14.4 Trends in alien plant species

Last Updated: 
May 29, 2015

This indicator tracks long-term changes in plant communities due to the introduction and spread of alien plant species in NWT’s ecosystems.

This indicator is also used to track the status of vascular plant species in the NWT General Status Ranking Program. All species tracked by this indicator are alien species, which are defined as species introduced into North America from Europe or Asia as a result of human activities.

The indicator also provides information on the level of invasiveness for each alien species. Invasive species are harmful alien plant species whose introduction or spread threatens the environment, the economy, or society. Plants can be invasive in one area or ecosystem and less in another. Synonymous with weed.

Predicted levels of invasiveness are preliminary and apply to the entire NWT, based on assessments in Canada1. These may change in future as more studies and information becomes available. The following categories are used to describe the level of invasiveness.

High = Invades man-made disturbed habitats and invades natural habitats quickly. Hard to eradicate. Has severe ecological impacts on physical processes, plant or animal communities and vegetation structure. Reproductive biology and other attributes are conducive to moderate to high rates of dispersal and establishment. Most are widely distributed ecologically.

Moderate = Invades man-made disturbed habitats and invades some natural habitats. These species are invasive but their ecological impacts are moderate or there was not enough information to justify a higher score. Ecological amplitude and distribution are generally limited, but these species may be locally persistent and problematic.

Low = Invades man-made disturbed habitats and some natural habitats with natural disturbances. These species are invasive but their ecological impacts are low or there was not enough information to justify a higher score.. Ecological amplitude and distribution are generally very limited, but these species may be locally persistent.

Potential = Can invade disturbed habitats if conditions favour this. These species can be invasive but there was not enough information to justify a higher score. Ecological amplitude and distribution are generally very limited, but these species may be locally persistent.

This indicator uses information collated from NWT residents and visiting experts as well as information summarized from the NWT General Status Ranking Program and projects undertaken for the Risk Analysis of Invasive Alien Species in the NWT funded in part by the AIS Partnership Program.

NWT Focus

Changes in the number of alien species in the NWT are monitored as their presence and abundance may affect the status of wild species native to NWT. Plant species are introduced to NWT as food crops, habitat remediation tools, landscaping varieties, or simply unintentionally. The majority of introduced (alien) plant species cause no damage to natural ecosystem because they need constant human assistance to survive and do not spread. NWT’s climate prevents many species from establishing themselves in NWT’s ecosystems. Some introduced plant species have, however, succeeded in spreading in some habitats, mostly those already disturbed by human activities. Some introduced plant species can spread and cause harm to natural habitats, out-competing native plant species.

Current view: status and trend

So far, there are no known alien plant species with a high level of invasiveness in the NWT.

In 2010, 116 alien plant species were known to occur in the NWT2. In 2005, 94 alien plant species were known to occur here. These are mostly found in or near communities, near roads and along disturbed areas such as cut-lines, pipelines and mine sites.

Of these 116 species, few have demonstrated that they can invade natural habitats1 (see table below). White and yellow sweet clovers are increasingly common in the NWT, from the Alberta border up to Norman Wells and Inuvik. These species are widespread in communities and along roads3. There are reports that they have been found outside man-made habitats in the NWT, but these need confirmation. These species are now known to invade river margins and sandy/muddy natural habitats in Alaska and Yukon4.

The predicted invasiveness of alien plant species is based on studies from across Canada1,5 and is applied to the NWT in a preliminary fashion. More studies on the distribution and biology of alien plants in the NWT are being conducted6.

