the mainland. With the predators that would normally keep them under control contained back on Isla Nublar, they flourish as pests and opportunistic predators, eventually gaining attention when a Procompsognathus swarm kills a baby. The novel ends with the reveal that several Velociraptor have managed to escape the napalm-bombing of Isla Nublar and are now thriving in Costa Rica, overcoming their implanted lysine deficiency by stealing from local farms.
  • Killer Species: The main antagonist of the series is a scientist who's genetically engineered new predatory species specifically to hunt down and destroy real-life cases of this trope, such as pythons and other large snakes in the Florida Everglades and lionfish in the coral reefs off the Florida coastline. Local authorities agree that he means well, but regard his creations as an even bigger example of the trope and call for their extermination, earning his wrath and his deciding that humans themselves are an invasive species that needs to be eliminated.
  • Perdido Street Station is based around one of these. Slake Moths are a near-invincible mind-devouring predator species with the ability to mesmerize prey with their wings, but are normally kept in check by the many hazards of their native environment — a Death World heavily polluted by the Torque. However, five larvae are brought to the city of New Crobuzon for use in producing Dreamshit for a crime syndicate... and when they get loose, the same traits that allow the Moths to merely survive their native environment make them damn near unstoppable in the city. With no competitors to stop them and no chance for the heroes to bring in competitors from their homeland without causing an even bigger disaster, the Moths are free to feast at will; worse still, they're smart enough to pick locks in pursuit of their prey. After seeing the bodies of their victims piling up over the next few days, both the heroes and the government are forced to take insane risks just to kill or capture the Slake-Moths before they can reproduce.
  • The Rolling Stones (1952) features a species known as Flat Cats. Essentially a Ridiculously Cute Critter, one is brought aboard a spaceship as a pet... only for problems to arise when the Flat Cat starts reproducing, aided by the lack of predators and environmental threats.
  • Star Wars Legends: The Republic hosts a strong exotic animal trade, which markets alien animals from throughout the galaxy to the core worlds' idle rich as pets and decorative animals. As is the case with this trade's real-life equivalents, these animals are frequently mismanaged and escape captivity, sometimes establishing stable populations on alien worlds. Coruscant in particular has served as the crux of this trade for millennia, and the deep bowels of its global urban sprawl are home to considerable populations of feral alien animals. This is often a considerable problem, as many of these creatures are dangerous predators and are the last sort of things anyone would want to be wandering around the neighborhood or nesting near important infrastructure.
  • Tuf Voyaging: The Ark was designed by the ancient Terran Federation to perform this using cell samples from a thousand worlds and cloning tanks with temporal acceleration. In one notable story Tuf is hired to produce gladiatorial animals for a feudal world and throws in some "harmless" animals for their beasts to prey upon, only for the rapid-breeding critters to ravage his clients' farmland (he despises cruelty to animals you know.)
  • The War Against the Chtorr: An unknown alien race infests the Earth with off-planet life forms to wipe out most of humanity and replace Earth's entire ecosystem with their own.
  • Worldwar: In the sequel series Colonization, the Race's colony fleet brings along animals from their homeworld. As they're adapted to their homeworld's arid environment, they devastate Earth flora in the regions controlled by the Empire and all the remaining human not-empires can do is shoot any that cross the borders.

    Live-Action TV 
  • Alien Empire discusses real world cases of this in detail, most prominently cockroaches. However, the episode "Replicators" discusses a hypothetical example just to show what happens when humans unwittingly introduce insects to perfect breeding environments: here, a fisherman decides not to get rid of the maggots he's been using for bait, and keeps them in his fridge while he goes on holiday, trusting that the cold will keep them under control. However, a fuse blows while he's out. As a result, the fridge — with plentiful food, no predators and no competitors — becomes the perfect environment for the maggots to metamorphose into bluebottle flies and start breeding. By the time the fisherman gets back, the fridge is now home to thousands of flies, which go on to infest the rest of the building as soon as the guy makes the mistake of opening the door.
  • The Goodies: In "Scatty Safari", the Goodies have started a celebrity zoo and are in search of a new star attraction. They eventually find it in the form of the last remaining Rolf Harris in Australia... and then pair it up with another Rolf kept in a Siberian zoo, resulting in the breeding pair and the result cub becoming their new attraction. Unfortunately, all three escape from the zoo, and due to their ability to both move and breed like greased lightning, they quickly overrun their new environment — resulting in a Pied Piper of Hamelin conclusion to the story.
  • Leverage: Invoked. The team destroys a mark's business by dumping an invasive species of clam into the waterworks of the mark's dam, forcing the dam to shut down operation to get rid of them before they have a chance to reproduce.
  • Star Trek: The Original Series: In "The Trouble with Tribbles," the Enterprise first makes the acquaintance of the eponymous creatures, initially believing them harmless pets. Unfortunately, Tribbles are a prey species on their native homeworld and have developed an incredible breeding speed to compensate; once aboard a spaceship with plenty of food and no predators, they quickly reproduce in such numbers that the crew is almost overwhelmed.

    Tabletop Games 
  • Pathfinder: Pest drakes are dragonets about the size of a pigeon that became major fad pets a few centuries in the setting's past. Many were released into the wild when they grew too big to care for, and more were freed when the fad passed, and they ended up becoming extremely common and destructive urban pests.
  • In Warhammer 40,000, the Orks technically started out as this: having been created by the Old Ones to serve as warriors in their wars against the Necrons, the Orks were engineered to not only possess great strength, psychic power and reality-warping technology, but also to reproduce via spores. This not only gives them an incredible advantage in numbers, but allows them to begin a planetary infestation again once they've apparently been exterminated. However, the Old Ones died out — in some versions of the story, helped along by their rebellious creations — and without masters or their traditional enemies, the Orks began to expand beyond all control: having been introduced to an environment ill-equipped to naturally repel them, the Orks soon become a persistent menace to civilizations across the universe. These days, however, it's most common for Orks to "introduce" themselves to a new environment.
    • Genestealers are a unique variation on this. They invade a planet by infecting species with sufficient biomass (though we only ever see this happen with humans in-game) with their genome, so that when the infected creature mates, the resulting offspring will be a genestealer hybrid. These hybrids themselves reproduce and grow until there are a great enough number of them on the planet. At which point they unknowingly form a psychic beacon for the Tyranids, letting them know there's a nice, biomass rich planet ready to be consumed.

    Video Games 
  • Dishonored: The Pandyssian rats were introduced to Dunwall by Hiram Burrows in an attempt to Kill the Poor. Pandyssia is a Death World full of nasty predators that keep the rat population manageable, but the biggest creatures in Dunwall are humans and hounds, both of which can be ganged up on and eaten by rat swarms. It wasn't long before the perpetrator lost control of the rats, who quickly broke free from quarantine and colonized the island. The icing on the cake is the plague the rats transmit to humans, creating a Plague Zombie Apocalypse.
    • Dishonored 2 confirms some Flavor Text from the first game claiming that the rats had been introduced to Serkonos some centuries prior. Unlike in Dunwall, Serkonos has a native predator that can manage the rats in the form of the bloodflies, bird sized insects that create hives and attack in swarms.
  • Kirby and the Forgotten Land has a rather extreme version of the trope with Fecto Forgo, the Ultimate Life Form behind the events of the game. In the past, they arrived in the Forgotten Land as an evil alien hell-bent on invading the land called Fecto Elfilis, attacking everything in its path until they were captured by scientists. It's mentioned on the Dream Discoveries Tour of Lab Discovera that Fecto Elfilis's actions were extremely disruptive to the planet's biosphere. For bonus points, Fecto Forgo's Boss Subtitles even call them an "Invasive Species".
  • In Knights of the Old Republic, a visit to Tatooine results in the Ebon Hawk being suddenly infested with Gizka after a spaceport official accidentally loads a crate of them onto the ship. With no natural pests or environmental threats to keep their numbers down, the creatures soon fill just about every single corner of the ship, and killing one only results in another appear to replace it. The player has the option of using Gizka poison to clear the infestation, convincing a buyer on Manaan to take them away, or just leaving them alone (they'll leave the Ebon Hawk when the ship crash-lands on the penultimate level).
  • Knights of the Old Republic II: The Sith Lords reveals that the Telos Restoration Project has had a problem with this in recent years. A carnivorous species had to be introduced in order to prevent the re-introduced herbivores from destroying the delicate ecology, and thanks to the restoration project's financial ties to Onderon, Cannoks were believed to be the ideal predators for the job... but unfortunately, they proved a little too adaptable: on top of being Explosive Breeders, Cannoks are highly aggressive and will attempt to eat just about anything they can fit in their jaws; all well and good on the Onderonian moon of Dxun, but with no predators on Telos to keep them in check, they had an insane advantage. By the time you reach the surface of Telos, they've wiped out the entire herbivore population and devastated the food chain. Worse still, they're now starving and even more aggressive than usual, making them the most common enemy on Telos next to Czerka mercenaries.
