Invasive Non-native Species

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).
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).
Source: adapted from Hudon et al., 200539

Locator map of the St. Lawrence River, Quebec. Click for graphic description (new window).

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