North America hosts hundreds of non-native animal species, from wild pigs tearing up farmland across the southern United States to Burmese pythons decimating mammal populations in the Florida Everglades. Some arrived centuries ago as deliberate introductions for agriculture, sport, or pest control. Others escaped from the pet trade, hitchhiked in ship ballast water, or simply followed human settlement. The ecological damage they cause is staggering, with economic costs in the United States alone reaching into the trillions of dollars over the past six decades.
Wild Pigs, Nutria, and Other Invasive Mammals
Wild pigs are among the most destructive non-native mammals on the continent. Descended from Eurasian wild boar and domestic swine brought by early European settlers, they now roam widely across at least 35 U.S. states and parts of Canada. They root through soil, destroy crops, contaminate water, and compete with native wildlife for food. Their populations have been growing both in number and geographic range in recent decades, and the scope of damage they cause still outpaces researchers’ ability to quantify it.1PubMed Central. Wild pigs, wild costs: the economic consequences of wild pig invasions in the United States Despite increasing research attention, significant gaps remain in understanding their basic biology, the full extent of their damage, and which control strategies actually work.2The Journal of Wildlife Management. Research priorities for managing invasive wild pigs in North America
Nutria are another import with outsized consequences. These large South American rodents were brought to the United States in 1899 for fur farming and quickly established feral populations in several states, particularly along the Gulf Coast and in the Chesapeake Bay region.3DigitalCommons@University of Nebraska – Lincoln. Nutria: An Invasive Rodent Pest or Valued Resource? Their voracious appetite for aquatic vegetation can destroy wetland habitats, accelerating erosion and reducing the habitat available to native species.
Japanese sika deer, introduced to the Delmarva Peninsula on the U.S. East Coast over a century ago as novelty game animals, present a different kind of problem. They hybridize with native white-tailed deer and compete aggressively with them for food and territory, raising concerns about genetic dilution and displacement of the native population.4Biological Invasions. A complete history of the establishment of Japanese sika deer on the Delmarva Peninsula: 100 years post-introduction
Burmese Pythons in the Everglades
Few invasive species stories are as dramatic as the Burmese python’s takeover of southern Florida. Native to Southeast Asia, these giant constricting snakes were released or escaped from the pet trade and have spread throughout Everglades National Park and surrounding areas. Their impact on native mammals has been catastrophic. Road surveys covering tens of thousands of kilometers found a roughly 99% decrease in raccoon sightings compared to pre-2000 levels, along with a nearly 99% drop in opossum observations and an 88% decline in bobcat sightings. Rabbits were not detected at all.5PubMed Central. Severe mammal declines coincide with proliferation of invasive Burmese pythons in Everglades National Park Surveys also showed that these mammals remain more common in areas where pythons arrived only recently and are most abundant outside the python’s current range, strongly implicating python predation as the cause.
The effects cascade beyond the animals the pythons eat directly. With mammals disappearing, mosquitoes that previously fed on raccoons and other mammals have shifted to feeding on other hosts, including reservoir species for certain diseases. The collapse of the mammal community is, in other words, reshaping disease ecology in ways scientists are still working to understand.6PubMed Central. Mammal decline, linked to invasive Burmese python, shifts host use of vector mosquito towards reservoir hosts of a zoonotic disease
European Starlings and Non-Native Birds
The European starling is probably the most famous deliberately introduced bird in North America. Native to the Palearctic region (Europe, northern Asia, and North Africa), starlings were intentionally released in New York City in the 1890s. Today they number in the hundreds of millions across the continent.7Biological Invasions. Global invasion history and native decline of the common starling: insights through genetics They compete aggressively with native cavity-nesting birds for nesting sites, sometimes evicting bluebirds, woodpeckers, and swallows from their holes. They also form enormous flocks that damage crops, particularly grain and fruit.
