The history of invasive species is usually one of unforeseen consequences. When an animal, fish, insect or plant is taken out of its original ecosystem and introduced to a new one—whether by accident or on purpose—it's less likely to have any natural predators. 

Which can lead to environmental havoc. 

Without anything to keep their population in check, some invasive species—especially the prolific breeders—often flourish. They can destroy native plants, gobble up native animal populations and introduce disease, upending the delicate balance of organisms that provide food or support for each other, or provide a check on each other’s growth. Extinctions have proliferated.

Globalization has fueled the problem of invasive species. When European colonizers sailed to the Americas, they disrupted existing animal populations while also introducing new ones. Invading creatures have long affected the United States, as people imported new animals for study, sport, fur or even the love of Shakespeare. (Yes, really.) Here are seven invasive species that still pose a threat to the U.S. today.

1. Feral Swine (Sus scrofa)

Feral Swine (Sus scrofa)
Kristian Bell/Getty Images

Other names: Wild or feral boars, hogs or pigs; Eurasian or Russian wild boars

Originate from: Parts of Europe, Asia and North Africa

Reason in U.S.: European settlers brought them for food beginning in the 1500s; others brought them for sport hunting in the 1900s

Destructive superpowers: Devour crops and native vegetation

Newsworthy moment: Twitter's 2019 viral meme of '30 to 50 feral hogs'

The very real problem of invasive feral swine went viral in August 2019 when Twitter user @WillieMcNabb of Arkansas tweeted: “How do I kill the 30-50 feral hogs that run into my yard within 3-5 mins while my small kids play?” The phrase “30 to 50 feral hogs” quickly became a meme; and while it’s unlikely that many wild hogs actually run into McNabb’s yard at once, the discussion did highlight the growing issue of wild hogs in the U.S.

Feral swine are the same species as the pigs found on farms, and are descended from farm escapees and/or Eurasian or Russian wild boars brought to the U.S. for sport hunting in the 1900s. The U.S. Department of Agriculture estimates there are at least 6 million feral swine spread throughout some 35 states. They have been a particularly virulent problem throughout the south, especially in Texas, where their incessant rooting and voracious eating destroy crops, erode soil and uproot tree seedlings, causing deforestation. They also carry disease like pseudorabies and swine brucellosis. The U.S.D.A. estimates, conservatively, that invasive swine cause upward of $1.5 billion in damage annually to all manner of agriculture, including rice, corn and grains.

2. Burmese Pythons (Python bivittatus)

Burmese Python (Python bivittatus)
Rhona Wise/AFP/Getty Images

Originate from: Southeast Asia

Reason in U.S.: Exotic pet trade

Destructive superpowers: Annihilating native small mammal populations

Newsworthy moments: Killed a two-year-old child; swallowed three deer

In 1979, staff members at Everglades National Park discovered Florida’s first recorded python, which was likely a Burmese python. By 2000, Florida was receiving reports of established populations of Burmese pythons in the state. These snakes, which can grow up to 20 feet long or more, were brought to Florida as part of the exotic pet trade. But many owners released the huge creatures, which reproduce rapidly; females are known to produce 50-100 eggs per year. With no predators on this continent, these slithery gluttons have since become a danger to native species, devouring more than 90 percent of small and medium-sized mammals in the Everglades. Some are more ambitious: One killed and swallowed three deer—a doe and two fawns—over a three-month period.

While not known to be a threat to humans, there have been isolated incidents. In 2009, a pet Burmese python broke out of its terrarium and strangled a two-year-old girl to death.

3. Domestic Cats (Felis catus)

Domestic Cats (Felis catus)
Malivoja/Getty Images

Other names: feral cats, outdoor domestic cats

Originate from: North Africa and Southwest Asia

Reason in U.S.: European settlers brought them as pets

Destructive superpowers: Kill native birds and mammals

Newsworthy moment: Author Jonathan Franzen saying 'cats must die'

Cats are a beloved pet in the U.S., which is why it comes as such a shock to many Americans that they are seen as a destructive invasive species here as well as in the U.S. and many other parts of the world. Historians believe ancient Egyptians were the first to domesticate cats, and that these pets spread to Europe during the Roman Empire. When Europeans colonized North America, they brought cats with them as pets.

Since then, these animals have flourished while harming native species. In 2013, a paper in Nature Communications estimated “free-ranging domestic cats kill 1.3–4.0 billion birds and 6.3-22.3 billion mammals annually.” And the American Bird Conservancy estimates that cats have contributed to the extinction of more than 60 species of birds, mammals and reptiles. Author and bird lover Jonathan Franzen, for one, has drawn criticism for his statements on this issue. Back in 2013, he told New York magazine: “The bird community’s position is, we need to get rid of the feral cats, and that means cats must die.”

4. European Starlings(Sturnus vulgaris)

Starlings (Sturnus vulgaris)
Al Hedderly/Getty Images

Other names: Common starling, English starling

Originate from: Parts of Europe, Asia and Africa

Reason in U.S.: A Shakespeare fan went too far

Destructive superpowers: Destroy crops

Newsworthy moment: In 1960, a swarm of 10,000 starlings flew into a passenger plane taking off from Boston’s Logan Airport, disabling the engines and causing a crash that killed 62 people.

Eugene Schieffelin was such a fan of William Shakespeare that he decided to introduce a bird mentioned in his play Henry IV into the U.S. In 1890 and 1891, Schieffelin unleashed approximately 100 imported Europeans starlings into New York City’s Central Park. 

Bad idea. 

Within 50 years, they had spread across the continent. Today, there are more than 200 million European starlings in North America; considered noxious and destructive, they compete with native species and destroy crops such as grains and pitted fruits. They swarm agricultural feeding troughs, contaminating food and water, and are linked with diseases like histoplasmosis, a lung ailment afflicting agricultural workers.

5. Nutria (Myocastor coypus)

Nutria (Myocastor coypus)
Alexander Sorokopud/Getty Images

Other names: Coypu, swamp rats, river rats

Originate from: South America

Reason in U.S.: Brought for fur trade

Destructive superpowers: Damage crops and natural resources

Newsworthy moment: A Louisiana hunter, likely incentivized by the state's $6-per-tail bounty on the swamp rats, handed in nearly 11,000 tails to the 2019 nutria culling program.

Described by some as a cross between a beaver and a sewer rat, with large orange buck teeth, nutria can grow as large as 20 pounds. They came to Louisiana in the early part of the 20th century as part of the fur trade, but as the market declined, traders released them into the wild, where they have wreaked havoc and spread to each coast. They not only damage crops and other property; they gnaw prodigiously through wetland plants, causing significant soil erosion and turning swamps into open waters; their extensive burrow systems can destabilize roads, bridges, levees and golf courses; and they can transmit diseases like tuberculosis. 

California actually eradicated nutria in the 1970s, only to discover them again in the state’s San Joaquin Valley in 2017. Since then, California has relaunched efforts to eliminate these rodents, which can be prolific breeders.

6. Asian Carp (multiple species)

Asian Carp
Benjamin Lowy/Getty Images

Originate from: Different parts of Asia

Reason in U.S.: Introduced into the U.S. in the 1970s to help keep fish farms clean

Destructive superpowers: Out-compete native fish for food and habitat

Newsworthy moment: Wildlife authorities cautioning against massive carp leaping into people’s boats

In the U.S., there are four invasive species collectively known as "Asian carp”: bighead carp, black carp, grass carp and silver carp. In the 1970s, the U.S. imported them to help keep aquatic farms clean, control weeds in canal systems and help with sewage treatment. However, they escaped into the Mississippi River basin, scarfing up the base of the aquatic food chain, with some species eating as much as 20 to 100 percent of their body weight daily. They have been outbreeding native species and pushing them out of their own ecosystem, while expanding into new waterways. Silver carp, which have a disconcerting habit of leaping out of the water, have been seen as a hazard to boaters and anglers.

7. Gypsy Moths (Lymantria dispar)

Gypsy Moths (Lymantria dispar dispar)
Ed Reschke/Getty Images

Originate from: Europe and Asia

Reason in U.S.: French artist and astronomer accidentally let them loose

Destructive superpowers: Strip trees of leaves and make them vulnerable to disease

Newsworthy moment: Defoliated more than 2 million acres in three years alone

French artist and astronomer Étienne Léopold Trouvelot first brought gypsy moths to the U.S. in the late 1860s to see if they could produce textiles better than silkworms. Unfortunately, they blew out of the building he was living in near Boston—oops!—and spread into the Massachusetts countryside. From there, the voracious gypsy moths, which appear initially as spiky caterpillars, have spread throughout the northeastern U.S., threatening more than 300 species of trees and shrubs, reducing animal habitats and presenting an increased fire hazard. They are particularly fond of oak trees.

They’re also an expensive pest to deal with: Between 1980 and 1994 alone, the U.S. spent an average of $30 million annually on gypsy moth eradication, suppression and research.


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