12 Invasive Animal Species That Have Destroyed Ecosystems

Invasive species rarely begin their reign of terror with malicious intent; they often arrive as accidental stowaways or well-meaning introductory problem solvers. Yet, these seemingly harmless newcomers have a remarkable ability to adapt, quickly establishing themselves in foreign territories. They outcompete native animals for resources, disrupt delicate food chains, and reshape entire ecosystems.  From the voracious appetites of predators that decimate native prey to the suffocating reproduction and rapid spread of prolific breeders that choke out native flora, the impact is significant, to say the least.

We're delving into the stories of the "dirtiest dozen" invasive species, dissecting their destructive mechanisms and exploring the profound damage they've inflicted upon ecosystems worldwide. These aren't just a few one-off incidents; they are ecological disruptors, setting up permanent residence in habitats where they don't belong, with devastating and often irreversible consequences. It might seem to some that releasing one little cat or snake into the wild is harmless, but there are potentially many ripple effects that little animals can cause to unravel entire ecosystems.

Originally hailing from across the Atlantic, feral swine (Sus scrofa) were brought over to the U.S. in the 1500s by Christopher Columbus and Hernando De Soto as a food source, but became a popular animal to import for sport hunting in the 1900s. However, their intelligence and ability to adapt to southern U.S. soils has made them a particular nuisance for farmers; they're voracious crop eaters, and they carry diseases that can devastate domestic pig populations. Feral swine will uproot tree seedlings, erode soil, and cause so much havoc that the USDA estimates it costs $1.5 billion annually to try and control their spread.

Feral swine, also called wild pigs or hogs, are also incredibly good at adapting to different climates, and are suspected to be in at least 35 U.S. states, though some believe they've spread to 39 and over into Canada. While they're often considered one of the worst invasive species to Florida, Texas seems to be the hotbed of wild hog populations, such that there are no state limits for hunting or trapping them year-round. Attempts to eradicate them have proven futile; now state agriculture departments seem to be resigned to just trying to control their spread. However, the USDA isn't giving up hope; since 2014, it has helped a dozen states completely eliminate their feral swine populations.

The Florida Everglades become a hotbed for Burmese pythons (Python bivittatus) in the 1970s. They are voracious hunters and have no natural predators in the U.S., essentially giving them free reign to eat whatever they want, whenever they want. Their massive size – up to 20 feet long – requires significant food resources, and the Everglades has been losing significant populations of smaller mammals and reptiles, decimating the food chain by as much as 90%. Snake catchers have removed thousands of Burmese pythons from the area, but female pythons can lay batches of 50 to 100 eggs at a time, making it a never-ending fight for control of the southern Florida wetlands.

The challenge with controlling these snakes is that they're ambush predators, so setting a food trap the way you might with an opportunistic eater isn't effective. Instead, researchers are attempting to reduce the population with a little bit of espionage. Male Burmese pythons can be lured into the open by using female python pheromones. Once caught, the males are implanted with radio transmitters and re-released with the hope that they'll seek out a female. Since female pythons are such prolific egg layers, they're the ones snake hunters are after and so these so-called male "Judas snakes" may be the key to eventually eradicating the population. 

However, Judas snakes haven't proven to be 100% effective as females have short mating windows, so it requires constant vigilance. The U.S. Geological Survey isn't deterred though, and is now attempting to implant female hormones along with the transmitters in these male pythons. Their hope is that the female scent will lure other males to the Judas snake, making it easier to bag multiple snakes at a time and stop one of the most invasive species the Florida Everglades has ever seen. 

Zebra mussels (Dreissena polymorpha) are an accidental addition to North America. Scientists believe they stowed away on European ships in the 1980s and made it to our waterways via ship's discharge waters. Once they got a foothold though, they dominated, annihilating other native mussel species by attaching onto them and outcompeting them for algae or plankton food sources. They're such a nuisance that they've even managed to make it into power plant water intakes, clogging them so thoroughly that it costs millions of dollars a year to undo the damage.

Zebra mussels are only around the size of a human fingernail but are prolific breeders; female mussels can release nearly 1 million eggs per year. As they mature, they can swarm native mussels, using their byssal threads to attach onto the native's shells until they're unable to move, eat, or breathe. The National Park Service believes that eradication is impossible at this point, simply due to their tiny size and highly mobile microscopic eggs. Instead, they've implemented programs to control or slow the spread, encouraging boaters to be aware of any potential hitchhikers on their hulls and to discharge water before leaving a lake or waterway. If a boat has traveled in infested water, the NPS recommends cleaning the boat thoroughly and letting it dry for five days before getting back into the water, since zebra mussels can't survive outside their aquatic homes any longer than that. 

The Asian carp origin story is a perfect example of how an invasive species starts with the best of intentions. Several species of carp were imported to North America and Europe in the 1970s to help keep waterways and aquatic farms clean of algae. But the North American imported carp made a daring escape into the Mississippi River basin, before it moved into the Illinois River and eventually the entire Great Lakes basin, where it outbred native fish while pushing them out of their territories and consuming up to 100% of their body weight in food daily. One species, the silver carp (Hypophthalmichthys molitrix), can reach 80 pounds and is even known to jump into fishing boats.

