Invasive Species


Invasive species were officially defined in the United States on February 3, 1999, by Executive Order 13112 signed by President Clinton. The order defines an invasive species as any species that is “1) non-native (or alien) to the ecosystem under consideration and; 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health.” Invasive species can be animals, plants, fungi, or microbes.

The 1999 definition of an invasive species is not as straightforward as might be supposed because some species considered beneficial in one ecosystem may be harmful in others. One example commonly given is the lake trout (Salvelinus namaycush), which is a desirable species in the Great Lakes, where it is native, but is regarded as an invasive species in Yellowstone Lake, where it has been introduced by humans and has displaced the native cutthroat trout (Oncorhynchus clarki bouvieri).

A hiker watches a herd of wild goats run past him in the Waianae mountain range in 2013.

A hiker watches a herd of wild goats run past him in the Waianae mountain range in 2013. In August of 2018, DLNR began it's aerial eradication project, in an effort to remove destructive animals in O'ahu's native watersheds in the Waianae Kai Forest Reserve.
(ª 2013 Kelly A. Quin)

In addition, the perception of “causing harm” may differ from one human group to another. Some environmentalists prefer the term pest species to describe organisms of this type. One example is the Canada goose (Branta canadensis), which is technically native to North America and was considered endangered as recently as the 1980s. Since 2000, however, the expansion of geese populations along the East Coast of the United States has led to complaints about the birds' aggressive behavior, their learned habit of begging for food, their droppings, and the bacteria carried in their droppings. As a result, some states now permit hunters to trap or shoot the geese for recreation. In addition to their nuisance behavior on suburban lawns and sidewalks, Canada geese are a threat to human life because they are the species most often implicated in bird strikes at airports. They caused the engine failure of United Airways Flight 1549, which was forced to ditch in the Hudson River in 2009 with no loss of human life, and they also caused the crash of an Air Force jet at Elmendorf AFB in Alaska in 1995 with the loss of all 24 crew members.


Invasive species can originate almost anywhere on Earth and find their way to other continents and countries by air and water transport. Although the movement of plant, animal, and microbe species began with the earliest human migrations from southern Africa into Eurasia and then the Americas, the relatively primitive methods of early human transportation (foot, cart, and horseback) were slow and restricted in their geographic range.

The development of reliable seagoing vessels as early as 3000 BC added another means of transport as well as encouraging trade, which provided an important financial motivation for travel and exploration. As is well known, many plant and animal species were introduced into Europe after the discovery of the Americas in the fifteenth century, and many European species were brought to the Americas by European explorers and colonists. One example of an invasive species in this context is the pigs introduced into what is now Florida by the Spanish explorer Hernando de Soto (c. 1495–1542). De Soto had brought the pigs from Spain to feed his troops, but some escaped, turned feral, and became the ancestors of presentday feral pigs in the southeastern United States.

The invention of the railroad, automobile, and airplane in the nineteenth and twentieth century not only vastly speeded up the movement of species from one location to another but also increased the range of species movements worldwide through affordable tourism as well as trade. As of 2018, there is no place on Earth that is exempt from the introduction of invasive species. Even Antarctica, long regarded as inhospitable to most forms of life, has some species now considered invasive, such as the king crab (Neo-lithodes yaldwyni) and the chironomid midge (Belgica antarctica), a type of small fly.

The Jackson's chameleon is an invasive species that was introduced to Hawaii in the 1970s, and primarly found at altitudes of 100 to 1,000 m (330 to 3,280 ft) in wet, shady places.

The Jackson's chameleon is an invasive species that was introduced to Hawaii in the 1970s, and primarly found at altitudes of 100 to 1,000 m (330 to 3,280 ft) in wet, shady places. They are a threat to native biodiversity of invertebrates and to endemic species like the critically endangered O'ahu tree snails, January 13, 2013.
(© 2013 Kelly A. Quin)

According to the U.S. Environmental Protection Agency (EPA), invasive species can enter or spread within the United States by any of the following means:


The global cost of invasive species runs into billions in all the world's currencies. One British document reports that INNS cost the United Kingdom at least 1.7 billion pounds (about $2.31 billion US) per year. The Invasive Species Centre in Ontario states that the cost of invasive species to the Canadian economy is between $16.6 billion and $34.5 billion per year. According to a 2017 report to Congress, over 50,000 non-native species have been introduced into the United States, with the cost of these species having a minimum price tag of $138 billion per year.