Species name Family Introduction Notes Habitat or Occurrence Notes Predicted Invasiveness

White Sweet Clover
(Melilotus alba)

Pea (Fabaceae) Introduced from Eurasia Roads, increasingly common in upper Mackenzie R., some sites along pipeline to Norman Wells. Taiga Shield and Taiga Plains. Moderate

Yellow Sweet Clover
(Melilotus officinalis)

Pea (Fabaceae) Introduced from Eurasia Recent in waste places, roadsides in southernmost Mackenzie River, along pipeline to Norman Wells. Taiga Shield and Taiga Plains (south).. Moderate

(Medicago sativa)

Pea (Fabaceae) Introduced from Eurasia(?) Widespread along NWT highways, and at least 1 site along pipeline to Norman Wells. Taiga Plains (south). Low/ Potential
Siberian Peashrub
(Caragana arborescens)
Pea (Fabaceae) Introduced Fort Smith, Fort Liard, Liard River at Petitot River, probably other places, escaped from gardens. Taiga Plains (south) Low/ Potential
Awnless Brome
(Bromus inermis)
Introduced from Eurasia Cultivated and found on roads and waste places. Taiga Shield and Taiga Plains. Moderate/ Low

Reed Canary Grass
(Phalaris arundinacea)

Grass (Poaceae) Presence of exotic genotypes among NWT sites is uncertain Sites in southern NWT and 14 sites along the pipeline to Norman Wells, part of seed mix applied in 1984. Taiga Shield and Taiga Plains (south). Moderate/ Low
Crested Wheat Grass
(Agropyron cristatum spp pectinatum)
Grass (Poaceae) Introduced from Russia (or Europe) Introduction near waste sites and townsites in south Mackenzie River area. Taiga Shield and Taiga Plains. Low/Potential
Flat-stem Blue Grass
 (Poa compressa)
Grass (Poaceae) Introduced from Eurasia Fort Simpson, Liard Trail (Hwy 7), west of Yellowknife, maybe elsewhere near southern Mackenzie River. Taiga Shield and Taiga Plains (south). Minor /Potential
Kentucky Blue Grass
(Poa pratensis)
Grass (Poaceae) Forms in the NWT are probably not native and introduced as lawn grass Near settlements, roads and one site on the pipeline to Norman Wells. Taiga Shield and Taiga Plains. Minor /Potential
Hoary False-alyssum (Berteroa incana) Mustard (Brassicaceae) Introduced from Eurasia First site discovered in 2006 along Hwy. 3. Taiga Shield. Low
Creeping Thistle
(Cirsium arvense)
Thistle (Asteraceae) Introduced from Eurasia Fort Simpson, Hay River, Fort Providence near highway and Yellowknife. Taiga Shield and Taiga Plains (south). Moderate/ Low
Common Tansy
(Tanacetum vulgare)
Thistle (Asteraceae) Introduced from Eurasia Along roads in southern NWT, as well as in Inuvik. Taiga Shield and Taiga Plains. Potential

Some plant species have both native and alien subspecies and varieties. These species may be used in seed mixes, especially if these varieties have been selected for “aggressiveness” or “pioneer capacity” that is their capacity to quickly grow, invade, and cover an area to offer some erosion control. However, as only the species name, not the full subspecies or variety name, is usually reported in re-generation programs, it becomes difficult to determine whether alien subspecies/varieties or native ones are being used. The invasiveness potential of these subspecies is mostly unknown for the NWT. Some of these subspecies are found today along seismic lines, pipelines or re-vegetated mines and other sites.

Looking forward

The North lags behind other North American jurisdictions in preventing introduction, controlling and eradicating invasive alien plants that could threaten native ecosystems, habitats, or species. We may have been complacent in our view of the threats of invasive alien species to the North’s ecosystems, assuming that our northern climate will prevent most species from establishing themselves here. With increasing development in the NWT, and an increase in habitat changes expected in the Taiga Plains, communities are preparing to increase their awareness of the risks related to alien plants and to help reduce that risk. NWT people have noticed an increase in plant species that are “out-of place” – or alien and “taking over some habitats” – or becoming invasive.

Every year, botanists collect plants, identify them and store them in botanical collections in museums in Canada, the U.S., England, and elsewhere. A project on plant data repatriation was initiated in 2007 to assemble a database of all plant locations and information on specimens collected in the NWT since the 1800s. Using this database, the NWT will be able to map all known locations of alien plants, analyse distribution patterns and identify future problems related to introduction of potentially invasive alien plant species.

