The Thousand Islands region in Canada is recognized for its rugged shorelines, wetlands, forests, and the wide variety of wildlife that call this area home. From the waters of the St. Lawrence River to inland wetlands and roadside meadows, these connected habitats support countless native plants and animals.
Not all the species found here however, are part of the area’s natural heritage.
Invasive species have become an increasingly familiar part of the Thousand Islands landscape over the past few decades. Some arrive through international trade, others hitch rides on boats, vehicles, or equipment, or are accidentally released. Once they gain a foothold, many spread rapidly and compete with native species for habitat and resources.
Some are easy to overlook. Others have become familiar sights along roadsides, shorelines, and trails. Over time, invasive species change the landscape and waterways – negatively impacting the ability of natural species that are key members of the eco-system – resulting in a degradation of the whole system.
Here is a look at the invasive species that we tackle, both in the water, and on land, here in the Frontenac Arch.
Unwelcome Guests Beneath the Water
Among the most successful aquatic invaders in the region is the round goby.
Originally from the Black Sea and the Caspian Sea, this small fish arrived in the Great Lakes via ballast water released by cargo ships. Since then, it has spread throughout the Great Lakes and St. Lawrence River system.
At first glance, the round goby doesn’t look particularly unusual, resembling other gobies, logperch, and some sculpin. It has a mottled brown-grey body and large frog-like eyes. A distinctive black spot on its dorsal fin is often the easiest way to identify it. Another giveaway is its fused pelvic fins, which form a suction-cup-like structure that helps the fish to cling to rocks in moving water.
Despite its small size, the round goby has had a significant impact on local ecosystems. It competes with native fish for food and habitat and feeds on both fish eggs and juvenile fish, altering aquatic food webs as its numbers increase.
The rusty crayfish is another invasive species causing concern in local waterways. Named for the rust-coloured patches on either side of its shell, it is larger and more aggressive than many native crayfish species. Rusty crayfish feed on aquatic plants, fish eggs, and small aquatic animals while pushing native crayfish from their preferred habitat.Â
For anglers and boaters, preventing the spread of these species begins with a few simple habits. Never release unused bait into the water, and always clean boats, trailers, and equipment before travelling to another lake or river.
Small Mussels, Large Impacts
Some of the most influential invasive species in the Thousand Islands are also among the smallest.
Zebra and quagga mussels arrived in North America in the late twentieth century and quickly spread throughout the Great Lakes basin. Today, they can be found attached to docks, rocks, boat hulls, water pipes, and almost any hard surface below the waterline.
These mussels can blanket rocks, docks, and water intake pipes in dense colonies containing thousands of individuals. While they may appear harmless, their impacts on aquatic ecosystems are far-reaching. By filtering enormous volumes of water, they remove microscopic organisms that form the foundation of aquatic food chains. This can alter nutrient cycles and food availability for native species.
Their impacts are not limited to the natural environment. They can clog water intake systems, damage infrastructure and equipment, and create significant maintenance costs for municipalities, industries, and waterfront property owners.
The best way to deal with them is to prevent them from spreading in the first place. The “Clean, Drain, Dry” approach encourages boaters and paddlers to clean all plants, mud, and animals from their boat and equipment, drain all water from compartments, and dry everything thoroughly before entering another body of water. These simple steps help prevent mussels and other aquatic hitchhikers from spreading between lakes and rivers.
The Green Invaders
Not all invasive species live underwater.
Along shorelines and wetlands, one of Ontario’s most recognizable invasive plants rises above the surrounding vegetation.
Phragmites, also known as European common reed, can grow up to five metres tall, forming dense stands that are difficult to walk through, reduce open water and change water flow, making wetlands less suitable for many birds, amphibians, and reptiles. What begins as a small patch can quickly expand into dense monocultures, crowding out native plants and reducing habitat for wildlife that depend on diverse wetland ecosystems.
Water soldiers present a similar challenge in aquatic habitats. This invasive plant forms floating clusters of stiff, sword-like leaves with sharp serrated edges. As infestations grow, they can interfere with boating, swimming, and fishing, disrupt wildlife movement, and displace native aquatic vegetation.
In forests and fields, invasive plants continue to reshape the landscape.
Common buckthorn, a shrub introduced from Europe, forms dense thickets that prevent sunlight from reaching the forest floor. As a result, many native wildflowers, seedlings, and understory plants struggle to survive.
Garlic mustard creates a different problem. Easily recognized by its heart-shaped leaves, small white flowers, and garlic-like scent when crushed, it spreads aggressively through woodlands. In addition to crowding out native vegetation, garlic mustard can alter soil conditions that many forest species depend upon.
Dog-strangling vine, often shortened to DSV, is another persistent invader. Despite its unusual name, its impacts are serious. The twining vine spreads rapidly, smothering surrounding vegetation and forming mats. It also poses a threat to monarch butterflies. Monarchs may lay their eggs on dog-strangling vine, but the caterpillars are unable to develop successfully on the plant.
When possible, removing small infestations before they produce seeds can help to slow their spread and protect nearby habitats.
A Hazard for People Too
Some invasive species affect more than just local ecosystems.
Wild parsnip has become increasingly common along roadsides, fields, and disturbed areas throughout Eastern Ontario. The plant produces clusters of bright yellow flowers and often grows more than a metre tall.