One important measurement of the demographics of invasive species is the so-called 10% rule, which holds that only 10% of non-native species introduced into a new ecosystem survive at all; and of those 10%, only 10% (or 1% of the original number of species) become invasive. Nonetheless, even that low final percentage adds up to thousands of invasive species in North America alone.


Few people introduce a new species into an established ecosystem intending to cause harm. They may do so, however, for a number of reasons:

Causes and symptoms/effects


Human travel, trade, and recreational activities are the basic cause of the introduction of invasive species, whether intentional or inadvertent.

In addition, however, invasive species share a number of traits that enable them to compete successfully with native species:


INNS have a number of negative effects on the environment, human and animal health, international relations, infrastructure, and military readiness:

The crossing of a first-generation hybrid plant or animal with one of its parental types.
Ballast water—
Water held within a specialized compartment, the ballast tank, within a boat or ship. Ballast water provides stability for the vessel when navigating along the coast or in the open ocean.
Refers to the variety and variability of living species. In general, biodiversity is highest in tropical climates, which cover 10% of Earth's surface but include 90% of its species.
Terrorism involving the intentional dissemination of bacteria, viruses, toxins, or other biological agents.
A viral infection characterized by fever, joint pain, headache, and rash.
Ecological competence—
The ability of an organism to survive and compete in a new habitat.
A community of organisms interacting with a specific environment that also includes such nonliving elements as air, water, and soil.
The intentional complete removal or destruction of an invasive species.
The phase in the hormonal cycle of female mammals when they are receptive to mating.
Describes an animal living in the wild but descended from domesticated individuals. A wide variety of animals can become feral, including pigs, goats, dogs, cats, parrots, doves, pigeons, donkeys, horses, water buffalo, and camels. Feral animals can become invasive by either preying on native species or by introgressive hybridization.
A bacterial disease of horses characterized by coughing, fever, and an infectious discharge followed by blood poisoning and death (in acute cases). Glanders can be transmitted to humans, although there were no human cases in the United States between 1945 and 2018.
Introgressive hybridization—
The spread of genes from one population into the gene pool of another as the result of hybridization between two populations of different sizes, in which backcrossing prevents the establishment of a single stable population.
The practice of eating invasive species in order to reduce, control, or eliminate their population.
Noxious weed—
Any plant that has been designated by an agricultural authority as harmful or injurious to crops, human or animal health, or natural habitats or ecosystems. In the United States, the Federal Noxious Weed Act of 1974 gave the secretary of agriculture the authority to declare plants noxious weeds and limit the interstate spread of such plants without a permit.
A general collective term for military supplies, including weapons, vehicles, maintenance equipment, and ammunition.
Pest species—
Any species, whether native to an ecosystem or introduced, that has become troublesome or harmful. The term is less precise than invasive species because an organism can be beneficial in one ecosystem but harmful in another.
Also known as cattle plague, a viral infection of cattle, antelopes, buffalo, and deer that was nearly 100% fatal. Although rinderpest was declared eradicated by the UN in 2011, stocks of the virus continued to be maintained in a few specialized laboratories and could be used as a biological weapon.
Ten percent rule—
A general rule for estimating the likelihood of an introduced species' becoming invasive. The rule states that only about 10% of new species introduced into a new ecosystem will survive at all; and of that number, only 10% (1 %) of the original number of non-native species will become invasive.
In epidemiology, any animal or microorganism that transfers an infectious disease agent into another organism.
Zoonosis (plural, zoonoses)—
Any of a number of diseases transmitted to humans from animals. Diseases that can be transmitted from humans to animals are called reverse zoonoses.

Countermeasures against invasive species

There is no single course of action against INNS that will cover all situations because there are so many different invasive species and because each threatened ecosystem is unique. Most plans, however, take one or more of the following approaches.

