Alien Species

The introduction of non-native species to an ecosystem is one of the major causes of decreased biodiversity.

Termed alien species, they are also known as exotic, introduced, non-indigenous, or invasive species. As the names imply, these species do not belong to ecosystems in which they are either intentionally or unintentionally placed. They tend to disrupt the ecosystem’s balance by multiplying rapidly. These species are often plants, fishes, mollusks, crustaceans, algae, bacteria or viruses.

Sea LampreyMany alien species are transferred into marine ecosystems through the ballast water transported during commercial shipping operations. Ship ballast water may transport up to 3,000 species around the world every day. Alien species are often introduced into freshwater ecosystems such as estuaries, rivers, lakes and streams by humans discarding animals or plants formerly held in captivity. In some cases, species used as bait can invade freshwater ecosystems. Well-known invasive species include the Northern Snakehead fish, the Zebra mussel, the Sea Lamprey and the Asiatic Clam, Corbicula fluminea.

Effects on Humans

The introduction of an alien species is often responsible for an increase in predation and competition, habitat reduction, a variety of diseases, extinction of native plants or animals and genetic change in populations. Certain strains of cholera have been transported in ballast water, ending up in oyster beds and infecting finfish destined for the dinner table.

Alien Species Aboard

Alien species are often transported to non-native habitats in the ballast of ships. The organisms are taken in when ships attempt to balance their load by letting water into their holding tanks. When they reach their destination, the ballast water is released and with it any organisms picked up earlier. Mollusks and other organisms whose habitat includes marine substrate also attach to the surfaces of ocean-going vessels at the point of departure and then fall into the water at the destination.

Unintentional Introductions

Invasive algae - CaulerpaAquarium plants and animals, such as the invasive algae Caulerpa, as well as ornamental plants like the purple loosestrife are released innocently into waterways by humans. They quickly overgrow and eventually choke native plants and even interfere with the water flow of lakes, rivers, estuaries, and streams. Unwanted exotic fish, such as the red lionfish, Pterois volitans, have invaded the waters of the Southeastern United States. The introduction of this non-native species may cause problems becaues of its poisonous spines that divers or swimmers may be unaware of and it may also pose a risk to native species through predation or competition.

Intentional Introductions

Alien species like the cane toad have been introduced intentionally to reduce the number of a native species in the area. Unfortunately, this plan can backfire when the animal multiples quickly and takes over the habitat and beyond. In Hawaii, for example, the mongoose has eliminated many species of birds but it was originally introduced to keep the rat population down. The kudzu plant that blankets much of the southern United States was the result of a program sponsored by the government to control erosion.


New seaways or cross-basin connections provide a way for alien species to cross over into novel territories. The Great Lakes became significantly invaded by alien species following the creation of the St. Lawrence Seaway in 1959.

Wetlands filled with foreign soil are invaded by seeds and roots of plants from other ecosystems.

The Food Industry

swamp eelSeafood like shrimp, oysters, and Atlantic salmon is farmed in non-native areas like the Pacific Northwest. When juveniles escape into waterways, the potential for spread of disease and harm to the ecosystem is created.

Animals like the northern snakehead fish and Asian swamp eel were introduced as a potential source of food, but they soon overwhelmed the ecosystem forcing local communities, scientists, and policy makers to find ways to control them.

Diseases, such as whirling disease, which infects rainbow trout, can infect native species when introduced by alien species, in this case the European brown trout imported from Europe.

Seafood is sometime packed in seaweed, which houses alien species that are subsequently introduced to new ecosystems.

The Fishing Industry

Recreational fisherman have introduced alien species to their favorite fishing hole so that there are an abundance of fish available to catch.

Fish bait, such as crayfish, minnows, and earthworms are often thrown overboard, introducing them to new ecosystems that cannot support them. Earthworms have depleted the topsoil in some northern U.S. forests reducing the organic matter available for native species.


Sewage and wastewater contain seeds and roots of invasive species that are discharged into waterways and transported by water flow to the ocean.

Voracious Invaders

Alien species are often able to survive better than native species, which results in increased competition among native species. Alien plants take over the areas with abundant sunlight and use up nutrients essential for other plants. They can also deplete oxygen in the water causing a hypoxic environment that suffocates other marine life.

Research and Prevention

Research known as vector ecology is currently taking place to determine exactly how alien species are introduced. Scientific studies in the population ecology of alien species is helping to understand why some species thrive in non-native environments and what impacts they’re having on native species. Research in biogeography provides important data about global distribution patterns of alien species and databases with organized information make it possible for scientists to compile and analyze the data to help shape future practices designed to avoid introduction of alien species.

EPA Biodiversity – Invasive Non-Native Species
EPA Invasive Species
EPA Invasive Species – Pathways for Invasive Species Introduction
The Marine Invasions Research Lab – a national and international center for research on biological invasions in coastal marine ecosystems.
Big Trouble from Little Squirts – WHOI scientist investigates a troublesome invasive species
What Triggers Mass Extinctions? Study Shows How Invasive Species Stop New Life