Invasive Non-native Species

Coastal marine ecosystems

Photo: colonial sea squirts © Gordon KingAlthough many non-native species have become established in Canada’s coastal marine waters,4-6 the impacts of invasive non-native species are most acute in the bays of P.E.I. Intensive agriculture and aquaculture activities have made P.E.I.’s coast more susceptible to the establishment and impacts of invaders. For example, since 1997, four species of sea squirts, or tunicates, have established, and are invasive, in P.E.I. Although established elsewhere in the southern Gulf of St. Lawrence, they are only invasive in P.E.I. There is also some evidence that another invasive species, the European green crab, preys on the predators of sea squirts, exacerbating the problem in P.E.I.6, 7

The European green crab is an aggressive competitor of native crabs and a predator of clams, mussels, juvenile fish, and many other species. It has recently become established on both the east and west coasts of Canada, although its establishment is too recent for its full impact to be known. The main source of coastal marine invasions in Canada has been transport on the hulls and in the ballast water of ships.5, 7, 8 New regulations on ballast water are designed to prevent further introductions through this pathway.

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Photo: zebra mussels © Jim Moyes, Environment Canada
Zebra mussels

Great Lakes

Invasive non-native species are responsible for the loss of much of the original biotic community of the Great Lakes.9 The demise of Great Lakes native biota started with the opening of the Welland Canal in 1829, the accidental introduction of sea lamprey in 1920, and the subsequent collapse of lake trout. Non-native species now dominate the Great Lakes, with enormous ecological and economic consequences.10 One study estimated the economic loss caused by non-native invasive species in the Great Lakes to be as much as $5.7 billion annually.11

Trends in non-native species in the Great Lakes

Cumulative number of species
Graph: cumulative number of non-native species in the Great Lakes. Click for graphic description (new window).
Long Description for Trends in non-native species in the Great Lakes

This bar chart shows the cumulative number of non-native species in the Great Lakes, categorized by ranges of years. The data are presented in the following set of points:

  1. prior to 1849, 9 non-native species were present;
  2. by 1899, 43 were present;
  3. by 1949, 86 were present;
  4. by 1999, 173 were present; and
  5. by 2008, 185 were present.
Source: data from Great Lakes Aquatic Nonindigenous Species Information System (GLANSIS), 200912

As of 2008, over 185 non-native aquatic species had been reported to have reproducing populations in the Great Lakes. Of these, at least 10% are considered to be invasive.12 Examples of the impacts include the collapse of:

  • the deepwater amphipod, Diporeia, and 33% of native mussels, after the introduction of zebra and quagga mussels;
  • many lake fish after the introduction of alewife.13

Prevention of future introductions, such as Asian carps from the Mississippi Basin, is a critical challenge.13

Native Mussel Declines

Number of freshwater mussel species before and after zebra mussel invasion
Map and graph: native mussel decline in Lake Erie and Lake St. Clair. Click for graphic description (new window).
Long Description for Native Mussel Declines

This map of Lake Erie, Lake St. Clair, the Detroit River, and the Niagara River shows sites that have data on the numbers of freshwater mussel species before and after zebra mussel invasion. The data, displayed on a bar graph, show steeply declining numbers of native mussels species at all sites where samples were taken pre and post-invasion. The data are presented in the following set of points, by site:

  1. Lake St. Clair: the sample site on the eastern shore of the lake had 5 species post-invasion. No sample was taken pre-invasion. A site along the southern shore of the lake had 11 species pre-invasion and 0 post-invasion. A site in the middle of the lake had 19 species pre-invasion and 5 post-invasion;
  2. the Bass Islands in southwest Lake Erie had over 20 species pre-invasion and 0 post-invasion;
  3. Rondeau Bay on the northwest shore of Lake Erie had 10 species pre-invasion, and approximately 1 post-invasion;
  4. Presque Isle Bay on the southeast shore of Lake Erie had approximately 15 species pre-invasion and 0 post-invasion;
  5. Port Maitland on the northeast shore of Lake Erie had approximately 15 species pre-invasion and approximately 5 post-invasion; and
  6. the Niagara River had fewer than 5 species post-invasion, with no samples taken pre-invasion.
Note: no “before” data for Niagara River and East Lake St. Clair.
Source: adapted from Metcalf-Smith et al., 200214

Native freshwater mussels are ecologically important as natural biological filters, food for aquatic species, and indicators of good water quality.15 Nearly 72% of the 300 freshwater mussel species in North America are vulnerable to extinction or are already extinct.15 Native freshwater mussels were virtually extirpated from the offshore waters of western Lake Erie between 1989 and 199116 and from Lake St. Clair between 1986 and 1994.17 Their decline has been attributed to a number of human stressors such as pollution, overexploitation, and habitat destruction by dams,18 in addition to declining water levels, and competition with non-native species such as zebra and quagga mussels.15 Free-flowing rivers can provide a refuge for native mussel species by limiting zebra and quagga mussel colonization. However, non-native mussels can still establish in regulated rivers with reservoirs.15 In a 2004/2005 survey, zebra mussels were noted at all sites sampled downstream from the Fanshawe Reservoir in the lower Thames River, a system that has one of the most diverse freshwater mussel communities in Canada.19

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Photo: purple loostrife © Steve Dewey,
Purple loosestrife

Terrestrial plants

Invasive non-native plants are one of the greatest threats to Canada’s croplands, rangelands, and natural areas. They degrade productivity and biological diversity; they are responsible for significant economic loss; and, they affect our trade with other countries. Approximately 1,229 (24%) of the 5,087 known plants in Canada are not native. Of these, 486 are considered weedy or invasive.36

Invasive non-native plants in Canada
Cumulative number of species, 1600 to 2005
Graph: cumulative number of invasive non-native plant species in Canada. Click for graphic description (new window).
Long Description for Invasive non-native plants in Canada

This line graph displays the cumulative number of invasive non-native plant species in Canada. From 1600 to 1800 the number increased, reaching approximately 50 by 1800. From 1800 to 1905 the number of new species increased at a more rapid rate, reaching over 200 by 1905. Over the next century the rate of increase was more moderate, reaching 245 in 2005.

Note: This graph represents an estimate of temporal trends for the 245 invasive plant species for which dates of introduction can be estimated.
Source: adapted from Canadian Food Inspection Agency, 201036

The most rapid accumulation of non-native plant species was between 1800 and 1900, a period of increased trade, immigration, and colonization. During this time many invasive plants were brought into Canada intentionally. The rate of new invasive plant introductions has slowed since the early 1900s, although range extension of established species is an ongoing problem. The geographic origin of most of the non-native plants in Canada is western Europe, reflecting dominant trade patterns of the past. Modern trade patterns point to new risks from the United States and Asia.36

Invasive non-native plants can cause ecological damage over a wide area and economic damage to multiple sectors. Some of the most damaging non-native plants include Canada thistle, leafy spurge, and knapweeds.37 Wetland plants are among the most aggressive invaders, changing vegetation structure, reducing the diversity of native plants and associated wildlife, and altering basic wetland functioning. Some of the most aggressive wetland invaders include purple loosestrife and European common reed.38

Expansion of the European common reed

St. Lawrence River, Quebec, 1980 to 2002
Three graphs: expansion of the European common reed. Click for graphic description (new window).
Long Description for Expansion of the European common reed

Three maps show the extent of the European common reed in 1980, 1995, and 2002, along the St. Lawrence River in Quebec. In 1980 European common reeds were found only in the river channel. By 1995 the distribution had expanded within the channel as well as on to land; by 2002 all locations that were already colonized by the reed had expanded in size.

Source: adapted from Hudon et al., 200539
Locator map of the St. Lawrence River, Quebec. Click for graphic description (new window).
Long Description for Locator map of the St. Lawrence River, Quebec

This map of Canada shows the location of the St. Lawrence River, Quebec.


European common reed, a subspecies of the native common reed, is one of the most dangerous non-native invaders of natural habitats in Canada.40, 41 It is currently a major problem in the east, where it forms dense stands to the exclusion of most native species.40 It first established in Nova Scotia in 1910,40 but spread most significantly from 1980-2002.39 Human- made linear wetlands, such as ditches, can act as dispersal corridors because they are rich in nutrients, extensively interconnected, and salt accumulation in them creates a competitive advantage for the salt-tolerant European common reed.42 Expansion of the European common reed jeopardizes ecosystem functioning because it reduces biodiversityPhoto: European common reed © Paul Catling and is of lower nutritional43 and habitat value44 than the native species it replaces. The European common reed is expected to expand its range to the Prairie provinces within one or two decades, where it could impede water flow in irrigation canals.40 Early knowledge allows for some time to conduct the research necessary to prevent its spread.40

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Pathogens and diseases of wildlife

Pathogens are disease-causing organisms. They come from a large spectrum of species groups, including worms, insects, fungi, protozoa, bacteria, and viruses. Many pathogens are native to Canada and the wildlife diseases they cause are part of the normal functioning of ecosystems. However, some recent disease outbreaks appear to be caused by invasive non-native pathogens or new strains of native pathogens. These include: a bacterium of poultry that also affects house finches; avian influenza, a usually benign virus of ducks that now exists in a strain deadly to poultry; duck plague, a virus native to Eurasia that can kill wild waterfowl; a chytrid fungus of amphibians; and West Nile virus, affecting mammals, birds, reptiles, and people.20

Distribution of birds testing postive for West Nile virus

2001 to 2003
Map: distribution of birds testing positive for West Nile Virus. Click for graphic description (new window).
Long Description for Distribution of birds testing postive for West Nile virus

This map of Canada with ecozone+ boundaries shows the locations of birds found from 2001 to 2003 that tested positive for West Nile virus. The highest concentration of positive birds was in the Mixedwood Plains Ecozone+, spanning southern Ontario and Quebec and east to the Atlantic Maritime Ecozone+. Positive birds were also found sporadically in New Brunswick and Nova Scotia and in the southern portion of the Boreal Shield Ecozone+. A high number of positive birds were also found dispersed across the Prairies Ecozone+ and along the southern border of the Boreal Plains Ecozone+.

Source: Leighton, 201020 adapted from Health Canada, 200321

West Nile virus cycles in nature between a wide range of wild bird species and a narrow range of mosquito species. It was transported to North America from Afro-Eurasia.22 First detected in Canada in 2001, it affected all provinces from Nova Scotia to Alberta by 2003, and by 2009, it had reached British Columbia. West Nile virus has killed thousands of corvids (crows, jays, magpies, and their relatives) and fewer non-corvid birds.23

Global spread of chytrid fungus of amphibians

Earliest occurrences of chytrid fungus in each major centre
Timeline: global spread of chytrid fungus of amphibians. Click for graphic description (new window).
Long Description for Global spread of chytrid fungus of amphibians

This timeline shows the date of the earliest occurrence of chytrid fungus in major global regions. The fungus first occurred in Africa in 1938, in North America in 1961, in Australia in 1978, in Central America in 1983, in South America in 1986, in Europe in 1997, and in Oceania in 1999.

Source: adapted from Weldon et al., 200424

A chytrid fungus of the skin has been linked to worldwide declines in amphibian populations25 and is generally believed to be the largest infectious disease threat to biodiversity.26, 27

The origins of chytrid fungus in North America are unclear. It may have originated in Africa and spread through trade of African clawed frogs, which were widely used in human pregnancy tests.24, 26 Trade of other species, such as the American bullfrog, may have contributed to its spread.24 There is some evidence that chytrid fungus has always been present in North America, but that environmental stressors, such as pesticides and climate change, have made amphibians more susceptible to it.28-30 The earliest record of chytrid fungus outside of Africa is from Quebec, in 1961.31Photo: northern leopard frog ©

Since then, chytrid fungus has been found in British Columbia,31 Alberta,20 Saskatchewan,32 Ontario, Quebec, New Brunswick, Nova Scotia,31 and, most recently, Prince Edward Island,33 Yukon,34 and the Northwest Territories.35

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