Prevention Early detection and rapid response

Early detection and rapid response (EDRR) includes monitoring of public and private lands and waters for evidence of invasive plant, animal, or insect species. Although much of EDRR monitoring is done by field biologists, veterinarians, and other specialists working for various state and federal agencies, anyone in the United States who notices signs of an infestation can take a photo or sample of the species involved and submit it to the closest APHIS office.

If complete eradication of the invasive species is feasible, it can be done by a variety of methods ranging from manual removal (digging, flooding, mulching, burning, removal of alternate hosts and destruction or removal of nests, egg masses or other life stages) to mechanical (mowing, chopping, tilling, and constructing barriers) and chemical methods (application of pesticides, herbicides, fungicides, insecticides, or sex hormones) to controlled burns and destruction of the plant or animal matter. In 2006, the government of Ecuador achieved complete success in eradicating the feral goats on the Galápagos Islands by sterilizing the females, injecting them with hormones to keep them in estrus, and then shooting the male goats when they approached to mate with the females.

Control and management

In many cases, however, eradication of the INNS is not possible either because the invasive species is well established in a sizable area or because different groups in a given society disagree about the need for complete removal of the invader. People may have different opinions about the extent of harm caused by a species that may be perceived as beneficial in some respects. One example is the disputes in many communities over feral cats, defined as “cats that live outdoors and have had little or no human contact,” as distinct from strays, which are cats that lived with humans at one time but are now homeless. While bird lovers often urge shooting or euthanasia of feral cats on the grounds that they prey on songbirds, other observers favor trapping and neutering followed by returning the cats to managed colonies—a strategy known as TNR. In some areas, TNR has proved not only to be a humane approach but has proved beneficial as a pest control strategy for keeping barns, military installations, and urban buildings free of rats and mice.

As of 2018, one new method of controlling invasive species was called invasivorism, or using invasive species as a human food supply. Chefs in a number of restaurants worldwide have begun to use such INNS as zebra mussels, Asian carp (Cyprinus carpio), lionfish, rusty crayfish (Orconectes rusticus), and the Chinese mitten crab (Eriocheir sinensis) to add variety to their menus.

Restoration and rehabilitation

The fourth approach to invasive species is the restoration of land areas, forests, or waterways that have been damaged by INNS. A major reason for ecological restoration is that INNS often take advantage of natural disturbances (floods, hurricanes, forest fires, volcanic eruptions, tsunamis, etc.) to invade ecosystems that were formerly resistant to them. In addition, invasive species often degrade an ecosystem to the point that some type of human intervention is needed to restore it to a healthy and stable condition. Third, restoration projects add to scientists' understanding of the ways in which species diversity contributes to making an ecosystem resistant to invasive species.



Anthony, Leslie. The Aliens among Us: How Invasive Species Are Transforming the Planet—and Ourselves. New Haven, CT: Yale University Press, 2017.

Jeschke, Jonathan M., and Tina Heger, editors. Invasion Biology: Hypotheses and Evidence. Boston: CABI, 2018.

Mazza, Giuseppe, and Elena Tricarico, editors. Invasive Species and Human Health. Boston: CABI, 2018.

Pitt, William C., James C. Beasley, and Gary W. Witmer, editors. Ecology and Management of Terrestrial Vertebrate Invasive Species in the United States. Boca Raton, FL: CRC Press, Taylor and Francis Group, 2018.


Baker, B. G., J. Bedford, and S. Kanitkar. “Keeping Pace with the Media: Giant Hogweed Burns—A Case Series and Comprehensive Review.” Burns 43 (August 2017): 933–38.

Bradbeer, D. R., et al. “Crowded Skies: Conflicts between Expanding Goose Populations and Aviation Safety.” Ambio 46, Suppl. 2 (March 2017): 290–300.

Carrion, V., et al. “Archipelago-wide Island Restoration in the Galápagos Islands: Reducing Costs of Invasive Mammal Eradication Programs and Reinvasion Risk.” PLoS One 6 (May 11, 2011): e18835.

