The United States is a big place. Spread over thousands of miles, the country boasts amazing natural landscapes. In them, varied ecosystems are home to an incredible number of species. From coast to coast, many climates offer unique grounds. The incredible biodiversity across the continent is ripe for research, and conservationists marvel at the diverse outdoor habitats found nationwide. But even with all the species endemic to America, invasive animals still wreak havoc.
In fact, some non-native animals have completely altered entire ecosystems. Wildlife experts do their best to limit damage, but the worst cases prove too difficult to stop. In these ten instances, foreign species were accidentally introduced to areas of the United States and then completely overturned the natural order.
Related: Top 10 Surreal Animals That Really Exist
10 Nutria (East Coast)
The nutria looks like a cute little critter. These oversized rodents resemble capybaras or beavers. Some say they even look like giant guinea pigs. And in some places, they are informally known by an eye-catching name: the swamp rat. But that’s where the fun stops. Their cute look belies their aggressive takeover.
Native to South America, these giant furry creatures were first imported to the U.S. in the 1930s. Ever since, their population has regrettably thrived. There is one big problem with these big-toothed herbivores: their diets! Nutrias eat their way through important wetlands and sensitive swamps. The giant rodents have unfortunately become experts at destroying critical habitats with their endless appetites.
Aquatic plants and swamp flora attract them the most, so the southeast is ground zero. But nutrias are now seen further north, too. There have even been sightings in California’s Central Valley. That’s an area previously thought too dry to attract them. It’s also America’s most productive agricultural space, so farmers and biologists are concerned by the spread.
Part of the problem is the nutria’s breeding patterns. Females can produce more than a dozen pups in each litter. Then, alarmingly, they are ready to breed again days after giving birth. The cycle has proven nearly impossible to stop. Populations have exploded while conservationists scramble to respond. That’s not to say people haven’t taken action.
In Louisiana, these unwanted swamp rats can fetch a big bounty. One intrepid bayou man collected nearly $60,000 from the state after he turned in more than 10,000 nutria tails from a hunting expedition. That move may have slowed the spread of these massive rodents, but it’s not stopping them. Ecologists continue to flounder against these ecosystem eaters.
9 Stink Bug (Mid-Atlantic)
The brown marmorated stink bug may appear to be an innocent (and fragrant) little thing. But the insect has been remarkably invasive across the eastern United States since being introduced three decades ago. Today, biologists believe the brown marmorated stink bug first came to America on cargo ships sailing out of Asia in the late 1990s.
In 1998, these bugs were first spotted in Pennsylvania. Since then, populations have boomed in the Mid-Atlantic. The bugs don’t have any natural predators. In China, they are kept in check by samurai wasps. But those insects don’t exist in the U.S. In America, that circle of life is thrown out of whack. And the stink bugs have flourished because of it.
Stink bugs have incredible appetites. Their diet consists of nearly every agricultural staple grown in America. Fruits are favorite targets. In 2010 alone, apple growers in the Mid-Atlantic reported nearly $40 million in losses from stink bug swarms. Unfortunately, controlling this invasive pest has proven nearly impossible. The bugs’ brown shell looks very similar to that of native stink bugs. But the American species doesn’t pose a threat to agriculture. Thus, ecologists are worried about wiping them out indiscriminately along with the Asian kind.
Plus, the bugs blend in perfectly with tree bark. Tracking their spread is costly and difficult. And their invasive misdeeds take a while to reveal themselves. Farmers often don’t know brown marmorated stink bugs have descended on crops until after they feed. “The damage from this pest often doesn’t appear until three or four weeks after it feeds,” one researcher noted. “It can come and go, and you wouldn’t even know it was there until it was too late.”
8 Northern Snakehead (Chesapeake Bay)
Northern Snakeheads are found across much of the far East Coast. The fish is native to coastal areas in eastern Russia, China, and Korea. But somehow, it’s gotten to America… and it’s destroying everything in its path. Snakeheads are fish, but they look like eels. As their name suggests, they have a long body similar to a python. With razor-sharp teeth and strong tails, they are good swimmers and relentless predators.
