Plantes exotiques et envahissantes au Canada Maritime

Aujourd'hui, les espèces exotiques constituent jusqu'à 25 à 35 % de la flore locale au nord-est de l'Amérique du nord (Whitney 1994), avec des pourcentages encore plus élevés dans les zones densément peuplées. Et avec l'arrivée de nouveaux envahisseurs, la proportion des exotiques continue à s'accroître. 

Les espèces de plantes envahissantes sont capables d'altérer profondément les processus des écosystèmes, leurs structures et leurs compositions, ce qui représente de sérieux problèmes pour la conservation des espèces indigènes et qui coûte souvent des millions de dollars en efforts de redressement et de contrôle.

"Invading plant species can profoundly
alter ecosystem processes, structure and composition, posing serious concerns
for the conservation
of native species and often costing millions of dollars in remediation and control efforts."

Exotic and Invasive Plants
in Maritime Canada

Sean Blaney
Botanist, Atlantic Canada Conservation Data Centre
Originally published in the 
Blomidon Field-Naturalist newsletter, Summer 2001.
April 2004

xotic, or alien, species are those species that have arrived at an area outside their natural range with deliberate or unintentional human assistance. Today few, if any, regions worldwide are free of alien species. The frequency of exotic introduction continues to increase with human alteration of natural ecosystems and with increasing opportunity for species' movement associated with greater global interdependence of economies. Vascular plant invaders are especially numerous. For example, Heywood (1989) estimates the introduced vascular flora of Australia at 1500 - 2000 species, Kent (1992) lists 1189 established exotic species in the British Isles and there are over 700 vascular plant species reported as exotic in at least one of the Maritime provinces (ACCDC database 2001). Exotic plants are not a new phenomenon in North America. The first alien species arrived with the earliest European settlement. Whitney (1994) cites early records showing that at least 40 species of European weeds were established around settlements in Massachusetts in 1672, with numbers rising to 140 species in the Boston area by 1840. Today, exotic species generally make up 25 to 35% of local floras in northeastern North America (Whitney 1994), with higher percentages in heavily urbanized areas. The proportion of exotics continues to increase as new invaders arrive.

Alliaria petiolata – Garlic Mustard

(Source: Website)

Invading plant species can profoundly alter ecosystem processes, structure and composition, posing serious concerns for the conservation of native species and often costing millions of dollars in remediation and control efforts. In deserts of the southwestern USA, tamarind shrubs (Tamarix spp.) have been shown to lower water tables due to their dense growth and relatively inefficient water use. The nitrogen-fixing shrub Myrica faya (a relative of our Sweet Gale and Bayberry) permanently changes Hawaiian grasslands into shrubland and forest by enriching the nutrient poor soil. In open rangelands in the western USA, annual grasses (especially Bromus species) increase fire frequency because they mature and dry out very early in the growing season. Closer to home, we have the Glossy Buckthorn (Rhamnus frangula, also known as Frangula alnus), a shrub which reaches tremendous densities in a variety of habitats, sometimes shading out native vegetation almost entirely. These are just a few of the hundreds of examples of ecologically significant plant invaders worldwide.

It is very important to note, however, that the exotic species which significantly alter ecosystems are in a very small minority. Most arriving plant invaders never become established, or have no noticeable impact if they do (Williamson and Fitter 1996a, 1996b), especially in cooler northern regions like the Maritimes. We are very fortunate that among our many exotic plant species, we have only a few which seem to have much negative effect on native biological diversity. Our exotic species are mostly restricted to open, human-disturbed situations such as agricultural lands, roadsides, and urban areas. Exotic species are relatively uncommon and insignificant in the more natural forests and wetlands of the Maritimes. Even at disturbed, open sites, most of the exotic species would not survive were the land left to return to forest. This contrasts somewhat with more densely populated areas to the south and west of us such as southern Ontario and the eastern and midwest US. In these regions, many natural areas are heavily penetrated by alien species to the point where native species appear to be losing out. Why this difference? A number of factors are probably important. Our cooler climate reduces the number of invaders suited to our region. Our much smaller and more thinly spread population results in less import and movement of exotic species. Perhaps most importantly, our Maritime landscape is comparatively much more intact. Where human-dominated habitats cover the majority of the landscape, small, fragmented natural areas are like islands in a sea of exotic-dominated communities. Native plant populations may become isolated and get eliminated by additional human disturbance or by random events. The dominant exotics meanwhile have a large and continuous numerical advantage in seed production that may swamp the native species.

So do we have reason to be complacent about the relative lack of invasive exotic species here in the Maritimes? Unfortunately not, I would say. First, new species will continue to be introduced to the continent and to spread from surrounding regions into the Maritimes. It is a virtual certainty that, over time, additional problematic invaders will arrive here. Second, the human impact on the landscape is increasing. The construction of new highways, pipelines and hydro rights-of-way are continuously creating new corridors for exotic plant spread. Urbanization and heavy forestry are also adding to the fragmentation and disturbance that tends to favour exotic species. Finally, we do already have some problematic invaders that are present and spreading.

In the section below, I look at a selection of the most important invasive exotics already present in the Maritimes, discussing their habitat, range and impacts. The list below is not meant to be a comprehensive list of the exotic species that do, or could, present problems for native species. I would be pleased to hear if readers have additional nominations.

Glossy Buckthorn (Frangula alnus, also called Rhamnus frangula)
Range: Locally abundant near Fredericton, NB, and Amherst, Wolfville & Digby, NS. It is scattered elsewhere in the Maritimes, but spreading rapidly around the above centres and in the vicinity of Miramichi, NB and Pictou, NS. Habitat: moist or wet old field, thicket and forest. Considered by many botanists to present the greatest threat to native species among all current Canadian invasive species. It can form such a dense understory shrub layer in moist or wet forests that succession or regeneration is virtually halted and herb diversity is greatly reduced. It is capable of tolerating nutrient poor and nutrient rich habitats and unlike many other invasives, it seems capable of spreading into otherwise undisturbed habitats. Near Fredericton, it is threatening rare silver maple swamp habitats along the Saint John River. The Common Buckthorn (Rhamnus cathartica) is also invasive and locally established in the Maritimes, but it prefers somewhat drier habitats. Both species produce abundant berry-like fruit dispersed widely by birds and by floodwaters along rivers.

Scots Pine (Pinus sylvestris)
Range: Throughout Maritimes near settlement. Habitat: Moist and dry open areas.
This species has been very widely planted and almost invariably escapes to the wild. It reproduces prolifically at a relatively young age for a tree and is capable of growing in very poor soils and dry conditions where many other trees have difficulty becoming established. It can threaten naturally open habitats such as shoreline dunes and bogs by shading out the native species adapted to open conditions.

Scotch Broom (Cytisus scoparius)
Range: NS, primarily southern but spreading north. 
Habitat: Fields, roadsides and open forest. Thought to be limited to the south by climate, this shrub may become more problematic with global warming. It is considered invasive in many regions with Mediterranean or temperate coastal climates worldwide. An accelerated spread in Nova Scotia has been reported in the last ten years.

Purple Loosestrife (Lythrum salicaria)
Range: Throughout most of Maritimes. 
Habitat: Marshes, shorelines, moist fields, ditches. Although this species has had the most media coverage of any exotic, it appears less problematic in our region than elsewhere. It never reaches densities in the Maritimes comparable to those to the south and west, and does not seem to spread well into undisturbed and nutrient poor wetlands. Biological control efforts using introduced European beetles appear to be having some success but it is unclear how the beetles will affect the related rare native Swamp Loosestrife (Decodon verticillatus).

Garlic Mustard (Alliaria petiolata)
Range: Abundant locally in the central Saint John River valley in NB; in the last two years, found to be well established in Prince Edward Island National Park at Cavendish and near Wolfville, NS. 
Habitat: Rich, deciduous forest, especially floodplains and shaded urban yards. This forest biennial stores energy in its roots over its first year and then sends up tall shoots early the next year, which can shade out native perennial forest herbs. It is a major invasive species throughout the eastern US and southern Ontario and is locally problematic in very rich floodplain forests of the Saint John River, where it grows with a number of other invasives of restricted Maritime range, i.e. Nipplewort (Lapsana communis), Town Avens (Geum urbanum), Celandine (Chelidonium majus) and the more common Dame's Rocket (Hesperis matronalis).

Japanese Bindweed (Polygonum cuspidatum)
Range: Essentially throughout Maritimes near settlement. 
Habitat: Moist open areas
This large, shrub-like herb forms dense patches from a massive, creeping root system that is almost impossible to eliminate. Elsewhere it is most problematic in floodplain habitats in river valleys, but it currently seems to be mostly restricted to roadsides in the Maritimes.

Finally, what can you do to reduce the impact of invasive species? 1) Become better informed about what species are potentially invasive. An excellent place to start a search for further information on invasive plants is the "Invasive Plants of Natural Habitats in Canada" website at
www.cws-scf.ec.gc.ca/publications/inv/cont_e.cfm.
2) Do not plant any non-native species known to be invasive in Canada or the northern US or at the very least, try to manage the spread of garden plants into wild habitats, especially for ground cover species. It is interesting to note that all the problematic species listed above were introduced into North America for horticultural purposes. 3) Avoid moving plant material or soil from areas that are likely to have invasive species present. 4) Eliminate invasive species from your property where possible and encourage others to do so as well.

LITERATURE CITED

HEYWOOD, V.E. 1989. Patterns, extents and modes of invasion by terrestrial plants. In: J.A. Drake, H.A. Mooney, F. DiCastri, R.H. Groves, F.J. Kruger, M. Rejmánek and M. Williamson (eds.). Biological Invasions: A Global Perspective, pp. 369-388. John Wiley and Sons, Chichester.

KENT, D.H. 1992. List of vascular plants of the British Isles. BSBI, London.

WHITNEY, G.G. 1994. From Coastal Wilderness to Fruited Plain: A History of Environmental Change in Temperate North America from 1500 to the Present. Cambridge University Press, Cambridge, UK. 451 pp.

WILLIAMSON, M. and A. FITTER. 1996a. The varying success of invaders. Ecology. 77: 1661-1666.

WILLIAMSON, M. and A. FITTER. 1996b. The characteristics of successful invaders. Biological Conservation. 78: 163-170.