Although it competes with native vegetation, wild parsnip is best known for the risks it poses to people. Its sap contains chemicals that can cause burns and blisters if it comes into contact with skin that is later exposed to sunlight. So, if you are planning to handle or remove wild parsnip, make sure to wear protective clothing, including long sleeves, pants, gloves, and eye protection. Avoid direct contact with the sap whenever possible.
If You Find an Invasive SpeciesÂ
One of the most effective tools in invasive species management is early detection.
When new infestations are discovered quickly, there is a much better chance of slowing their spread and reducing long-term impacts.
If you encounter a suspected invasive species:
- Take photographs.
- Record the location.
- Report the sighting through Ontario’s invasive species reporting programs or to a local conservation organization.
Many successful management efforts begin with observations from residents, boaters, anglers, hikers, and landowners who take the time to report what they see.
Protecting the Thousand Islands
Invasive species remain one of the most significant conservation challenges facing ecosystems across Ontario, including the Thousand Islands region. While their impacts can be substantial, there are still plenty of ways people can help.Â
Every boat that is cleaned before entering a new waterbody, every invasive plant removed before it begins to seed, and every reported sighting contributes to the protection of native habitats and wildlife.
This year, TIWLT’s wildlife biology crew has spent many hours removing invasive plants from several of our protected properties, including garlic mustard and dog-strangling vine. While eliminating invasive species entirely is rarely possible, targeted removal helps native plants begin to recover, improves wildlife habitat, and slows the spread of existing infestations while preventing new ones from becoming established. Removing invasive plants also creates space for native wildflowers, shrubs, and trees to regenerate naturally, supporting healthier habitats for birds, pollinators, reptiles, and other wildlife.Â
This ongoing stewardship work is an important part of TIWLT’s mission to conserve and care for the natural landscapes that make the Thousand Islands such a remarkable place.
The next time you’re out on the water, walking a shoreline trail, or enjoying a day at the cottage, take a closer look at your surroundings. You might be surprised by how many invasive species are already part of the landscape. Recognizing them is a small step, but it helps protect the habitats and wildlife that make the Thousand Islands special.Â
By Alexis Osborne
Thousand Islands Watershed Land Trust
References
Government of Ontario. Invasive species in Ontario. Ontario.ca. Available at: https://www.ontario.ca/page/invasive-species-ontario
Government of Ontario. Wild parsnip. Ontario.ca. Available at: https://www.ontario.ca/page/wild-parsnip
Government of Ontario. Dog-strangling vine. Ontario.ca. Available at: https://www.ontario.ca/page/dog-strangling-vine
Government of Ontario. Garlic mustard. Ontario.ca. Available at: https://www.ontario.ca/page/garlic-mustard
Government of Ontario. Common buckthorn. Ontario.ca. Available at: https://www.ontario.ca/page/common-buckthorn
Government of Ontario. Water soldier. Ontario.ca. Available at: https://www.ontario.ca/page/water-soldier
Government of Ontario. Invasive phragmites best management practices. Ontario.ca. Available at: https://www.ontario.ca/page/invasive-phragmites-best-management-practices
Invasive Species Centre. Species profiles and identification resources. Invasive Species Centre. Available at: https://www.invasivespeciescentre.ca
Ontario Federation of Anglers and Hunters. Invading Species Awareness Program. Invading Species Awareness Program. Available at: https://www.invadingspecies.com
Fisheries and Oceans Canada. Round goby (Neogobius melanostomus) species profile. Government of Canada. Available at: https://www.dfo-mpo.gc.ca/species-especes/profiles-profils/round-goby-gobie-taches-noires-eng.html
Fisheries and Oceans Canada. Zebra mussel species profile. Government of Canada. Available at: https://www.dfo-mpo.gc.ca/species-especes/profiles-profils/zebramussel-moulezebree-eng.html
Fisheries and Oceans Canada. Quagga mussel species profile. Government of Canada. Available at: https://www.dfo-mpo.gc.ca/species-especes/profiles-profils/quaggamussel-moulequagga-eng.html
Fisheries and Oceans Canada. Rusty crayfish species profile. Government of Canada. Available at: https://www.dfo-mpo.gc.ca/species-especes/profiles-profils/rusty-crayfish-ecrevisse-taches-rouge-eng.html
Government of Canada. Aquatic invasive species program. Fisheries and Oceans Canada. Available at: https://www.canada.ca/en/fisheries-oceans/services/aquatic-invasive-species.html
Ontario Ministry of Natural Resources. Invasive phragmites best management practices. Government of Ontario.
Ontario Ministry of Natural Resources. Water soldier. Government of Ontario.
Nature Conservancy of Canada. Invasive species and habitat conservation resources. Nature Conservancy of Canada. Available at: https://www.natureconservancy.ca
Parks Canada. Wildlife safety in Thousand Islands National Park. Government of Canada. Available at: https://parks.canada.ca/pn-np/on/1000/securite-safety/faune-wildlife
Thousand Islands Association. Invasive species. Thousand Islands Association. Available at: https://www.thousandislandsassociation.com/invasive-species/
Ontario Parks. Invasive species in Ontario. Ontario Parks Blog. Available at: https://blog.ontarioparks.ca/invasive-species-in-ontario/