They are big, too. They can grow up to three feet long. And a very unique respiratory system allows them to survive out of water for several days on end. In Asia, snakeheads are popular staples at fish markets. Locals catch them in reservoirs, ponds, and rice paddies and serve them as a delicacy. But as fascinating as these facts may be, the fish is brutal and aggressive.
In the United States, northern snakeheads have recently taken up residence in several areas. Notably, they are thriving in the murky waters of the Chesapeake Bay. Brackish inlets and slow-moving streams are their ideal habitat. Once settled, they feed on everything they can find. Snakes, mammals, reptiles, fish, and even birds are on the menu. Female snakeheads reproduce like crazy, too. Some carry up to 100,000 eggs apiece. Mating has meant population booms nationwide as ecologists desperately try to get control.
Snakeheads have even been spotted as far away as California and Florida. The problem is so bad that biologists now counsel people to “kill it immediately” upon sight. But even amid this open season, northern snakeheads continue to thrive.
7 Asian Swamp Eel (Freshwater Lakes)
Asian swamp eels aren’t actually eels at all, but they look the part. They are scaleless fish with long bodies. Their snake-like figure tapers into a tail similar to eels, hence their name. But these predators are aggressive and unwanted by any classification. Sharp, bristle-like teeth and a voracious diet make them a threat to all lake-based fauna. Frogs, fish, turtle eggs, and even crustaceans are favorite delights for these unforgiving aggressors. They can breathe air, too, giving them habitat flexibility unseen in most fish.
While they are native to swampy waters in Asia, these predators came to the United States inadvertently. Immigrants introduced them as an imported food source. Some were even bought and sold as pets in the aquarium trade. Soon, they were being discarded in waterways across America. Quickly, their population grew.
Today, ecologists are working overtime to control this aggressive predator. Their endless appetites are resistant to nearly all human interventions. Some scientists even call them the animal equivalent of a kudzu vine due to their relentless run. Vegetation removal and electrical barriers have been used to curb the swamp eel spread. But it’s a difficult road for humans as these fish continue to thrive in unwanted spaces.
6 Tegu (Florida and Georgia)
The tegu is a lizard the size of a small dog that is native to South America. It can grow up to four feet long from head to tail, and the biggest ones weigh more than ten pounds. Naturally, they flourish in Argentina, Paraguay, Uruguay, and southern Brazil. Their behavior is like that of many other lizards. And although they are good climbers, they typically dwell on the ground. There, they feed on pretty much anything they can catch.
In those warm climates, these omnivores are kept in check by many natural predators. Birds of prey, large snakes, and pumas all feed on the large lizards. But in North America, those same predators don’t exist. Thus, stateside tegus run amok in swamplands and forests across Florida and the southern parts of Georgia.
Researchers aren’t sure how tegus first came to the U.S. Because of their behavioral profile, it’s possible people wanted them as exotic pets. Biologists believe many were shipped illegally into the country. Once here, owners were likely shocked at the big lizards’ food aggression. So many may have been dumped in the wild. In the relatively warmer climates of the deep South, they have thrived.
Their 20-year lifespan allows for plenty of mating opportunities. Female tegus lay about three dozen eggs per year, too. Thus, offspring multiply quickly. With no real natural predators, the lizards feed as they please on birds, reptiles, fish, and plant life. Wildlife officials are trying their best to keep down these invasive populations, but tracking and capturing them has proven very difficult.
5 Peacock (Los Angeles)
If you’ve ever been to a major zoo almost anywhere in America, you’ve seen peacocks. The big birds are native to India and Africa, but zoos across the continent have imported them. Once inside, they roam freely on the grounds. They are very territorial, but they typically coexist among enclosures. Visitors marvel at how they peck, squawk, and strut. And since they don’t fly very well, peacocks live out (mostly) peaceful lives within the zoo.
