What are invasive alien species?


Terminology around invasive alien species is diverse, and definitions may be ambiguous. The work of ISANS aims to support Canada’s federal invasive alien species initiatives, and as such defines invasive alien species as:

Plants, animals, aquatic life or micro-organisms that outcompete native species when introduced outside of their natural environment and threaten ecosystems, economy and society (Government of Canada 2009).

Probably the best known invasive alien species in Nova Scotia is the European brown spruce longhorn beetle (Tetropium fuscum). Discovered in 1999 in Point Pleasant Park in Halifax, this insect is the ongoing focus of a Canadian food Inspection Agency (CFIA) containment and eradication program. The Halifax population is the only known infestation of European brown spruce longhorn beetle in North America!

There are many other invasive alien species that are impacting Nova Scotia right now, and more will likely be introduced. Visit the Terrestrial Invasive SpeciesAquatic Invasive Species, and Future Threats pages to learn more.


Alien species originate on another continent, in another country, or in another part of Canada. The means by which a species arrives at a new location is referred to as a pathway. The most common pathways vary by the type of alien species, and include the following, as identified by the Government of Canada:

firewood/unfinished wood products, 
plant products, 
wood packaging
EXAMPLE: The European brown spruce longhorn beetle (Tetropium fuscum) arrived in Halifax in 1990 (though not correctly identified until 1999) in wood packing materials.
live food imports
aquarium and horticultural imports
EXAMPLE: Purple loosestrife (Lythrum salicaria) is an escaped garden ornamental.
vehicles (ground, air, and aquatic)
ballast water of shipping vessels
EXAMPLE: One of Canada’s most notorious invaders, the zebra mussel (Dreissena polymorpha), was introduced to the great lakes in discharged ballast water in 1986.
diseases in wildlife


Outside of their home ranges, invasive alien species lack the natural controls that keep their populations in check. Secrets of their success in new environments include high productivity, good dispersal, long growth periods, and lack of natural controls (predators, disease, etc).

Only a very small percentage of alien (introduced) species become invasive. Approximately 10% of alien species become established in their introduced ranges, and only 10% of those, or 1% of the total, become invasive. Unfortunately, it is difficult to predict which alien species will become invasive, and new species are introduced every day. Some alien species may be present in their introduced ranges for a long time before they become invasive. Just because a species does not exhibit invasive characteristics when it is introduced does not mean that it will remain innocuous in its new range forever. Those introduced species that do become aggressive invaders may cause serious, costly, and possibly irreversible damage.


It is widely accepted that invasive alien species are the second greatest threat to biodiversity worldwide! In Canada alone, more than 20% of our species-at-risk are threatened with extinction by invasive alien species. According to the Government of Canada, in addition to threatening biodiversity and the survival of native species, invasive alien species may have the following harmful impacts:
contribute to soil degradation and erosion
degrade water quality
degrade terrestrial and aquatic habitats
alter infrastructure
introduce disease
reduce terrestrial and aquatic recreational opportunities
reduce productivity in forestry, agricultural, and fishing industries
affect export and trade opportunities
reduce property values

In general, the cost of managing and/or controlling an invasive alien species once it has become established is very high. Prevention of new introductions is the most cost-effective means of mitigating the potential impacts invasive alien species.