Invasive Species

Invasive species are species of plants, animals, or insects that are not native to the area. They can invade our meadows, woodlands, wetlands, and waters, damaging and pushing out the native plants and animals that belong there.

Why Should We Care?

Invasive species pose the greatest threat to biodiversity after habitat loss. They have few natural predators, can grow, and reproduce quickly, and can adapt to new environments faster than native species.

Their presence disrupts habitats and food sources for the animals and plants that already live there.

Invasive species are troublemakers in our environment, proving costly and difficult to eliminate. It has been estimated that the financial impact for Conservation Authorities and municipalities to manage invasive species in Ontario is $51 million a year.

As the largest urban centre in Canada, the Greater Toronto Area, has many invasive species.

Since invasive species management requires significant time and resources, TRCA focuses on monitoring and management actions for all high priority invasive species in all high priority areas.

TRCA staff monitor the Lake Ontario Waterfront for the presence of invasive carp
TRCA staff monitor the Lake Ontario Waterfront for the presence of invasive carp.

Where Do They Come From?

Many invasive plants came to North America from overseas hundreds of years ago, when plants and seeds were brought here to be grown for food or displayed in ornamental gardens.

Invasive plants are introduced and spread by infested packaging material, seed dispersal by both environmental and human sources, and by escaping from gardens.

Invasive fish can be introduced and spread in various ways, including ballast water, movement of bait, the aquarium and water garden trades, live food fish, unauthorized introductions, and canals and water diversions.

Forest pests and pathogens are generally associated with the import and export of wood packaging materials, shipping containers, and the movement of firewood.

Meet Our Top Invaders

The Toronto region is home to numerous invasive plants, animals, and insects. You may have seen some in your garden or encountered them when out hiking or fishing in natural areas.

Invasive Plants

Some of the invasive plants that pose the biggest problems for our local environment include European buckthorn, dog-strangling vine, garlic mustard, wild parsnip, Japanese knotweed, and phragmites.

Dog-strangling vine
Dog-strangling vine
wild parsnip
Wild parsnip

These are determined plants, with intricate root systems that allow for rapid spread across ecosystems.

Simply pulling them up often makes the problem worse. For example, if you try to pull garlic mustard out of the ground, an emergency chemical response allows microscopic parts of the plant to remain in the soil and regenerate.

Some invasive plants can even be dangerous. If the sap from wild parsnip gets on your skin, and your skin is then exposed to sunlight, serious burns can occur.

Invasive Fish and Aquatic Species

Invasive fish and other aquatic species such as crayfish can be found in our lakes and rivers.

The ones posing serious issues are: Asian Carp, Goldfish, Round Goby, and Sea Lamprey.

grass carp discovered at Tommy Thompson Park in 2015
Invasive carp discovered at Tommy Thompson Park in 2015.

These species reproduce quickly and can thrive in many habitats and conditions. Asian Carp and the Round Goby have big appetites and can reduce or eliminate food sources and habitats for native fish such as trout and salmon.


Sea Lamprey are parasitic, attaching themselves to other fish and feeding on their internal fluids, eventually killing the host.

Forests Pests and Diseases

The species that pose the most serious threats to the health of our local forests include the Emerald Ash Borer and hemlock woolly adelgid.

Emerald ash borer
Emerald Ash Borer

The Emerald Ash Borer was introduced to North America through untreated wooden shipping pallets. Adults lay their eggs in hardwood trees, and larvae then tunnel through the living tissue of native ash trees stopping the flow of water and nutrients, killing the trees.


Hemlock woolly adelgid (HWA) is an aphid-like insect (aphids suck fluid from plants) that attacks and kills hemlock trees by feeding on nutrient and water storage cells at the base of needles. It has no natural controls.

This species has yet been detected in the GTA, but has been found in Port Colbourne, Ontario.   We need increased public awareness so that when it does arrive, we are able to find out early and mitigate the damage.

Forests can withstand occasional disturbances such as insect outbreaks, but their resilience is strengthened by having a diversity of healthy trees that can tolerate different stressors.

Continuous long-term monitoring in our regional forests enables conservation managers to track the health and function of these important ecosystems over time.

TRCA and Invasive Species

Toronto and Region Conservation Authority’s (TRCA) Invasive Species Management Strategy guides our response to invasive species. Examples of TRCA monitoring and management include:

  • Long-term monitoring of aquatic and terrestrial species, including the presence of invasive species, to assess the health of our regional watersheds and the Lake Ontario waterfront.
  • Specific monitoring programs to help our partners proactively survey and remove invasive species, such as Sea Lamprey and Asian Carp
  • Invasive species management in the management and habitat restoration plans for our conservation areas. In 2023, TRCA managed 184 ha of invasive plants to reduce the extent and density of these unwanted species.
  • School education programs and online learning resources related to invasive species
  • Community events, including local volunteer efforts to remove invasive plants, and informative webinars on invasive species.

Community volunteers help to remove invasive garlic mustard
Community volunteers help to remove invasive garlic mustard.

Emerging threats such as Asian Carp, Oak Wilt, and Spotted Lanternfly are already on TRCA’s radar. Prevention remains the best defense against invasive species, which is why we must work together to become aware and proactive to counter their spread.

What You Can Do

Two key steps you can take to reducing the spread of invasive species are:

  1. Plant native wildflowers, trees, and shrubs.
  2. Clean off your boat and fishing gear before traveling to a new area.

You can also get more information on reporting and dealing with invasive species from Ontario’s Invasive Species Centre and the Canadian Food Inspection Agency.


Additional Resources

Visit TRCA’s Reporting Hub to learn more
about environmental conditions in Toronto and region.



TRCA Watershed Planning and Ecosystem Science: