Native species: a species that, other than as a result of an introduction, historically occurred or currently occurs in an ecosystem.


Alien/Introduced species: A species introduced and occurring in locations beyond its known historical range. Such a species can have either positive or negative effects.


Noxious species: A species that has been introduced to an area and has detrimental effects, but has not become widespread.


Invasive species: A species which has been introduced to an area, has detrimental effects, and has become so dominant it can out-compete native species.



A species can be introduced intentionally or accidentally. Intentional reasons for introduction include landscape restoration, biological pest control, sport, pets, and food processing.


PATHWAYS are the means by which species are transported from one location to another.


Natural pathways include wind, currents, and behavioral, possibly biological, adaptation by a species.


Man-made pathways are created by human activity. These can be:


Intentional. Living seeds, whole plants, or pets are sometimes intentionally introduced to an area.

Unintentional. Examples of unintentional pathways are ballast water discharge, organisms living in soil associated with the trade of gardening materials, plant pests living in introduced fruits and vegetables, and generally by the international movement of people.


Humans are the primary agents of dispersal of non-native species, both by accidental and deliberate introductions.


Major pathways include:


Aquarium trade: Wholesale importers, culture facilities and retail pet stores transport and sell non-native fresh and salt water plants, fish and invertebrates. These can escape or be released into the environment.


Biological control: Selected non-native species, usually target predators, are intentionally introduced in an effort to control the growth and spread of other introduced species. This doesn't always work out as planned as these selected species can become noxious or invasive.


Boats and ships: Ballast discharge and hull fouling are two ways boats and ships can introduce organisms. Ballast water is water pumped into tanks of ships to provided stability and balance when the ships are not carrying a full cargo load. It can contain aquatic plants, animals and pathogens. Recreational boaters and sea or float planes transport species in bait buckets or boat wells, often without realizing it. Hull fouling refers to the way organisms can attach themselves to the hull of a ship during a voyage and transport themselves long distances this way. Aquatic plants, in particular, are easily transported when plant fragments get tangled on boat propellers and fishing gear of recreational boats.


Channels, canals, locks: The building of channels, canals and locks creates artificial connections between waterways, allowing the free movement of species across physical barriers. It also facilitates the transport of species by vessels.

Local example: Before the development of canals connecting the Mississippi River's tributaries with Lake Michigan, a natural protective barrier between the two watersheds existed, preventing the interchange of aquatic organisms. Construction of artificial waterways destroyed the natural barrier, providing a transportation route for travel of non-native species from the Great Lakes to the Mississippi River.


Live bait: Recreational fishermen buy live worms and other aquatic organisms. Both the bait species and its packing material can result in introductions through intentional and accidental release.


Nursery industry: Nurseries, garden centers and mail-order catalogs sell non-native plants for aquatic gardens and ponds. Non-native plants are dispersed when discarded in public waterways, or when accidentally attached to other horticultural species.


Scientific research institutions, schools and public aquariums: Private and public research laboratories, schools and aquariums use non-native species for testing, teaching and research. Accidental release of specimens can occur when strict protocols for animal management aren't followed or when protocols do not exist. Specimens may also be intentionally released or may escape.


Recreational fisheries enhancement: U.S. federal and state agencies have in the past imported fish species to enhance recreational fishing, resulting in accidental release and unplanned spread of some species. Private citizens may also transport and release fishes into a body of water in hopes that the fish will breed, allowing the fishermen to catch a fish they wanted to but normally couldn't have caught.


Restaurants, seafood retail and processing: Packing materials for live seafood such as seaweed and sea water contain a number of living organisms and provide an opportunity for species introductions when unused product, packing materials and shipping containers are disposed of improperly. Live organisms either in or on live seafood may pose an additional threat.

(information gathered from: http://www.wsg.washington.edu/outreach/mas/aquaculture/pathways.html)


Interestingly, the recent improvement in water quality of lakes and rivers (due to the environmental movement) has made it easier for introduced species to dominate an area. In some areas, the years of poor water quality significantly reduced the population of native species. As the water quality improves, an introduced species can move into the area without much competition.





Second only to land clearing and development, invasive species pose the greatest threat to the native species in the United States, and many other areas of the world.

Cornell University research shows that there are approximately 50,000 foreign species in the United States and the number is increasing. About 42% of the species on the Threatened or Endangered species lists are at risk primarily because of non-indigenous species.



Some of the more common ways these species affect native species include:


Introduced species have few natural predators, parasites, or diseases and can out-compete native species, resulting in reduction of biodiversity and possibly extinction of the native species.


Native plants and native animals have co-evolved over long time periods. The presence of some introduced species generally alters these co-evolved relationships and as a result, the ecosystem, sometimes to a point where it can no longer support the native species.


Humans sometimes attempt to restore diminishing populations in natural habitats by bringing in species that are native to the area, but come from a non-local source. In such a case, it is sometimes overlooked that species tend to adapt to their local environment over many generations in ways that could possibly alter their genes. When the introduced natives breed with the local natives, the genetic makeup of both can be altered. This hybridization may result in the locals being less well adapted to and fit for their own environment. This concern comes into play especially when dealing with endangered or threatened species.


Introduced species can degrade the water quality by changing the nutrient level in the water and by overpopulating lakes and rivers.


Introduced species can also increase soil erosion by stripping the land of its natural vegetation.


Introduced species don't always have negative effects, nor do they all become classified as invasive or noxious. Species are sometimes intentionally introduced because the benefits outweigh the costs. For example, most agricultural food crops in the United States have been introduced.




According to research done by Cornell University, Invasive species in the United States cause damages in excess of $138 billion per year.


However, it is also important to note that nearly all of the crop and livestock species are introduced and are essential to the agriculture and economy in the U.S. Introduced species, such as corn, wheat, rice, and other food crops, and cattle, poultry, and other livestock, now provide more than 98% of the U.S. food system at a value of approximately $800 billion per year.





Getting rid of noxious or invasive species can be impossible. Reduction and containment are more probable goals.

Solutions can include introducing a natural predator of the problem species. However, this requires introducing another non-native species which could potentially become a problem of its own.

Chemicals that adversely affect the problem species could be used, however they will also most likely affect other species as well.

The U.S. Army Corps of Engineers is currently researching and attempting to reconstruct natural barriers that had previously existed. For example, construction of an electrical dispersal barrier at Chicago is underway to reduce the opportunity for species to flow between the Great Lakes and the Mississippi River. The dispersal barrier consists of a series of electrodes attached to the bottom and walls of the canal which will create a graduated, pulsed, direct current electrical field to maintain a continuous barrier throughout the water column. This is not intended to stun or kill fish but to deter their continued movement beyond the barrier.

Essentially once a species is introduced into an area, it is near impossible to undo the consequences. Nature takes over and it becomes survival of the fittest. The problem lies in that many of the introduced species are extremely fit and adaptable.

On an individual level, what you can do to help



Though damages from introduced species can reach into the billions, it is difficult to be compensated by legal action because it is nearly impossible to assign blame. It is also difficult to pass legislation to deal with the problems these species create.

For more information: