5.2 How do Invasive Species Spread?

Pathways are the routes by which invasive species are introduced to an area. Invasive species can spread through natural pathways or man-made pathways. Natural pathways, including wind, water, and animal dispersal, are common in helping an invasive species spread once they are established in a new habitat. The most common ways for invasive species to get introduced to new areas are through man-made pathways, including:

Travel

Many invasive species are accidentally introduced as humans travel. They can hitchhike on vehicles, clothing, or cargo and can be easily transported around the world. The first invasive species in North America were unintentionally brought by European settlers. For example, brown rats (Rattus norvegicus) were spread here and all around the world as stowaways on the ships of settlers.

Today, people are unintentionally bringing new invasive species to Canada through travel and tourism. The Canada Border Services Agency (2023) plays an important role in helping to keep harmful foreign species from entering Canadian ecosystems: “various food, plant and animal products are restricted or prohibited entry because they can harbour invasive species, foreign animal diseases, and plant pests”.

Global Trade

Global trade has made it more common for invasive species to cross international borders, often unintentionally. Ships can transport aquatic invasive species in their ballast water. For example, zebra mussels (Dreissena polymorpha) first arrived in the Great Lakes in ballast water carried by ships from Asia (Upper Thames River Conservation Authority, n.d.-b). There are now Ballast Water Regulations that require ballast water to be cleaned before release to reduce the spread of aquatic invasive species.

Terrestrial plants and animals are also spread by global trade. For example, the emerald ash borer (Agrilus planipennis) is an invasive wood-boring beetle that is native to Asia. It was likely introduced to Detroit in the 1990s, hidden inside wooden shipping crates. The Emerald Ash Borer spread to Ontario in 2002, killing up to 99% of ash trees in its path. To this day, it continues to devastate the ash tree population as it spreads in all directions across North America, where ash trees are present (National Resources Canada, 2024).

Now, there are regulations for wood packaging materials used in international trade requiring wood to be treated to kill any possible pests to prevent their spread.

Recreation

Various recreational activities can spread invasive species. For example, after the zebra mussels were introduced into the Great Lakes through global trade, they were then spread locally by boaters and anglers. The mussels or their larvae could attach to boats and equipment and were transferred from contaminated sources to other water bodies around Ontario. Public education campaigns like Clean Drain Dry are used to help prevent the spread of aquatic invasive species via recreation.

Another common way for people to unintentionally spread invasive species recreationally is through moving firewood. Firewood can contain larvae for invasive pests, like the Emerald Ash Borer. When you transport that firewood to other locations, you are also unwittingly helping the invasive species spread to new areas. There are regulations that restrict the movement of firewood to slow the spread of the Emerald Ash Borer and other invasive pests.

Horticultural Practices

Horticultural practices are one of the most significant sources of invasive species through the intentional and unintentional movement of plants and soil. Gardeners and landscapers often introduce exotic plants for their aesthetic appeal or hardiness without realizing these species can escape cultivation and invade local ecosystems. Once they have established in a new area, natural dispersal mechanisms, such as wind, water currents, and animal movement, can further spread these invasive species.

Common ornamental garden plants like Lily of the Valley, goutweed, Japanese knotweed, and honeysuckles, among many others, have spread aggressively into natural habitats and outcompete native flora. Additionally, soil and mulch used in gardening can harbour invasive plant seeds, insects, and pathogens.

Although horticulture is the most significant pathway for the introduction of invasive plants, Canada’s legislative framework is out-of-date and fragmented. Many plants that are known to be invasive and damaging to local ecosystems are still being sold for landscaping. In Ontario, it is illegal to sell certain plants (e.g. knotweeds, phragmites, dog-strangling vine), but it doesn’t regulate many other known invasives (e.g. goutweed, Lily of the Valley, Japanese barberry, English ivy, honeysuckles, Norway maple, etc.). Canada is in need of a comprehensive database with invasive plant ratings and regulatory tools to prevent invasive plants from being sold to the public. The Canadian Coalition for Invasive Plant Regulation is a group of volunteers that are working to raise awareness about the negative impacts of invasive plants and are actively advocating for improving regulations.

Pets

Many animals and plants sold for aquariums are not native to Canada. Some owners release pets into the wild when they become too large, too difficult to care for or when they simply don’t want them anymore. These released pets may survive, reproduce and spread, becoming aquatic invasive species. For example, disinterested owners often release goldfish into creeks and ponds when they no longer want the fish at home. Feral goldfish were first detected in London in 2001, and the population has since boomed, causing significant damage to our local aquatic ecosystems. Under federal law, it is illegal to release goldfish and other aquatic species in the wild, but many people ignore this law. Further public education is needed to explain the impacts of releasing aquatic pets in the wild to try to prevent further release and subsequent destruction.

Intentional Introduction

Humans have intentionally introduced species for various reasons, including economic benefits, aesthetic enhancement, pest control, recreational activities, and cultural or historical significance. For example, many species, including Norway Maple, Common Buckthorn, and House Sparrows, were brought to Canada to make European settlers feel more at home by recreating familiar landscapes.

The intentional introduction of species has led to unintended ecological consequences, as the introduced species become invasive and disrupt ecosystems. A prime example of this is the introduction of the European starling to North America. In 1890, approximately 100 starlings were released in Central Park, New York, by a group led by Eugene Schieffelin. This group aimed to introduce all bird species mentioned in the works of William Shakespeare to North America. The starlings quickly adapted to their new environment and spread across the continent.