Jones, H. P., et al. “Invasive Mammal Eradication on Islands Results in Substantial Conservation Gains.” Proceedings of the National Academy of Sciences of the United States of America 113 (April 12, 2016): 4033–38.

Lama, J. K., and D. S. Bachoon. “Detection of Brucella suis, Campylobacter jejuni, and Escherichia coli Strains in Feral Pig (Sus scrofa) Communities of Georgia.” Vector Borne and Zoonotic Diseases. Published electronically April 26, 2018. doi: 10.1089/vbz.2017.2187.

Lampert, A., et al. “Optimal Approaches for Balancing Invasive Species Eradication and Endangered Species Management.” Science 344 (May 20, 2014): 1028–31.

Löhmus, M., et al. “Rodents as Potential Couriers for Bioterrorism Agents.” Biosecurity and Bioterrorism 11, Suppl. 1 (September 2013): S247–257.

Ravet, K., et al. “The Power and Potential of Genomics in Weed Biology and Management.” Pest Management and Science. Published electronically April 24, 2018.


Centre for Agriculture and Biosciences International. “Invasive Species: The Threat to Livelihoods.” This is a 6-page illustrated brochure that explains the economic impact of invasive species on developing countries. (accessed May 2, 2018).

Congressional Research Service. “Invasive Species: Selected Laws and the Role of Selected Federal Agencies.” (accessed May 3, 2018).

Department of Defense. “Department of Defense: Invasive Species Challenges and Solutions.” (accessed May 3, 2018).

Environmental Protection Agency (EPA). “Invasive Non-Native Species.” (accessed May 1, 2018).

Global Invasive Species Programme. “Invasive Alien Species: A Growing Global Threat.” (accessed May 2, 2018).

Live Science. “On the Menu: Taking a Bite Out of Invasive Species.” (accessed May 4, 2018).

National Invasive Species Council. “NISC Management Plan, 2016–2018.” (accessed May 1, 2018).

National Invasive Species Information Center. “What Is An Invasive Species?” (accessed April 20, 2018).

National Wildlife Federation. “Invasive Species.” Threats-to-Wildlife/Invasive-Species (accessed April 30, 2018).

U.S. Forest Service Invasive Species Program. “Playing Smart Against Invasive Species.” This is a 26-minute video about the importance of people taking preventive measures against introducing invasive species when enjoying outdoor recreation areas. (accessed April 30, 2018).


Center for Invasive Species and Ecosystem Health, University of Georgia, Warnell School of Forestry and Natural Resources, 180 East Green St., Athens, GA, 306022152, (706) 542-2686, Fax: (706) 542-8356,, .

Centers for Disease Control and Prevention, 1600 Clifton Rd., Atlanta, GA, 30329, (800) CDC-INFO, .

Centre for Agriculture and Biosciences International, Nosworthy Way, WallingfordOxfordshire, OX10 8DE, United Kingdom, +44 (0) 1491 832111, Fax: +44 (0) 1491 833508, , .

Global Invasive Species Programme, United Nations Avenue, PO Box 633-00621, Nairobi, Kenya, +254 20 722 4462/50, Fax: +254 20 712 2150,, .

Invasive Species Centre, 1219 Queen St. East, Sault Ste. Marie, P6A 2E5, CanadaOntario, (705) 541-5790,, .

National Invasive Species Council Secretariat, U.S. Department of the Interior, Office of the Secretary, 1849 C St. NW, Mailstop 3530, Washington, DC, 20240, , .

National Wildlife Federation, 11100 Wildlife Center Dr., Reston, VA, 20190, (800) 822-9919, , .

U.S. Department of Agriculture, National Invasive Species Information Center, Room 109-HH, National Agricultural Library, 10301 Baltimore Ave., Beltsville, MD, 20705, (301) 504-5755,, .

U.S. Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC, 20460, (202) 564-4700, .

U.S. Forest Service Invasive Species Program, 1400 Independence Ave. SW, 3rd Fl. SW, Mailstop 1153, Washington, DC, 20250-1153, .

Rebecca J. Frey, Ph.D.

  This information is not a tool for self-diagnosis or a substitute for professional care.