But that’s not the case in Los Angeles. Decades ago, real estate developers in southern California imported peacocks from Africa and Asia. World-famous moguls like William Wrigley and Hugh Hefner got in on the act too. The richest Angelenos wanted expensive peacocks to strut around the grounds of their decadent estates. They got that… and then some.
Over the years, these feral peacocks spread from living on private grounds to moving into the public realm. Today, peacocks have taken over northeast Los Angeles. They occupy parks and strut along streets. They are certainly not endemic to the area, but the good weather makes it work. Plus, there’s no shortage of food available between local fauna and discarded human scraps. Over time, the peacocks have come to thrive in LA’s gritty urban core.
Some locals love the unique charm of peacocks roaming the streets. Others hate the birds and wish they’d be removed. The city has made it illegal to feed peacocks. If Angelenos are caught doing so, they face a $1,000 fine and up to six months in jail.
Still, residents have gotten aggressive in trying to rid LA of the birds. News reports note some Angelenos have tried to poison them. Others have shot at them with pellet guns or tried to run them over in cars. But their squawks persist. Perhaps these peacocks just have their own Hollywood hopes and dreams?
4 Norway Rodent (Nationwide)
Norway rats are one of the longest-lasting invasive species in North America. They were first introduced to the continent in the 1770s, a few years before the United States existed. Slowly, they have spread across the land. Today, they are even found in Alaska and Hawaii. Their name is a bit of a misnomer. They didn’t originate in Norway but rather Asia.
However, centuries ago, they wiggled their way onto ships crossing large oceans. Their resilience as a species allowed them to flourish on board. Once these ships docked all over the world, the rats scurried off and made new homes. In the U.S. today, most people casually call them sewer rats or brown rats. They are often famously associated with the urban rodent population in places like New York City. But their habitats branch out far beyond that into rural locations too.
These rats are the ultimate foragers. They can track down food on city streets and in far-flung natural areas. They love to sift through trash for tasty treats. But they can hunt too. Their prey includes lizards, fish, other rodents, and chicks. Armed with quick feet and a nose for food, they will eat nearly anything. Studies have found the remnants of thousands of different things in their stomachs.
Today, they are so invasive as to be uncontrollable. Across the U.S., these rats breed endlessly. On average, females produce up to eight pups per litter. And they can have as many as seven litters per year. Unstoppable at this point, Norway rats have become a key component in nearly every North American ecosystem.
3 Brown Tree Snake (Guam)
Guam is thousands of miles from the mainland United States. In fact, the isolated Pacific Ocean island is far from any other occupied territory. But that doesn’t mean it can’t experience its own animal invasion. During World War II, American and Australian forces fought Japanese soldiers in the Pacific. The fighting was brutal, endless, and everywhere.
To prepare for long-range battles, Allied ships would load up on supplies in places like Papua New Guinea and the Admiralty Islands. Their intention was to bring ammunition, food, and medical care to Guam and other outlying islands. But they also brought brown tree snakes. The aggressive serpents quietly stowed away in supply containers. Soldiers unknowingly transported them on Navy ships.
Once in Guam, the snakes slithered away from human eyes and into forests. As they settled in, they thrived. The reason for their population boom was simple: Guam has no natural predators to keep them in check.
Back in the wilds of Indonesia and Papua New Guinea, brown tree snakes suffered losses from larger predators. But the monitor lizards, feral pigs, and pythons that lived there didn’t exist in Guam. Having inadvertently found themselves at the top of the food chain, these brown tree snakes ate everything they could find. Populations of lizards, small mammals, and virtually every single native bird species on Guam have been decimated.
The snakes are able to lasso their bodies up tree trunks and pipes too. Thus, even birds hiding out high up in nests aren’t safe from attack. Ecologists have tried to tamp down brown tree snake habitats, but they move fast and breed quickly. Today, even Guam’s unique flying fox population is at risk of decline.
