Species Invading the United States

There are thousands of non-native species in the United States, many of them invasive. The National Invasive Species Council provides information on invasive plants, animals, and aquatic species, as well as state-by-state information. Below are two examples of species that have already been established in the United States and two species that are emerging threats.

Already Established

While some potentially invasive species have been kept at bay, countless others have found their way into the United States where they are causing extensive ecological and environmental harm.

Species Invading the United States
Eurasian Wild Hogs
Eurasian wild hogs (Sus scrofa, also commonly known as feral swine or feral hogs) threaten ecosystems across the continent and on islands ranging from Hawai`i to the Caribbean. There are an estimated 2.4 to 6.9 million feral hogs spread across 37 states. If no effective management actions are taken, and feral hogs spread to all available habitats, it is estimated that the U.S. feral hog population could reach 21.4 million.

Feral hogs are highly adaptable and breed prolifically. They damage vegetation and soil by rooting and wallowing. They compete with native wildlife for food, especially nuts. Pigs also eat insects, worms, crustaceans, bird and reptile eggs, even small animals. Furthermore, feral hogs can carry human and animal diseases, including coliform bacteria, Giardia, African swine fever, and Pseudorabies.

Feral hogs’ spread has been assisted by sportsmen deliberately releasing them for hunting opportunities. States have been urged to classify feral hogs as harmful and illegal to possess, move, or sell in order to reduce the deliberate release of additional feral hogs. Using these legal mechanisms in tandem with a state-led eradication program, Colorado reported in 2020 that it had eradicated feral hogs from the state after a 20-year effort.

Species Invading the United States
White-nose Syndrome
White-nose syndrome (WNS) has killed millions of bats across North America over the last two decades, making it one of the deadliest wildlife diseases in recent history. This disease is caused by a fungus, Pseudogymnoascus destructans (Pd). Federal and state natural resource agencies believe that the strain of Pd found in North America likely came from Europe and was introduced by humans who inadvertently stored Pd spores on their clothing and gear. Biologists first discovered bats sick with white-nose syndrome in caves near Albany, NY in 2007 and further evidence indicates that the disease likely arrived in North America in 2006 or earlier.

As of February 2020, bats with WNS have been found in 33 U.S. states and seven Canadian provinces. Thirteen bat species, including three threatened and endangered species, have been confirmed with white-nose syndrome in North America. Pd has also been detected in five additional states — California, Mississippi, North Dakota, Texas and Wyoming.

For the most up-to-date information about WNS, see whitenosesyndrome.org.

Emerging threats

A number of likely invasive species have made their way into the country but have not yet become widespread. Diligence, attention, and concerted efforts will be necessary to keep these species from becoming established.

Species Invading the United States
Brown Tree Snake

The brown tree snake (Boiga irregularis) caused an alarming number of bird extinctions when it was introduced to the once snake-free island of Guam during World War II. If brown tree snakes become established in Hawaii, many endemic birds will likely disappear.  The snake is particularly adept at stowing away on planes—a number have been found hidden away in landing gear after trans-Pacific flights. Ongoing efforts to prevent snakes from leaving Guam and from entering Hawaii have so far been successful.

Species Invading the United States
Asian Longhorned Tick
Native to several eastern Asian countries, the Asian longhorned tick was first detected in the United States in 2017, when a sheep in New Jersey was found infested with them. In addition to livestock, humans, pets, and wildlife have all been found carrying this tick. In other countries, the Asian longhorned tick is known to spread germs through its bites that can make people seriously ill. However, there is not yet evidence of Asian longhorned ticks transmitting disease to humans or animals in the U.S. As of February 7, 2020, longhorned ticks have been found in twelve U.S. states. The CDC recommends using EPA-approved insect repellents and permethrin-treating clothing to protect yourself from tick bites in infested areas. If you think you have found an Asian longhorned tick, please follow CDC recommendations on removing it and contacting relevant health organizations.