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Why Are Invasive Alien Species a Problem?


Common reed (Phragmites australis) © Ken Allison, Canadian Food Inspection AgencyIntroduction

According to the World Conservation Union, invasive alien species are the second most significant threat to biodiversity, after habitat loss. In their new ecosystems, invasive alien species become predators, competitors, parasites, hybridizers, and diseases of our native and domesticated plants and animals.

It takes the right conditions for an alien species to become established and grow. In fact, most introduced species do not become permanently established in their newly found environment. They either find the wrong type of conditions for survival, or there they are unable to produce enough offspring to maintain a viable population. For those species that are able to reproduce and survive, most never cause significant problems. They survive, spread and reproduce but generally do not pose a risk to Canada’s economy, environment or society.

Some introduced species however, are able to thrive and even flourish at their new location. They have the right biological properties that allow then to grow in numbers quickly. In addition, they are usually able to reproduce and spread quickly, often out-competing native plant and animal species for food water and space.

Invasive alien species generally share common characteristics which can make them difficult to control and contain, including:

  • Higher rates of reproduction – it has been estimated that one Purple Loosestrife plant can produce 3 million seeds.
  • Fewer natural predators – newly introduced species are often free of predation and disease, two major factors that keep native plant and animal populations in balance.
  • Ability to thrive in different environments – most invasive alien species are generalists, meaning they can survive in a variety of different habitat types and climate regions.

The impact of invasive alien species on native ecosystems, habitats and species is severe and often irreversible, and can cost billions of dollars each year.


Invasive Alien Species Impacts

Environmental ImpactsPacific tree frog found in plants shipment © Diana Mooij, Canadian

  • Can negatively impact biodiversity
  • Can cause species extirpation and even extinction
  • Can cause soil degradation, and erosion
  • Can alter fire cycles

Societal Impacts

  • Can cause disease
  • Can cause human or animal suffering
  • Can reduce land and water recreational opportunities

Economic Impacts

  • Control and management costs can be significant
  • Can reduce productivity in forestry, agricultural, and fishing sectors
  • Can cause export and import trade restrictions
  • Can reduce property values

The following are just some examples of invasive alien species impacts in Canada and abroad. Check regularly as this list will be continuously updated.

Canadian Impacts

Table 1: Types of Species and their Impacts to Canada
SpeciesType of impact (economic, environmental, societal)Description of impact

Zebra Mussel

Dreissena polymorpha

EnvironmentalZebra muscles will infest almost every hard surface including rocks, boat hulls, and water intake pipes. They can even grow on the shells of our native clam species, reducing their ability to feed and as such increase mortality.

Japanese Knotweeds

Fallopia japonica & Polygonum cuspidatum

EnvironmentalJapanese Knotweed is a perennial shrub from Asia, often considered one of the most persistent of all weeds. Once established in disturbed areas or along stream banks, this highly invasive species can displace virtually all other types of vegetation. 

West Nile Virus

Family: Flaviviridae

SocialThe West Nile Virus originated in Africa and Europe. Since its discovery in North America, public health agencies have been waging an aggressive campaign to inform the public about the risks associated with contracting this disease.

Emerald Ash Borer

Agrilus planipennis

EconomicThe cost of eradication and control efforts associated with this invasive insect have totalled in the millions per year.

Global Impacts

Table 2: Types of Species and their Impacts Globally
SpeciesType of impact (economic, environmental, societal)Description of impact

Comb Jellyfish

Mnemiopsis leidyi

EconomicThis small jellyfish, native to estuaries of eastern North and South America, was almost certainly exported to the Black Sea in ship ballast water. Since it was first discovered in 1992, it has  single-handedly caused the collapse of the entire anchovy fishery, an industry once worth millions of dollars per year.

Brown Tree Snake

Boiga irregularis

EnvironmentalThis invasive alien species first introduced to Guam shortly after World War II, has caused immeasurable damage to the Island's ecosystem. The Brown Tree Snake has been directly linked to the extirpation of several native species of birds (8), lizards (3), and mammals (2 bats).

Water Hyacinth

Agrilus planipennis

SocialOriginally from the Amazon basin, this water plant has spread to many aquatic ecosystems around the world. Whole communities around Lake Victoria, Africa’s largest lake, have had to move to gain better access to open water and fish stocks. 
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