Degradation of Natural Areas

Garlic Mustard PlantsNatural areas such as forests, prairies, wetlands and lakes provide many services and benefits to the economy, society and the environment. Natural areas provide shelter and food for wildlife, remove pollutants from air and water, produce oxygen through photosynthesis and provide valuable recreational and educational opportunities. Invasive plants can have a large impact on natural areas and threaten these important services that they provide.

Invasive plants impact species diversity and species richness by competing heavily for resources such as light, moisture and soil nutrients that native plants require to establish and grow. These changes in species composition may affect wildlife that are adapted to native plant communities. Ultimately, invasive plants affect the intricate linkages that make ecosystems strong and resilient. For example, garlic mustard (Alliaria petiolata) often out-competes native woodland herbs like white trillium (Trillium grandiflorum) by changing the soil chemistry and making it less suitable for these native species (Queen’s Printer for Ontario, 2012). Changes in these linkages can affect the services and benefits that natural areas provide.

Interference with Agriculture

Invasive plants can have a wide range of impacts on the agricultural industry. Invasive plants can act as new or additional hosts for new or existing crop diseases and crop pests, they can cause reductions in crop yields and may require increased use of pesticides to control them. This increases costs for farmers and reduces crop values. Invasive plants, such as leafy spurge (Euphorbia esula) or some knapweed species (Centaurea spp.) (both listed as provincial noxious weeds), can take over farmlands, reducing crop production and foraging space. The estimated annual economic impact of invasive plants on Canadian agriculture is $2.2 billion (Environment Canada, 2010)

Reduced Forest Regeneration and Productivity

Invasive plants can reduce forest regeneration through direct competition with tree seedlings, resulting in reduced density and slowed growth rate. Reduction in forest regeneration results in the loss of wildlife habitat, and decreases the diversity of a stand, making it more vulnerable to insects and disease.

Invasive plants can also affect forest productivity by impacting beneficial soil organisms and changing soil chemistry. Some invasive plants can form dense mats of vegetation, hindering natural forest regeneration and hindering or preventing access by forestry workers and equipment (e.g. dog-strangling vine (Cynanchicum rossicum)).

Danger to Human Health and Safety

Some invasive plants, like giant hogweed (Heracleum mantegazzianum) and wild parsnip (Pastinaca sativa), cause human health concerns because their sap is toxic to skin. Other plants can cause physical injuries to the body; common buckthorn (Rhamnus cathartica) branches end in a short, sharp thorn which may inflict injury. Human safety may also be impacted by fast growing invasive plants. For example, Phragmites australis (hereafter referred to as Introduced Phragmites) grows large and rapidly, and may reduce visibility at rights of ways, increasing the risk of car accidents. Dead, dry stalks of Introduced Phragmites are also highly combustible and can become a fire hazard.

It’s important to note that many of our native plant species can pose a risk to human health and safety, but a key difference with invasive plants is that they become widespread and prevalent much faster than our native plants, which means their risks may occur at a much higher rate. This makes preventing their spread and controlling them and the risks they pose to humans that much more difficult, but that much more important.


Invasive plants can have a large economic impact on individual landowners and municipalities. A recent study shows that property values for shoreline residences in Vermont affected with Eurasian water-milfoil (Myriophyllum spicatum) were down as much as 16.4 % (OMNRF, 2012). Due to the explosion of leafy spurge (Euphorbia esula), Manitoba has experienced a $30 million reduction in land values (CFIA, 2008). Leafy spurge infests 340,000 acres of land in Manitoba, costing taxpayers an estimated $19 million per year to protect grazing land, public land, and rights-of-way (CFIA, 2008). In Ontario, the MNRF has been involved with invasive Phragmites control pilot projects since 2007 and to date control costs range between $865 and $1,112 per hectare (OMNRF, 2012). Invasive species have an impact on approximately 20% of Species at Risk in Ontario (OMNRF, 2012).

Invasive plants directly affect municipalities with reforestation projects and recreational trails.  They increase management costs (e.g. project planning and monitoring) and they increase operational costs (e.g. mowing, pruning and hand pulling). They also complicate reforestation projects as they need to first be removed, and then the gaps created through removal must be addressed by using large, potted plant stock, or additional site maintenance to prevent the risk of re-invasion.

The Trinational Commission for Environmental Cooperation reported that economic losses and the costs of environmental impacts caused by invasive species exceed $100 billion annually in the U.S. alone (OMNRF, 2012). In Ontario, over $30 million has been spent by the Canadian Food Inspection Agency (CFIA) to slow the spread of emerald ash borer (EAB) (OMNRF, 2012). On a municipal scale, the City of Toronto has estimated EAB management costs for 2013-2020 to be $71.2 million for tree removal, wood disposal, pesticide injection, replacement plantings and staff resources (City of Toronto, 2012).

Adverse Impacts on Recreation and Aesthetics

Natural areas in municipalities support a wealth of recreational activities including hunting, fishing, swimming, hiking, bird watching, and mountain biking. Invasive plants that invade recreational areas often reduce the area’s attractive and enjoyable qualities. For example, invasive plants may reduce native plant biodiversity, affecting the number of songbirds in the area; walking through dense vegetation can prove difficult; and popular swimming areas may become unusable with the presence of invasive aquatic plants. Seeds and other plant parts can hitch rides on hiking boots, clothing, pets, birds and vehicles, resulting in new infestations, potentially over great distances.