  • Mass Effect:
    • Played for tragedy in the case of the Krogan. Deliberately uplifted to serve as a trump card in the war against the Rachni, things started going wrong once the war was won: the traits that had allowed the Krogan to survive on their home world of Tuchanka soon proved highly dangerous once they ended up on less-hostile worlds: numerous accounts exist of Krogan overwhelming the natural environments of the planets they were allowed to colonize, eventually forcing them to expand further. Worse still, Krogan are not only intelligent beings but also a Proud Warrior Race, so it didn't take long for tensions between an overpopulated Krogan army and a stubborn galactic government to erupt into conflict. Consequently, a new war started - one that only ended when the Salarians and the Turians infected the Krogan with the Genophage. Krogan still exist by the start of the series, but as a demoralized Dying Race reduced to squabbling over any female capable of consistently bearing live young.
    • Vorcha are an accidental example of this: a primitive species that started out on the brutal planet of Heshtok, they were introduced to the rest of the galaxy when explorers landed on their homeworld, unwittingly giving the Vorcha a chance to stow away. Aggressive, fast-healing, fast-breeding and short-lived, they flourished away from their homeworld - to the point that parties on Omega go out of their way to cull their numbers before they grow too numerous. Thanks to their aggressive streak, they're popularly stereotyped as violent criminals and scavengers, especially given that the Blood Pack exploits their breeding rates to employ them as shock troops.
  • Metroid:
    • The titular creatures are considered to be a dangerous threat to whatever environment (barring cold temperatures) they appear in, being near-unstoppable while consuming the life force of all organisms in their path. Despite this, Metroids are extremely desired by both the Space Pirates and the Galactic Federation for their potential applications, even if it means risking the lives of many to get their hands on even a single one. It is because of these factors that Samus is ordered to wipe out all Metroids from existence lest they fall into the wrong hands and become an invasive species throughout the galaxy.
    • However, it turns out the Metroids were created by the Chozo to combat an even bigger threat to the galaxy known as the X-Parasites, organisms native to SR388 who consume their prey on a cellular level and take on modified versions of their forms, being just as unstoppable as their former predators.
  • Prey (2017): The Typhon Mimics reproduce by killing intelligent life forms, generating healthy adult offspring in seconds. The Talos 1 space station is quickly overrun, and there is fear that a single Mimic reaching Earth will doom the planet. The ending shows that they are right, and Earth has been completely colonized and terraformed by the Mimic, with fate of Humanity unknown.
  • Pokémon Sun and Moon: Rattata were introduced to Alola after stowing away on cargo ships, quickly establishing themselves as vermin. As a result, Yungoos was imported from another region to help control their population. However, the Rattata adapted by becoming nocturnal and moving to urban habitats, thus creating a new variant of the species. Meanwhile, Yungoos itself would go on to become a scourge in Alola.
  • The Secret World: The Ak'ab are a monstrous race of giant insects imported from Guatemala by Mayan cultists to use as living weapons in the Darkness War on Solomon Island. The Buzzing specifically states that the native Wabanaki had no defence against the Ak'ab, nor did any of the magical beings on the island until the arrival of Excalibur. Unfortunately, the Ak'ab escaped the counterattack and hid underground; awakening in the 21st century, the insects emerged to find themselves with no masters, no predators and no competitors adequately equipped to stop them. As such, it's not long before they begin devastating the already-beleaguered environment, dominating local forests, pushing the Sasquatch to the brink of extinction and overwhelming any human efforts to oppose them.
  • Shortly before the events of Temtem, Digital Temtem were invented in Cipanku, who then escaped into the wild and started displacing the native Electric Temtem from their natural habitats. In a sidequest, some scientists introduce Mimit, which can breed with most species, into the highlands of Iwaba in an attempt to rectify this.

  • Freefall: Sam expresses concern about ducks, of all things, getting loose on his homeworld and destroying the ecosystem. Though given the wide number of Terran animals that have attempted to eat him (and his need for higher oxygen than Earth's atmosphere) his concern might not be unjustified.
    Sam: Life started on your planet hundreds of millions of years before it started on mine. We don't have beetles! None of our insects can fly! You want to cause mass extinction on my world? You don't need nukes. Just drop down some ant colonies.

    Web Original 
  • Green Antarctica: Pretty much any Antarctic species that makes it to the rest of the world becomes horribly invasive, some of them managing to thrive even in the tropics. Interestingly, it seems to not work the other way, since while rats, pigs, and dogs have been introduced to Antarctica, the native wildlife is more than a match for them.
    • Devilsnote  were introduced to China to control rat populations, unfortunately, by 1936, their population had skyrocketed, leading to large parts of Shanghai being firebombed to destroy the swarm. It's been theorized that the government never fully exterminated them, and a mega-swarm is predicted to break out around 2012-2014.
