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Exotic Species
To learn more about the major threats to aquatic biodiversity please select from the following:
A leading cause of biodiversity loss in many aquatic ecosystems is the
introduction of exotic species. An exotic species is a non-native plant
or animal deliberately or accidentally introduced into a new habitat.
Such species include plants, fishes, algae, mollusks, crustaceans, bacteria,
and viruses. Such species that are able to reproduce and survive outside
of the habitats where they evolved are also referred to as alien, introduced,
invasive, non-native, or non-indigenous. (See the Minnesota DNR in their "Exotic
Aquatic Plants and Animals Guide". 
and EPA's Web Site on "Invasive Species")
There are many ways that exotic species are introduced into freshwater
areas including the release of pet fish or foreign species brought in
for entertainment in public or commercial aquariums or for education.
In addition, the demand for bait fish, the captive rearing of fish and
the unintentional transfer of species by ship
ballast water (link is to the Coast Guard "Ballast Management" Web site) all contribute to the introduction of exotic species in freshwater areas (Crossman
1991). However, typically exotic species are introduced into marine
areas by means of transplanting or commercial shipping. It has been reported
that ship ballast water is responsible for the transport of approximately
3,000 species worldwide each day.
Exotic species can have many negative impacts on the environment, the economy, and human health. When species are introduced into an area, they may cause increased predation and competition, disease, habitat destruction, genetic stock alterations, and even extinction. Approximately 68% of fish species lost in North America over the last century were caused by an invasion of exotic species (Miller et. al, 1989). The invasion of exotic species has also caused the economy to suffer through the obstruction of industrial and municipal water pipes and the displacement or elimination of important commercial and sport fishing species. Public health may also be negatively impacted. For example, in a number of coastal areas in the United States, cholera strains carried in the ballast water of some commercial trade ships contaminated numerous oyster and fin-fish populations, making them unsafe for consumption.
Read about one of the latest threats to aquatic ecosystems in many states - the Northern Snakehead fish.
Learn about the Exotic
Zebra Mussel from the U.S. Department of Agricultural Library.
How Exotic Species Reach Rivers, Streams, and Lakes
The most common ways for exotic species to arrive in aquatic habitats are:
-
Ship ballast. Ships take on water in special holding tanks, to balance their loads. When they reach another port and take on more cargo, they release their ballast. That ballast water mixes with the harbor water, bringing with it any organisms it contains. According to a Sea Grant study, approximately 2.4 million gallons of ballast water arrive in U.S. harbors from foreign ports every hour.
Ship surface. Mollusks and other clinging organisms may attach themselves to a ship in one port, then either fall off or be removed during cleaning at another port, entering the water. Canals have provided additional entry.
-
Deliberate introduction for sport. Some sport fishers have illegally imported favored species to a lake or river closer to their homes, in hopes of gaining for successful, convenient fishing.
-
Unintentional introduction. Cultivated plants (originally imported from other countries) sometimes spread to the wild; aquaculture and aquarium organisms may reach more open water.
How Exotic Species Harm Rivers, Streams, and Lakes
Non-native species have earned their negative reputation as "invader species" because of their great success at survival. A non-native animal may survive better than a native, not only because it has no natural enemies in the new environment, but because it grows more quickly or in less favorable conditions than natives. This causes increased competition for resources by native species.
Exotic plants will:
- Crowd out sunlight and nutrients from other plants
- Crowd out other plants, jeopardizing animals dependent on native vegetation
- Overgrow, leading to excessive plant growth, which in turn leads to decay and excess oxygen depletion, which results in fish losses.
- Crowd out navigation channels
- Clog machinery
Exotic animals tend to:
- Consume food sources that native species would eat, leaving insufficient food.
- Occupy safe or supportive habitat, leaving a reduced amount of habitat for natives.
- Serve as food for native species but lack certain essential nutrients, leading to death of native offspring.
- Consume eggs, young, and adults of native species
Losses for humans from exotic species include:
- High transportation and navigation costs
- Reduced food availability for subsistence fishing
- Reduced commercial fishing
- Reduced sport fishing
- Reduced water quality or supply
Additional Resources
- To find out more about the impacts of introduced species in the United
States visit the U.S.
Global Change Research Information Office (GCRIO).
- If you would like to see what some exotic species look like, visit the U.S. Environmental Protection Agency's Exotic Species page or learn more about exotic species through EPA's watershed academy module on Invasive Non-Native Species.
- Learn more about exotic species at the Aquatic
Nuisance Species Task Force website.
- U.S. Fish and Wildlife Service
Invasive Species Program and their Division of Environmental
Quality's Invasive
Species page.
- USGS South Florida
Restoration Science Forum
- National Sea Grant Non-indigenous
Species Sites
- The Nature Conservancy's America's Least Wanted: Alien Species Invasions of U.S. Ecosystems
- Great Lakes Panel on Aquatic Nuisance
Species
- Sea Grant Nonindigenous
Species Site
- hosts various papers including 'Nonindigenous Aquatic Species in a United States Estuary: A Case Study of the Biological Invasions of the
San Francisco Bay and Delta'