Blog & News

Pathways for invasive species

Published on Saturday July 17, 2021
Authored by PEIISC

An invasive species is any organism (plant, animal, insect, disease, etc.) that has been introduced to an area outside of its native range, has a tendency to spread, and threatens the environmental, economic or social health of the area to which it has been introduced.

We move species around the world every day, both intentionally and unintentionally, through our social and economic activities, and naturally through environmental pathways. Invasive species tend to show up more around urban centres and transportation hubs.


Historic Introductions

Many well-established invasive species on PEI arrived here long ago. Some of them arrived with early settlers that brought plants with them, to their new home, as a memory of their homeland or for their medicinal properties.

Outdoor Recreation

Many invasive species have traits that make them excellent invaders. These traits include small seed mass and seed adaptations that allow for greater dispersal and rapid expansion of invasive populations. Some adaptations can allow seeds to easily attach to clothing or our pet’s fur, or get stuck in our boot treads. If our equipment or pets are not properly cleaned before outings, we could be spreading invasive plant seeds to new areas. Anytime you are hiking, biking, or walking your pet in a natural area, be sure to clean all of your equipment, and your pet, before exploring a different natural area.


Recreational boating is a significant pathway for the spread of aquatic invasive species. Small animals (e.g. zebra mussels (Dreissena polymorpha)) can attach themselves to boat hulls and aquatic plants (e.g. Canada waterweed (Elodea canadensis)) can get tangled in propellers and trailers. If boats are not properly cleaned, they can end up giving these invasive “hitchhikers” a ride to a new waterway.

Bird seed

Feeding wild birds is a popular pastime. Unfortunately, the seed mixes we buy often contain invasive weed seeds. Seeds that end up on the ground underneath of bird feeders can establish, flower and spread. Research from Oregon State University found that 10 common brands of seed sold in retail stores contained more than 50 weed species.

Firewood movement

The movement of firewood is another important pathway for invasive species. Some invasive insects can bore into wood (e.g. emerald ash borer), while others lay their eggs on wood (e.g. gypsy moth), making the movement of firewood an effective pathway for invasive species.

Garden Waste

Gardeners may be contributing to the spread of invasive species by dumping unwanted plants in natural areas. Often, when an invasive takes over a garden patch, gardens will remove the plant and dump the plant debris in a nearby woodlot or field. Those invasive species often survive and give rise to new invasive species infestations.



Agriculture is a major vector for invasive species introductions. Many invasive plants have been introduced intentionally as agriculture crops and then later escaped cultivation. An example of this is white sweet clover (Melilotus albus), which was intentionally planted as a forage crop. Now, white sweet clover is ubiquitous on PEI – you can find it growing around farmers’ fields and along roadsides across the Island.

Invasive species have also been introduced unintentionally through agriculture via contaminated seeds and soil.

White sweet clover (Melilotus albus). Photo by Rob Routledge, Sault College,


The horticulture industry is a significant pathway for invasive species. Most of the flowers and trees that are sold at greenhouses and garden centres are not native; and most are not invasive, however some are. On PEI, there is no legislation prohibiting businesses and individuals from selling or trading invasive species – except for The Weed Act, which only regulates one species, purple loosestrife (Lythrum salicaria). Many of the invasive species we have on PEI are consequences of the horticulture industry. For example, Himalayan balsam (Impatiens glandulifera) was brought to North America from Asia as an ornamental plant. It escaped cultivation and is now on invasive species lists across the country.

Additionally, imported flowers and trees can carry new diseases and insects that also have the potential to become invasive.


The forestry industry is a major pathway for invasive species. Equipment used for land-clearing, if not properly cleaned, could result in the spread of invasive species. Sometimes trees planted for commercial forestry are non-native and may become invasive. The movement of raw lumber and wood products can be a vector for the spread of invasive forest insects or disease.

Games Species/Pet Trade

Pets and game species are sometimes released into the wider environment, and sometimes they become invasive. Pets are often released because their owners no longer can or want to care for them, and game species are released for recreational hunting or fishing purposes. An example of this is the release of goldfish into pond systems by pet owners. Goldfish introductions have become a major issue in some jurisdictions in Canada and the U.S.

Goldfish (Carassius auratus). Photo by U.S. Fish and Wildlife Service ,

Global trade and travel

Our world has become incredibly connected. The goods we buy at the grocery store may be from half way around the world. As we move goods around, we are potentially also moving invasive species. The shipping process is a significant pathway for invasive species. Many invasives that are present on PEI arrived here as a result of the shipping industry. Common Reed Grass (Phragmites australis ssp. australis) is a serious issue for wetlands across Canada, and arrived in North America accidentally via ballast materials. Some invasive, wood-boring insects have been introduced to new areas by the movement of wood pallets used for shipping.

As individuals travel, they could unknowingly be carrying invasive plant seeds on their shoes and clothing, and furthering the spread of invasive species.



Many invasive plants have special adaptations that enhance their ability to disperse seed and spread further.  Some of these adaptations take advantage of the movement of wildlife.  In this way wildlife can contribute to the spread of invasive species.  For instance, common buckthorn (Rhamnus cathartica) produces berries which contain its seeds.  The berries often persist into the winter months, making it an attract food source for wintering birds.  However, the berries also contain a natural laxative so seeds are quickly spread by birds who eat the berries.

Common buckthorn fruit cluster. Photo by Green Thumb Photography.

Water and Wind

Another common seed adaptation of invasive species is small seed mass.  A small mass allows seeds to be carried on the wind.  Invasive fungi spores and flying insects may be moved as a result of wind as well.  Some invasive species’ seeds are buoyant, allowing seeds to float and disperse naturally via waterways.

Climate change

As our climate changes, we are seeing extended springs and late winters. We are changing the conditions for all living things around us. The extended growing season is particularly beneficial for invasive plant species, which are adaptable enough to change their flowering schedule to emerge and bloom earlier in the spring. This gives invaders the benefit of being able to take advantage of limited resources before native species get a chance to. Native species are slower to adapt to these changing conditions.