9 Most Dangerous Invasive Species in Ontario

Nature is a wonderful thing. Our planet’s biodiversity is unparalleled in the universe (at least, as far as our knowledge goes), and this means that every region has its own unique plants and animals.

However, due to human movement, whether intentional or otherwise, invasive species end up in another region’s environment and cause harm to that region’s native species.

1. Top 9 Invasive Species in Ontario

1.1 Zebra Mussels and Quagga Mussels

invasive species in ontario
Photo by CHUTTERSNAP on Unsplash/Copyright 2018

Zebra and Quagga Mussels are freshwater molluscs indigenous to the Black Sea region of Eurasia. Both species were thought to have arrived through the ballast water of transoceanic ships.

Quagga Mussels have spread beyond the southern Great Lakes and can be found in various parts of the United States, including Texas, Colorado, Utah, Nevada, and California.


  1. These invasive mussels remove plankton from the water, and native species no longer have access to it as a food source.
  2. Large colonies may encroach on beaches and fish-hatching grounds, slashing swimmers’ feet.
  3. Due to their large colonies, they also obstruct water intake lines.
  4. They can increase toxic algal blooms, which lowers the quality of the water and adversely impacts native fish and aquatic wildlife.

1.2 Phragmites

Also called the European Common Reed, these aggressive phragmites outcompete native species for water and nutrients and spread swiftly. Its roots exude biochemicals into the soil, preventing neighbouring plants from growing.

Invasive phragmites typically grow up to 5 meters tall.


  1. They lower plant biodiversity due to natural plants being crowded out, as they can grow so densely that they crowd out other species.
  2. It offers limited habitat and food resources to the general wildlife, thus causing issues within the region’s food chain.
  3. They cause lower water levels resulting from rapid growth because they shed moisture more quickly than native plants would.
  4. It increases fire risks since the reed stands contain many dead stalks.
  5. They can adversely influence agriculture, provide a risk to traffic safety, and affect leisure activities like swimming, boating, and fishing.

1.3 Water Soldier

This is a submerged, perennial, invasive aquatic plant, and it is believed that vegetative reproduction is the main method of dissemination in North America.


  1. Water soldiers can be underwater up to 5 meters, but during the summer, they float to the surface. Summertime pastimes like swimming, boating, and fishing are put in danger by this.
  2. They create stagnant waterways by forming dense floating vegetation mats, reducing biodiversity by displacing indigenous aquatic species.
  3. They also can change the chemical of the nearby water, which could hurt phytoplankton and other significant aquatic creatures.
  4. Swimmers and anybody who touches these plants risk getting hurt by the plants’ sharp, serrated leaf edges. So, be careful handling them if you come across them.

1.4 Purple Loosestrife

invasive species in ontario
Photo by Liana Mikah on Unsplash/Copyright 2018

Nearly every province of Canada and practically every state in the United States now has Purple Loosestrife, which has spread quickly throughout the continent.


  1. It can produce up to two million seeds in a single growing season, resulting in dense stands that outcompete natural plants for habitat.
  2. Due to these populations, ecosystem services are altered, which includes fewer places for birds to nest, shelter, and food, as well as a general reduction in biodiversity.
  3. Large patches of this plant can ruin farmland, obstruct irrigation channels, and lower the fodder value of pastures.

1.5 Dog Strangling Vine

Black Swallow-Wort and Pale Swallow-Wort, two invasive plants that are native to Eurasia, are both referred to as “Dog-Strangling Vines.” These perennial vines have been aggressively colonizing central and southern Ontario in recent years.


  1. This vine aggressively spreads by draping itself along the ground or wrapping itself around other plants and trees, causing them to become “strangled” by dense vine patches.
  2. It grows into dense stands that suffocate and choke out new trees and other plants, hindering the regeneration of forests.
  3. It is challenging to eradicate because the seeds are quickly dispersed by the wind, and new plants can arise from root pieces.
  4. Colonies create mats of intertwined vines that are difficult to navigate and obstruct recreational activities in the forest.
  5. Leaves and roots may poison livestock. Deer and other animals avoid it as well, putting less desirable native plants under increased grazing strain.
  6. The monarch butterfly, a species in danger in Ontario, is in danger from the vine. The butterflies lay their eggs on it, but the larvae cannot finish their life cycle and die.

1.6 European Buckthorn

invasive species in ontario
Photo by Yulia Khlebnikova on Unsplash/Copyright 2020

European Buckthorn, also known as Common Buckthorn, is a native of Europe. It was frequently planted for fencerows and windbreaks in agricultural areas after being brought to North America as an ornamental shrub.

Since then, it has spread rapidly. It can be found throughout Canada, from Saskatchewan to Nova Scotia.


  1. It outcompetes native plants, lowers biodiversity, degrades the quality of wildlife habitats, and negatively impacts a variety of industries and the environment.
  2. It grows in thickets that push out native species.
  3. It can change the amount of nitrogen in the soil, improving circumstances for its own growth and inhibiting the growth of native species.
  4. It produces a lot of seeds, many of which germinate quickly and stop the growth of local trees and plants.