    • Hive monkeys have displaced raccoons and other small-to-medium sized omnivores in North America, and are capable of killing humans.
    • Gugsnote  have established populations in the Alps and the sewers of New York.
    • Southern koalas have infested and caused the depopulation of large portions of the Amazon, Congo, and Southeast Asia. They can survive in the tropics, it just makes them more aggressive.
    • Likewise with wasp-weasels, a venomous mammal that some people theorize were deliberately introduced as biological terrorism.
    • Fire ants (that is, ants that can start small fires) have been introduced to other continents, where they cause wildfires.
  • Hamster's Paradise: When the isolated continent of Borealia collides with the continent of Nodera multiple species were able to cross over the connecting landbridge. The most devastating of which was the Borelian saberleaf grass crossing into Nodera, the plant incorporated silica into its blades which makes it razor sharp and too dangerous for herbivores to eat or even just walk through and its prolific breeding outcompeted native grasses and drove several species of Noderan grazers into extinction or forced them into other habitats. However, an inversion of this trope happened when the ungulope, another native of Borealia, entered Nodera, their mouths, throats and stomachs were designed to safety consume saberleaf and when they fed on it they reduced its numbers and allowed the native grass and herbivores to recover.
  • Serina: The Kyran Islands were an isolated group of islands that where home to various species of small burrowing birds called snuffles that hid from large predatory birds by hiding underground, but when the islands became connected to the mainland it allowed the small, mammal-like tribbetheres to enter the region, and unlike the native flightless birds they were able to follow the snuffles into their homes and eventually hunt them into extinction.
  • TierZoo ranks animals on a tier list based on how well they can perform in their environment. By these criteria, invasive species are almost always ranked high precisely due to the capability to dominate and proliferate in their introduced habitat, such as the Lionfish at A-rank and the Cane Toad at S-rank.

    Western Animation 
  • Molly of Denali: In "It Came From Beyond," Trini encounters one bird vetch and thinks it would be perfect for Molly's garden. Soon, more and more of that plant start spreading around the garden. Trini learns that it is an invasive species to Alaska.
  • The Simpsons:
    • In "Bart vs. Australia", Bart brings his pet bullfrog to Australia; this results in a population explosion later on in the episode. At the end of the episode, a koala is seen hanging onto the skid of the helicopter taking the Simpson family to a ship that will take them back to America, implying that America will soon have the same thing happen to it with koalas.
    • "Bart the Mother" has two Bolivian Tree Lizards hatching in Springfield, where their population explodes and they end up eating almost all of Springfield's pigeons, much to the delight of the population of Springfield (who view pigeons as disease-ridden "flying rats"). When Lisa points out that now Springfield is full of Bolivian Tree Lizards, Skinner outlines Springfield's solution; first, they will import large numbers of lizard-eating Chinese Needle Snakes, and then when the snake population becomes uncontrollable, they will import snake-eating gorillas; they don't need any animals to deal with the gorillas, because the gorillas will freeze to death when winter comes.
  • The Legend of Tarzan: In "The Seeds of Destruction", Jane plants a sweet-smelling non-native flower near the elephant pond, but the vine grows out of control, driving the elephants out of their home and forcing them to share with the gorillas. When tensions between the two threaten to lead to all-out war, Jane must find a way to fix her mistake.

    Real Life 
  • The Other Wiki has a list. See below for more detailed examples...
  • Australia has some of the most infamous examples of introduced pests, many of which are so adaptable that they have proved a serious threat to Australian biodiversity, ecological stability, and in some cases, even to the economy in such an isolated ecosystem. That's why the country takes biosecurity so seriously.
    • Rabbits, once introduced to Australia as game for shooting hunts, have since proved to be one of the single biggest nuisances in the country. With local predators being wiped out by the settlers, their breeding rate proved extreme enough to endanger native species like Bilbies, and then to native plants, and then to the crops of British settlers. Centuries onwards, Rabbits are still a major problem in Australia — to the point that an attempt to wipe them out via germ warfare failed once they started adapting to the Myxoma virus.
    • Foxes were also introduced as game for foxhunting. Unfortunately, with dingoes learning to avoid larger urban centres, they quickly became apex predators and began preying on smaller native animals — to the point that they've been observed climbing trees in pursuit of koalas and sugar gliders, and may have actually played a part in the extinction of several species.
    • Cane Toads are possibly the most notorious example to be found in Australian history. Originally imported to eliminate the native beetles that were damaging cane crops, the toads not only proved completely useless as biological pest control, but they ended up becoming a serious problem for native wildlife. Cane Toads are very poisonous, and because native predators haven't adapted to their toxins, birds and mammals attempting to prey on them usually end up dead; as such, they reproduce at a much greater rate than they would in their native territory... and worse still, they have a habit of eating the young of goannas and crocodiles, further damaging biodiversity.