2 Wild Horse (American West)
Wild horses living in the American West aren’t invasive in quite the same way as other animals on this list. In fact, some would argue they aren’t invasive at all. Fossil records show prehistoric horses lived in what is now North America about 50 million years ago. They appear to have thrived in those times. About 13,000 years ago, the last of those horses crossed the land bridge into Asia.
However, horses have been absent from American ecosystems ever since. That is until Spanish explorers reintroduced them in the 1500s. Native American tribes quickly took to the animals. Warriors learned to ride them with incredible grace. Horses spread across the Great Plains and Southwest to help with hunting and transportation. Today, horses are inextricable from the culture of the Old West.
Modern wild horses have had a different run, though. They aren’t only found in the West. In fact, bands roam as far north as Nova Scotia. One notable group of wild horses lives on the sandy beaches of North Carolina’s Outer Banks. But there is an invasive component to their existence. In the Southwest, ranchers have grown tired of wild horses. Landowners argue the horses compete with cattle for sparse grazing lands. And with major irrigation issues in the dry region, farmers are concerned that precious assets are being allocated to these stallions.
The ranchers may be worrying for nothing, though. Unlike other unwanted species on this list, even if wild horses are invasive, their numbers are dwindling. Biologists are worried about the future of these maned marvels. The hope is that they have enough cultural capital among Americans’ imaginations to inspire conservation. As the country continues to develop, wild horses remain a romantic vestige of the old American West.
1 Tiger (Nationwide)
We got your attention with this one, didn’t we? Don’t worry! Vicious tigers aren’t roaming the streets of America as an invasive species on the loose! (Uh, we hope?) Instead, this phenomenon has to do with the number of wild tigers living as exotic pets in the U.S. Perhaps Americans can blame Netflix’s hit series Tiger King for this one.
When the show first aired in March 2020, it became the most-watched program ever produced by the streaming giant. Millions of people spent millions of hours watching the story of amateur tiger wrangler Joe Exotic. Apparently, many viewers really related to Joe’s big cat dreams. Tiger King came out at a time when captive tiger populations were swelling across the United States.
The World Wildlife Federation has been sounding the alarm on pet tigers in America for years. Unfortunately, many municipalities haven’t been listening. Today, about 5,000 captive tigers are living in the United States. When one considers that there are only about 4,000 remaining wild tigers worldwide, that number becomes shocking. Even more surprisingly, only 6% of captive tigers live in zoos that are staffed by professional veterinarians and keepers.
Thus, roughly 4,500 wild tigers live in poorly-suited backyards and homemade shelters across the country. Some states outlaw exotic pets altogether. But other states have limited restrictions, and owners take advantage. Today, tigers live in private breeding facilities and cheap roadside attractions. Environmentalists worry about the danger they present to caregivers, as well as worst-case scenarios for neighbors should they escape.
+ BONUS: Asian Citrus Psyllid (Sun Belt)
The Asian citrus psyllid may be a remarkably small bug, but it is incredibly damaging to large-scale farms. The insect carries a bacteria that causes Huanglongbing disease. More commonly called yellow dragon disease, it is a bacteria that kills citrus plants. Their carriers are very tiny and thus tough to find. The origins of this troublesome pest are in southeast Asia and India.
Scientists aren’t exactly sure how they reached the United States. However, it’s most likely they were imported among illegal agricultural shipments. Once here, the bug spread, and its carrier bacteria has caused mayhem.
There is no known cure for yellow dragon disease. Infected trees must immediately be removed from groves. If not, the entire grove risks falling prey to the relentless and deadly bacteria. Grapefruits, oranges, mandarins, lemons, and other citrus plants are constantly under threat. So farmers across Florida, the deep South, and even southern California are forever on guard against the Asian citrus psyllid.
Not wanting to lose entire sections of valuable cash crops, the USDA and others routinely enact quarantines to protect trees. The state of California also has its own procedures to try to keep Huanglongbing at bay. Even so, growers from the Golden State to Louisiana to the Sunshine State report losses every year. Until a cure can be found, farmers must fight in vain against this nearly-unseen invasive pest.