House sparrows are another familiar non-native. Originally from Eurasia, they were introduced to Brooklyn in the 1850s and spread rapidly across the continent. Like starlings, they compete with native species for nesting cavities. Rock pigeons, the common pigeons found in every city, are also non-native, brought from Europe and North Africa centuries ago as domesticated birds. The Eurasian collared dove, a more recent arrival, has been expanding its range northward and westward since the 1980s after being introduced to the Bahamas.
Freshwater and Marine Invaders
North America’s freshwater systems face serious pressure from multiple non-native fish species. Asian carp, a group that includes silver carp, bighead carp, grass carp, and black carp, are among the most consequential. Originally imported in the 1960s and 1970s to control algae and vegetation in aquaculture ponds, they escaped into the Mississippi River basin and have been spreading ever since. Silver carp in particular have expanded rapidly, with growth rates that have surprised researchers.8Fisheries. Growth patterns of invasive Silver Carp in the Mississippi River basin Their arrival has had measurable effects on native fish. In reaches of the Mississippi invaded by silver carp, catch rates for native gizzard shad dropped roughly in half, and bigmouth buffalo declined similarly, while the condition of both native species worsened over time, suggesting the carp were outcompeting them for food.9PLoS ONE. Incorporating basic and applied approaches to evaluate the effects of invasive Asian Carp on native fishes Black carp, which eat mussels and snails, have been found in the wild in the Mississippi basin since the early 1990s, with genetic evidence pointing to multiple separate introductions from aquaculture facilities.10Biological Invasions. Genetic analysis of invasive Asian Black Carp (Mylopharyngodon piceus) in the Mississippi River Basin
Zebra mussels, tiny freshwater mollusks from the Caspian Sea region, arrived in the Great Lakes in the late 1980s in the ballast water of cargo ships. They attach in dense clusters to any hard surface, clogging water intake pipes for power plants and municipal water systems. Damage estimates have soared into the billions of dollars.11PubMed. Zebra Mussel Invasion Threatens U.S. Waters They also strip so much plankton from the water that they reshape the entire food web, starving native mussels and fish of nutrients.
In marine waters, the Indo-Pacific lionfish has become one of the most damaging reef invaders along the Atlantic coast, throughout the Caribbean, and into the Gulf of Mexico. Believed to have been released from home aquariums, lionfish are voracious predators with no natural enemies in their new range. Research on their stomach contents shows they preferentially target certain families of native reef fish, eating some species far more often than their abundance would predict.12Global Ecology and Conservation. Patterns of predation of native reef fish by invasive Indo-Pacific lionfish in the western Atlantic Because many of their prey are juvenile herbivorous fish, lionfish can indirectly promote algal overgrowth on coral reefs, compounding existing threats from warming and acidification.
Invertebrates You Might Not Expect
When people think about invasive animals, they rarely picture earthworms. But most of the earthworms in the northern United States and southern Canada are not native. Glaciers wiped out native earthworm populations across much of the northern part of the continent during the last ice age, and European species arrived with colonists in the soil of potted plants, in ship ballast, and through the fishing bait trade. These European earthworms have dramatically altered northern hardwood forests. They consume the thick layer of leaf litter that many native plants and fungi depend on, fundamentally changing soil structure and nutrient cycling.13PubMed Central. Ecosystem responses to exotic earthworm invasion in northern North American forests The result is measurable declines in plant diversity, with species losses continuing even at sites that have been invaded for more than two decades.14PubMed Central. The long‐term effects of invasive earthworms on plant community composition and diversity in a hardwood forest in northern Minnesota