  • Prickly Pear cactus was introduced to serve as a fence, but quickly spread over 15,000 square miles, rendering the land useless for grazing. Fortunately, the Cactoblastis moth was introduced as the caterpillar eats the cactus (hence the name) and quickly brought it under control, in a rare instance where introducing a predator of an introduced species didn't make things worse.
    • However, when Cactoblastis was introduced to South Africa to control a similar prickly pear infestation, it started to consume a spineless species of cactus that was commonly used as livestock feed too. Then it somehow spread to the Caribbean and United States, where it's now endangering native cactus species too.
  • In many cases it was predation by introduced rats, dogs, and pigs, not hunting as is commonly assumed, that led to the extinction of the dodo and many other flightless birds native to islands. Special mention goes to the Stephen's Island Wren of New Zealand, which was supposedly wiped out in the space of a few weeks by a lighthouse keeper's cat.
  • The Spanish would often leave breeding pairs of pigs in places they explored. The pigs could be relied on to survive without human supervision, and would reproduce so quickly that future voyages would be able to use these pigs to resupply. However, this also damaged many native ecosystems, and spread plagues throughout the native populations. Extra History has a video looking at the impact introducing pigs to the Americas had here.
  • The Giant African land snail was accidentally spread to a number of Pacific islands, where they wreaked havoc. An attempt was made to introduce another snail species, the carnivorous rosy wolfsnail in the hope they would prey on the Giant African land snails. The wolfsnails, however, instead began attacking native snail species, rendering many of them extinct, and is now an invasive species itself.
  • Eugene Schieffelin released 60 European Starlings into New York City's Central Park, because the bird was mentioned in one of William Shakespeare's plays; the Starling is now considered one of the worst invasive species in North America.
  • Until the 1950s, Lake Victoria in Africa was home to a huge number of cichlid fish species, which had diversified to fill virtually every environmental niche available. Unfortunately, someone then decided to introduce a new species to the lake to improve the game fishing, and settled on the Nile perch, a very large — and very hungry — predator, which proceeded to chomp down on every cichlid it could find. So far at least 40% of Victoria cichlid species are now extinct and most of the others are critically endangered, which has caused a colossal disruption to the lake's ecosystem — not least of which being an unchecked overgrowth of algae, choking out most other plant life and coating the lake bed with rotting detritus. Oh, and by the way, the lives of over 30 million people depend on the resources of this lake.
  • Argentine Ants. In their native habitat, the floodplains of the Parana river in South America, their numbers are kept in check by the fierce competition from two other native ant species, fire ants and army ants, and because whenever they establish a new colony, genetic mutations result in the different colonies seeing each other as rivals and competing with each other, so their survival hinges on them being highly aggressive. When a small number of queens stowed away on human ships and set up new colonies in Madeira and New Orleans, there were no species that could compete with them, and lack of genetic diversity meant that the mutations that make them compete with themselves didn't establish as quickly, meaning they were more likely to co-operate together and form mega-colonies. Today, they're the most numerous ant species on earth, have established colonies on six continents, and are displacing and devouring many native ant, invertebrate and plant species in their new environments. As an added bonus, they also know how to co-operate with aphids, and aphids that ally with them also profit from their success, growing into swarms that kill off human crops.
  • Bacteria and viruses that are lethal to humans are a variation of this. They're typically suited to life inside a specific host species, and they try not to kill the host that provides them with warmth, shelter and nutrition for the same reason humans don't burn their own houses down. Measles, tuberculosis and smallpox, for example, are minor nuisances to a cow because the inside of a cow's body is their natural habitat - it's their difficulty adapting to human biology that makes them so dangerous if/when a human gets infected. And that's just transmission from animals to humans. With modern air travel, diseases can spread quickly around the world. The COVID-19 Pandemic is a spectacular modern example.
  • An example that had nothing to do with humans occurred about 3 million years ago when North America connected to the once isolated South America and caused what's known as the Great American Interchange. Species from both continents would cross into the other but it was definitely more one-sided in favor of the north as many of the native predatory marsupials, terror birds, giant rodents and native ungulates were outcompeted and replaced by canines, big cats, camelids and elephants. However there is some debate on whether it was as simple as competition as some ground sloths, terror birds and giant armadillos managed to make it all the way into North America, some theorize that the extinction of the South American megafauna was more to due with the ice age that was starting up adversely affecting the southern ecosystems and the northern animals just took advantage of the now vulnerable natives.