With increases in development and climate change, the NWT can expect more alien plant species to arrive and some to move to natural habitats and become invasive. A risk analysis and management options plan is being developed to help detect these plants early, and to eradicate or control them as necessary.

There is increasing demand in the NWT for seed mixes containing native plant species for work in revegetation plans and mitigation activities.

Looking around

In areas where there is greater native plant diversity (number of plant species), more species of alien plant species are usually also found.7 For example, the NWT has about 1035 native vascular plants; this is more than Nunavut (618) and less than the Yukon (1,081). The number of alien plant species found in the NWT (116) also falls between the numbers of alien plant species detected in Nunavut (20) and Yukon (118). This relationship is linear for all western and northern jurisdictions in Canada. The NWT has about the number of alien plant species expected for a northern jurisdiction considering the number of native species present here. Note that if a province/territory is rich in native plant species, it tends to be “better” for exotics. Conversely, if it is poor in native plants, it is worse for exotics7 . In BC, which is rich in plant species, 23% of vascular plants are alien species, whereas in the NWT, which is relatively poor in plant diversity, only 10% of vascular plants are alien species.

A different ratio of alien to native plant species is observed in eastern Canada, where three times more species of alien plants are observed compared to what we would expect in western and northern Canada. Possible reasons for this include the longer history of habitat use and human settlements in eastern Canada resulting in more disturbed habitats that are presumably more favourable to alien plants7, and higher rates and more points of entry for plant introductions for food and gardens.

Find out more

  • Find out more about invasive alien species in Canada and around the world with the IUCN Invasive Species Specialist Group. A complete list of all alien plant species known to occur in the NWT can be found in Oldham (2006), and in the NWT Species Infobase.
  • Additional information on the biology and global distribution of all alien plant species found in the NWT can be accessed on the Global Invasive Species Database, managed by the Invasive Species Specialist Group (ISSG) of the IUCN Species Survival Commission.

Other focal points

  • Information on alien insects and on alien mammals, birds and fish species is in the WILDLIFE focal point

Technical Notes

  • The definition invasiveness is based on the definition contained in the Convention on Biological Diversity Decision VI/23 and the and the Invasive Alien Species Strategy for Canada.
  • All photos are from Mike Oldham © ENR, GNWT, except Reed Canary Grass photo from Chris Evans, River to River CWMA, Bugwood.org, Crested Wheat Grass photo from Dave Powell (USDA Forest Service), Hoary False-alyssum photo from Field Guide to Noxious and Other Selected Weeds of British Columbia, Creeping Thistle C Lapina - image from Alaska; Common Tansy photo from https://commons.wikimedia.org/wiki/File:Common_Tansy_-_Tanacetum_vulgare_(38240015522).jpg


Ref. 1. Canadian Botanical Conservation Network. 1997. Invasive Herbaceous Species List

Ref. 2. Working Group on General Status of NWT Species. Current. NWT Species Infobase.

Ref. 3. Olham, M. 2007. The 2006 survey of exotic plants along Northwest Territories highways. Report to the GNWT. 

Ref. 4. Conn, J.S., K.L. Beattie, M.A. Shephard, M.L. Carlson, I. Lapina, M. Hebert, R. Gronquist, R. Densmore, and M. Rasy. 2008. Alaska melilotus invasions: distribution, origin, and susceptibity of plant communities. Arctic, Antarctic, and Alpine Research 40(2): 298.

Ref. 5.  Lake, J.C. and M.R. Leishman. 2004. Invasion success of exotic plants in natural ecosystems: the role of disturbance, plant attributes and freedom from herbivores. Biological Conservation 117(2): 215-226.

Ref. 6. ENR. Invasive Alien Species Program.

Ref. 7. Stark, S.C., D.E. Bunker, and W.P. Carson. 2006. A null model of exotic plant diversity tested with exotic and native species-area relationships. Ecological Letters 9:136-141.


Updated: May 29, 2015