The Argentine ant, native to South America, has invaded coastal and Mediterranean climates across the United States, particularly in California. It displaces native ant species and in doing so disrupts seed dispersal, a mutualism many native plants depend on. At invaded sites, native ant seed dispersers drop by about 92%, and the Argentine ant does not pick up the slack. Seed removal rates and seedling establishment both fall.15PubMed Central. Quantitative analysis of the effects of the exotic Argentine ant on seed-dispersal mutualisms Argentine ants are too small to carry most native seeds effectively, and the native harvester ants they replace are the only ones that can.16PubMed. Invasive Argentine ants (Linepithema humile) do not replace native ants as seed dispersers of Dendromecon rigida (Papaveraceae) in California, USA In some cases, Argentine ants even preferentially disperse the seeds of invasive plants over native ones, because the invasive plants tend to produce smaller seeds that the ants can actually carry.17PubMed. New mutualism for old: indirect disruption and direct facilitation of seed dispersal following Argentine ant invasion
Even the European honey bee, so deeply embedded in American agriculture that people assume it belongs here, is an import. Colonists brought honey bees from Europe in the 1600s. Research shows that when honey bee hives are placed near native bumble bee colonies, the bumble bees shift their foraging from pollen collection to nectar collection in response to increased competition, and their reproductive success drops as a result.18Ecology. Competitive interactions between the invasive european honey bee and native bumble bees A separate study found that honey bee introductions indirectly decreased pollination of native wildflowers by reducing nectar and pollen availability and crowding out visits from native bees that were actually more effective pollinators.19PubMed. Honey bee introductions displace native bees and decrease pollination of a native wildflower
Free-Ranging Cats
Domestic cats were brought to North America by European settlers and have since become one of the continent’s most prolific predators of native wildlife. A widely cited estimate puts the annual toll at roughly 1.3 to 4 billion birds and 6.3 to 22.3 billion small mammals killed per year in the contiguous United States alone. The vast majority of this killing is done by unowned or feral cats rather than household pets.20Nature Communications. The impact of free-ranging domestic cats on wildlife of the United States Those numbers make free-ranging cats likely the single greatest source of human-linked mortality for birds and mammals in the country.21PubMed. The impact of free-ranging domestic cats on wildlife of the United States This reality sits awkwardly with how most people think about cats, but from an ecological perspective, a feral cat colony functions much like any other introduced predator.
Rats and the Cities They Built In
Both the brown rat and the black rat are Old World species that arrived in North America aboard European ships. Researchers using isotopic and protein analysis of archaeological rat remains from eastern North America, spanning from the 1550s to 1900, have traced the arrival and competition between these two species in granular detail. Brown rats arrived earlier than previously expected and rapidly outcompeted black rats in coastal urban environments, despite the two species occupying different positions in the food web.22PubMed Central. The ratting of North America: A 350-year retrospective on Rattus species compositions and competition Today brown rats dominate most North American cities, while black rats persist mainly in warmer, more southern areas. Both species carry diseases relevant to human health, making their invasion history more than an academic curiosity.
Imported Diseases That Devastate Native Wildlife
Not all non-native threats are animals themselves. Some are pathogens that humans have inadvertently moved across oceans. White-nose syndrome, caused by the fungus Pseudogymnoascus destructans, provides a stark example. Genetic evidence strongly suggests the fungus was introduced to North America from Europe, where bats have long coexisted with it without suffering mass die-offs. North American bats, having never encountered it before, had no resistance. The disease has killed an estimated five to six million bats across the continent, devastating multiple species and leaving some on the brink of collapse.23Current Biology. European Origin of White-Nose Syndrome Fungus in North America Since bats are major consumers of insects, including crop pests and mosquitoes, their decline has ripple effects across agriculture and human health.
When “Native” Gets Complicated
The question of what counts as native to North America is straightforward for species that showed up in a shipping container last century, but gets genuinely tricky when you zoom out in time. The standard definitions used by conservation organizations generally define a native species as one found within its natural range, sometimes with the added requirement that it arrived without human assistance.24Biological Conservation. What constitutes a ‘native’ species? Insights from the Quaternary faunal record That seems simple enough until you consider the deep history of the continent.