1.7 Giant Hogweed

A member of the carrot family, the Giant Hogweed is grown for decorative purposes in gardens. It can grow in ditches, streams, and roadside edges. It is spreading across North America and is becoming more prevalent in southern and central Ontario.


  1. Giant hogweed possesses a toxic sap that can severely burn human skin when exposed to light. Blisters that hurt appear within 48 hours as symptoms. Purple scars can develop and remain for years.
  2. Because of this, removal can be risky; therefore, it shouldn’t be composted or burned.

Pulling giant hogweed when it is still very young and small can be done, but it carries a high risk due to the plant’s sap, which can cause severe burns when exposed to sunlight. Therefore, it is recommended to use herbicides and seek professional help to control giant hogweed.

Garden gigantic hogweed should not be planted, and sightings should be reported.

1.8 Emerald Ash Borer

An invasive wood-boring beetle that is native to areas of Asia is called the Emerald Ash Borer (EAB). It has been found in the Detroit, Michigan, and Windsor, Ontario, regions.


  1. Millions of Ash trees in southwest Ontario and the Great Lakes States have been devastated by this forest pest, causing the loss of expensive wood used to make furniture, buildings, and leisure products.
  2. When its larvae burrow through a tree’s vascular system, which distributes water, nutrients, and sugars throughout the tree, it kills Ash trees.
  3. There are some known natural enemies, such as wasps and bark-foraging birds, that may limit population growth or spread and cause loss of food sources and habitat for other animals.

The Canadian Food Inspection Agency has declared it illegal to move firewood and any other items made from Ash trees outside of the areas classified as Infested Places Orders due to the significant economic and environmental damage they pose.

1.9 Beech Bark Disease

A new hazard to beech (Fagus grandifolia) trees in Canada’s hardwood and mixed forests is beech bark disease, first identified in North America.

Since then, it has rapidly spread and is currently present in several American states as well as Ontario.

An imported European beech scale insect and the nectria fungus work together to create this illness.


  1. Beech bark disease causes older beech trees to die back, potentially posing a serious danger to Ontario’s wildlife, biodiversity, and sustainable forestry.
  2. It reduces the number of trees that provide food to wildlife. Beechnuts are a crucial food source for wildlife, particularly for black bears.
  3. It seriously weakens trees, putting them at risk of additional challenges.

While this new disease seriously threatens the majestic beech stands of Ontario, not all beech are affected, and individual beech tree prevention is achievable.

In Pursuit of Invasive Species: Ontario Forests

2. Frequently Asked Questions (FAQs)

2.1 What are Invasive Species?

Invasive species refers to any plants, animals, or other organisms that are not native to an ecosystem and that cause harm to the environment, the economy, or human health.

2.2 How Many Types of Invasive Species in Ontario Are There?

Ontario has the most invasive species in all of Canada – more than 180 aquatic invasive species, approximately 500 non-native plants, 39 known forest insects and 10 tree diseases.

2.3 What Is the Most Invasive Species in Ontario?

Depending on the region, the answer to this will differ, making it a very subjective question. .

2.4 How Can These Invasive Species Be Managed?

Managing invasive species that have already established themselves is difficult, but some steps can be taken. To help stop the spread of aquatic invaders, make sure that any vegetation, animals, and dirt are removed from boats and trailers before leaving a location.

Avoid boating through infected areas, clean any sporting goods, and never release or compost unwanted aquarium plants to stop the spread of this disease. Find out more here.

2.5 What Steps Is the Government Taking to Control Invasive Species in Ontario?

The Invasive Species Act, which governs the prevention and management of invasive species in Ontario, was introduced by Ontario’s provincial government in 2015. This Act created two categories – prohibited and restricted. Initially, there were 22 prohibited and 11 restricted invasive species. In January 2022, 13 new invasive species were added.

3. Closing Notes

Invasive species in Ontario have travelled here from great distances and are causing huge amounts of damage to the native wildlife habitats and the ecosystems of this region.

It is important to be careful during outdoor activities to prevent spreading them through human action, especially during water activities and hiking. Clean and dry boats and shoes, and make sure to check and clean the fur of your pets.

Learning to identify this species so that you can report it to the authorities will make all the difference.

Muskaan grew up with her nose always buried in a book, and that love of reading has now spilled over into a love of writing. She also has a passion for traveling and enjoys sharing her experiences. Her research skills as a law student have come in handy when planning adventures. Muskaan is dedicated to delivering high-quality content that informs and entertains and is always looking for new and exciting writing opportunities.
Ananya, with a background in commerce, is a passionate writer and has written articles covering a wide range of topics for various different websites. She is particularly interested in travelling and exploring various places and cultures across the world. Ananya also works as an editor, guiding fellow writers and helping provide the audience with content that is useful to them.