The name "Asian carp" covers four species of carp: the aforementioned silver, grass (Ctenopharyngodon idella), black (Mylopharyngodon piceus), and bighead (Hypophthalmichthys nobilis). Their decimation of the Great Lakes' native flora and fauna have required the pooling of resources from multiple federal, U.S. state, and Canadian agencies, including the U.S. Fish and Wildlife Service, Environmental Protection Agency (EPA), and a 26 organization-wide coalition from the U.S. and Canada called The Invasive Carp Regional Coordinating Committee. Absolutely no egg, hybrid, gamete, or anything resembling a live Asian carp species is allowed into the U.S. without a permit from a federal agency, while waterways and lakes are extensively monitored to quarantine and control carp populations. 

The European starling (Sturnus vulgaris) has an interesting introduction to becoming an invasive animal. During the 1890s, Shakespeare superfan Eugene Schieffelin decided to bring 100 starlings over to the U.S. because they were mentioned in "Henry IV" and apparently the U.S. needed a little iambic pentameter bird culture. Within 50 years, the North American population of European starlings rose to over 200 million and swarmed agricultural feeding troughs, consuming much of the food and water meant for farm animals, and contaminating any remaining with diseases like West Nile Virus, E. coli, and histoplasmosis, which can attack the lungs of agricultural workers. Suffice to say Schieffelin's homage was not "a good jest for ever." 

The effect of European starlings on the ecosystem is horrifying, with their giant swarms causing absolute mayhem. In 1960, a flock of starlings that was 10,000 strong flew into a plane departing Boston's Logan airport, killing 62 people. Attempts to control the populations are usually advised at local levels, as these birds tend to congregate around manmade structures. Installing anti-perch spikes on windowsills, audio devices containing distress calls, and even small-scale pyrotechnics are all ways used to deter starlings from buildings.

Introduced in the early part of the 20th century, Nutria (Myocastor coypus) have become invasive in 17 states around the U.S., and are semi-aquatic monsters that will gnaw down crops and wetland plants with abandon, destroying natural defenses against soil erosion and irrevocably decimating swamps. They're also excellent burrowers, and have been known to create tunnel systems 150 feet long that destabilize streets, bridges, and levees. Worse still, they are carriers of diseases like tuberculosis.

Removing nutria from their current habitats is complicated due to their listing as protected species by some states. California eradicated them from the state in the 1970s, only to have them come back decades later, which isn't surprising since one mating pair can produce up to 200 babies. But they and other states are not taking this new generation of nutria lightly. California is investing $10 million to re-eradicate nutria from their wetlands, while Maryland has partnered with federal agencies like the U.S. Fish and Wildlife Service to aggressively combat the population and revegetate marshes around the Chesapeake Bay destroyed by nutria.

The cane toad (Rhinella marina) came to the U.S. from its native South American habitat in the early 20th century as a potential candidate for controlling agricultural pests in sugar cane fields, hence its name. But over the decades, cane toads escaped their enclosures, possibly with a little help from importers, and now can be found in abundance all along Florida's I-4 corridor, and have even made it to Australia. The invasive spread of cane toads is not necessarily due to competition with other toads, though they're known to eat whatever they can catch, but because they emit a toxic ooze over their skin as a defense mechanism. Consequently, they have no natural predators, so will happily grow their toad families to the point that the local ecosystems are overwhelmed.

Since they're such a threat, there are few regulations for eradicating cane toads, as long as it does not violate any animal cruelty laws. For residential areas dealing with cane toad infestations, it's recommended to place barriers that are about 20 inches high around ponds and dams to disrupt their ability to get near water. Meanwhile, researchers at the University of Sydney have begun testing the introduction of a parasitic worm that can infect the cane toad's lungs, killing it over time. Other scientists in Australia have begun testing other biological methods to thin the populations, including testing potential viruses that can disrupt their development and reduce their ability to reproduce.

The small Indian mongoose's (Herpestes auropunctatus) origin story is a little like the nursery rhyme about an old lady who swallowed a spider, then continued to swallow animals of an increasingly larger size to rid herself of the last.

In the 1800s, Puerto Rico had an infestation of black rats (Rattus rattus), which were accidental imports that were destroying sugar cane fields. So small Indian mongooses were imported to eat the rats, but were so agile and adaptable that they too spread to the point of being invasive. They're opportunistic hunters that will eat anything, ranging from small spiders to young white-tailed deer. They've been responsible for at least one bird species' extinction and are known to carry rabies, making them dangerous to pets and humans. They never ended up eradicating the rats, though. 

Small Indian mongooses are invasive in many parts of the world, but they can be beneficial by hunting animals dangerous to humans, such as venomous snakes like the fer-de-lance and habu pit vipers. However, their populations need to be carefully monitored or their voracious appetites will decimate native mammals and birds. Trapping programs have been successful but require constant management to ensure that other opportunistic hunters that the mongooses keep in control, like feral cats and rats, can't rebound. Toxins have been used to eradicate populations, although native animals can also be harmed by this approach.

Lionfish (Pterois spp.) are native to the Indo-Pacific waters, but ended up making their way to the southeastern U.S. coast and Caribbean waters in the 1980s. Since then, lionfish have become as problematic for the seas; since they're so far from home, they have no natural predators and will eat prey that groupers and snappers rely on, which also affects commercial fishing. Their predation of herbivore fish around coral reefs drastically affects the reef's ecosystem, as fewer herbivores means more algae, which adds additional stress to the struggling reefs that are being crippled by climate change. Furthermore, lionfish have spines full of ciguatoxin, a poison that can be incredibly painful if ingested.

Scientists are realizing that lionfish are nearly impossible to eradicate from their non-native waters, relying instead on close monitoring with attempts to control the populations as best as possible. This includes educating hobby aquarists about harm that dumping unwanted lionfish into seawater can have on the native fish and wildlife. The NOAA is also pushing for commercial fisheries to begin harvesting them from the sea for food, though careful preparation needs to be done to prevent poisoning from the spines. The Reef Environmental Education Foundation (REEF) even has workshops teaching people how to make jewelry from lionfish skin.

The brown tree snake (Boiga irregularis) is another one of many invasive species that probably originated as a ship's stowaway. Native to Australia and Indonesia, a brown tree snake most likely snuck into some cargo headed for Guam during the 1950s, where it found a paradise all to itself. 

Since the species doesn't have any natural predators on the island, it ravaged the ecosystem, causing the extinction of 12 animal species while preying on the native lizards and birds that controlled the insect populations. Now, not only are disease-carrying insects thriving out of control, but the brown tree snake is an absolute nuisance for the human population; it's known to climb power lines and nest in transformers or electrical boxes, causing an estimated 200 power outages per year and costing Guam's economy $4.5 million a year.

To date, the brown tree snake has not made its way to the mainland of the U.S., though eight specimens have landed in Hawaii, most likely hitchhikers on military equipment from Guam. The USDA estimates that if the brown tree snake set up shop in Hawaii it would cost the islands up to $1.7 billion per year, so the Hawaiian government takes snake control very seriously. But the federal government hasn't just left Guam to the snakes, either. In 2020, the U.S. Department of the Interior earmarked $3.4 million in grant funding for controlling the population in Guam and ensuring rapid responses to detecting and eradicating any stray brown tree snakes that may make their way to other U.S. states or territories. 

You might think Mr. Whisker's jaunts into the backyard are harmless, but if they go feral, cats (Felis catus) are a serious threat to local ecosystems. Cats are excellent hunters and are believed to kill up to 2.4 billion birds and 22.3 billion mammals every year. In fact, they're such effective predators that it's estimated they have contributed to the extinction of at least 63 bird, mammal, and reptile species. But it's not just the animal populations we need to worry about; feral cats are the number one carrier of rabies among domestic animals and are more likely than any other animal to expose humans to the virus. So save yourself a potentially hefty medical bill by keeping your cat indoors.

Previous attempts to control feral cat populations have been hit or miss. The method of "Trap, Neuter, and Release" was a popular compromise conservationists tried, but its results have proven to be ineffective and do nothing to curb the spread of feline-carried diseases, such as feline leukemia virus, toxoplasmosis, and the aforementioned rabies. 

Organizations like the American Bird Conservancy (ABC) instead advocate for keeping cats closer to home and providing indoor enrichment or outdoor "Catios" that keep your cat safe in an enclosed space. If you do need to let your cat outside, the ABC recommends leash training or strollers to ensure your kitty can't escape. Not only will you turn heads with your cat harness, but you'll be doing more good for your cat than you think; it's estimated that free-roaming cats only live an average of five years, while indoor cats stick around an average of 17 years.

In 1869, gypsy or spongy moths (Lymantria dispar) broke free from an entomologist's backyard in Medford, Massachusetts, where they were being evaluated as potential replacements for silkworms, and made their way into in the countryside. From there, they spread across the Northeast U.S., munching on the leaves of oak trees and over 300 other tree and shrub species, destroying native flora and the homes of beneficial fauna in the process. Gypsy moths are highly mobile, thanks to the females preferring to lay their eggs on outdoor surfaces used — and moved — by humans. And so, while they began their invasive spread in Massachusetts, the moths have hitchhiked as far south as North Carolina and as far west as Minnesota. 

Federal and state governments are taking this particular invasive species very seriously, spending on average $30 million yearly just to curb their spread. The USDA and several state departments of agriculture launched a national "Slow the Spread" (STS) campaign, which focuses on heavily monitoring quarantined areas and eradicating populations when possible. To date, USDA data estimates that concerted outreach and monitoring efforts have prevented the gypsy moth from infesting 7 million acres per year.