Horses are the classic case. The family Equidae evolved in North America and lived here for tens of millions of years. Genetic work on Pleistocene horse remains confirms that New World horses were genuinely North American endemics, not recent arrivals from Eurasia.25PLoS Biology. Evolution, Systematics, and Phylogeography of Pleistocene Horses in the New World: A Molecular Perspective But they went extinct on the continent roughly 10,000 years ago, and the horses here today descend from domesticated animals reintroduced by Spanish colonists in the 1500s. Are modern feral mustangs “native” to North America? Genetically they are the same genus that evolved here, but ecologically they are functioning as reintroduced animals in landscapes that have changed dramatically since the Pleistocene.
Camels are an even more surprising example. The family Camelidae originated in North America about 45 million years ago, diversified here, and eventually gave rise to two branches: one that crossed into Eurasia and became the camels we know today, and one that dispersed to South America and became llamas, alpacas, and their relatives.26Nature Communications. Mid-Pliocene warm-period deposits in the High Arctic yield insight into camel evolution Fossils of giant camels have been found as far north as the Canadian High Arctic. Like horses, camels disappeared from North America at the end of the Pleistocene.
These deep-time examples matter because they inform an active scientific debate. Some conservationists have proposed “Pleistocene rewilding,” the idea of introducing living relatives of extinct North American megafauna, like African or Asian elephants, lions, and cheetahs, to restore ecological processes that disappeared with the Pleistocene extinctions.27PubMed. Pleistocene rewilding: an optimistic agenda for twenty-first century conservation Critics point to obvious risks: altered disease dynamics, unpredictable ecological consequences, the enormous cost of secure fencing, and the social challenge of getting people to accept large predators on the landscape. It remains a fringe idea, but it illustrates how the concept of “native” can bend depending on the timescale you choose.
Even on shorter timescales, the formation of the Isthmus of Panama about three million years ago triggered what paleontologists call the Great American Biotic Interchange, a massive two-way migration of mammals between North and South America. Armadillos, opossums, and porcupines moved northward; bears, cats, and deer moved south.28PubMed Central. The Great American Biotic Interchange: Dispersals, Tectonics, Climate, Sea Level and Holding Pens The interchange was so extensive that it reshaped the fauna of an entire hemisphere, with South American species eventually colonizing tropical North America.29Annals of the Missouri Botanical Garden. THE GREAT AMERICAN BIOTIC INTERCHANGE: PATTERNS AND PROCESSES Nobody calls armadillos or porcupines “invasive” today, even though their ancestors walked in from another continent. Time, it turns out, is the unstated variable in every definition of “native.”
The Trillion-Dollar Price Tag
The economic scale of animal invasions in North America is difficult to overstate. One analysis estimated that biological invasions cost the North American economy at least $1.26 trillion between 1960 and 2017, with costs climbing from about $2 billion per year in the early 1960s to over $26 billion per year by the 2010s. Invasive vertebrates were associated with the greatest costs, and terrestrial habitats bore the heaviest financial burden.30NeoBiota. Economic Costs of Biological Invasions within North America A United States-specific analysis found that from 1960 to 2020, reported costs reached $1.22 trillion when counting only the most reliable estimates. About three-quarters of that total came from direct damages and resource losses rather than management spending. Mammals and insects were the costliest groups, and agriculture was the hardest-hit sector.31PubMed. Economic costs of biological invasions in the United States
Despite these enormous costs, federal prevention spending has historically been modest. The U.S. government’s primary tool for blocking harmful animal imports, the “injurious species” listing process under the Lacey Act, has operated on less than $500,000 per year, a tiny fraction of the damage invasive animals cause.32Biological Invasions. Invasive animals and wildlife pathogens in the United States: the economic case for more risk assessments and regulation The mismatch between the cost of invasions and the investment in preventing them is one of the more persistent frustrations in conservation policy. Every species that establishes a self-sustaining population becomes exponentially harder and more expensive to control, which means the most cost-effective action is always the one taken before the animal arrives.