An overview of invasive species present and future in the Kawarthas

Peterborough Examiner  – September 15, 2023 – by Drew Monkman

In addition to the epic wildfires and floods that climate change has helped deliver this summer, another environmental crisis continues to unfold: the huge impact of invasive species. These are animals, plants, fungi and micro-organisms introduced accidentally or deliberately by human action outside of areas where they occur naturally.

Invasive species are a threat that’s always on my mind in early fall when I see the legions of dead ash trees across the city and county. Where once these trees added stunning reddish-purple foliage to fall roadsides and woodlots, all that remains are their lifeless skeletons – all courtesy of the invasive emerald ash borer. 

A further reminder of this crisis came on September 4, when the International Platform on Biodiversity and Ecosystem Services (IPBES) released a report showing how invasive species pose a growing and costly threat to biodiversity, food security, the global economy and human health. They are causing biological havoc and economic woe across the planet. Invasive species have also contributed to 60 per cent of global plant and animal extinctions and are leading to a species homogenization of much of the world. Not only are humans accelerating the inadvertent movement of species through activities like shipping and air transport, but now global heating is amplifying their spread and impact.

Just last week the invasive species emergency went from a news story to something personal when I learned of the tragic death of Dr. Mike Cranfield. He was a Peterborough native and public school friend of mine. Mike was also a wildlife veterinarian and world-renown conservationist who led the Mountain Gorilla Veterinary Project (Gorilla Doctors). He died from West Nile Virus, an invasive pathogen first found in Uganda that has spread nearly worldwide.  It’s believed he contracted the virus from mosquito bites while working on his cabin north of Peterborough.

So, with invasive species weighing heavy on my mind, I want to provide an overview this week of some of the Kawarthas’ most egregious invasives and what newcomers we might expect.   

Top (L to R): dead ash trees (Drew Monkman), phragmites (Drew Monkman), garlic mustard
Bottom (L to R): round goby (Michael Fox), Norway rat (Sonja Barker), Asian tiger mosquito

Invasive plants

Ontario is home to hundreds of invasive plants, most of which have arrived from Europe and Asia. Many were introduced deliberately but most simply showed up as seed hitchhikers in shipments of grain or other goods. Some, like ox-eye daisy, are generally harmless, while others have come armed with aggressive reproductive qualities that enable them to displace our native plant communities.  

Most people are familiar with purple loosestrife, an invasive plant that adds touches of pink to summer wetlands. In some rare good news, loosestrife has declined thanks to the successful introduction of the loosestrife beetle which eats the plants. However, other invasives like phragmites, dog-strangling vine, garlic mustard, Japanese knotweed, European buckthorn and, in our freshwater ecosystems, Eurasian water-milfoil and starry stonewort, continue to spread almost unabated.  

Phragmites (common reed) has the dubious honor of being Ontario’s worst and most visible invasive plant. It is particularly abundant at the corner of the Parkway and Sir Sandford Fleming Drive. Phragmites is a perennial grass that grows up to 15 feet tall with bushy, golden panicles that turn grey and fluffy in fall. It can completely take over a marsh or damp meadow community, crowding out native plants and seriously degrading the area as wildlife habitat. Even turtles struggle to walk through it. Its seeds are easily wind-dispersed, allowing further spread.

Invasive invertebrates

The impact of invasive invertebrates is equally concerning. The emerald ash borer, for example, has killed millions of ash trees across Ontario and thousands in Peterborough. Many of these can be seen in the same location on the Parkway (see above) as the phragmites. Woodlands that lose significant numbers of ash soon become more vulnerable to an onslaught of other invasive species. As for the loss of urban canopy trees, it is heart-wrenching. A huge white ash was cut down just this week on Conger Street. Fortunately, the City of Peterborough is planting hundreds of trees on public property every year to replace ash trees.

Other invasive insects that wreak havoc on our trees include spongy (formerly gypsy) moths that defoliated huge areas of forest mostly north of Peterborough just two years ago; woolly beech scale which, in association with a fungus, is killing beech trees; the European elm bark beetle, which spreads the fungus that causes Dutch elm disease; and Asian long-horned beetles that attack a wide variety of hardwood trees like maples. Another invader, the hemlock woolly adelgid, is expanding northward from the Hamilton area and leaving dead hemlocks in its wake.

Zebra mussels, too, continue to be a problem in the Kawartha Lakes. They clog pipes, affect fish spawning areas, and are linked to toxic algal blooms. They are especially harmful to native mussels, many of which are species at risk.

When it comes to impacts on human health, black-legged (deer) ticks are one of the main offenders. They were previously unheard of in Ontario, fended off by cold winters. But, with shorter and milder winters, this species is now well-established in the Kawarthas and is spreading Lyme disease.

Looking ahead, there is concern that new mosquito species could bring exotic diseases. According to recent modelling done by Toronto’s BlueDot, a company that maps the spread of infectious diseases, southern Ontario could soon become host to Asian tiger mosquitoes. They are already established in much of the eastern U.S. and are considered one of world’s worst invasive species. The tiger mosquito is an important vector for the transmission of many viral pathogens including Zika, Chikungunya, dengue, yellow fever, and West Nile. It can also transmit a parasitic heartworm that causes heart failure in dogs and cats.  

Mammals, fish & fungi

            Peterborough and the Kawarthas is also home to invasive mammals, fish and fungi. Friends of ours who live near the hospital are now dealing with a Norway rat invasion. We’ve also learned that Peterborough’s homeless encampment is overrun by these invasive mammals. Originally from Asia, rats transmit disease, cost businesses hundreds of millions of dollars in damage each year and contribute to the extinction or range reduction of native animals.            

Feral and free-roaming cats also qualify as invasive species. Originally from Africa, our native wildlife did not evolve to defend themselves against cat attacks. It’s estimated that 100 million birds fall victim to cats each year in Canada. Wild boars, too, are a growing problem. Native to Eurasia, they have been reported across southern Ontario including a handful of sightings in Peterborough County and Kawartha Lakes. They can destroy native ecosystems and decrease biodiversity.

Round gobies, an invasive fish, are now well established in the Otonabee River. According to Graham Raby, an assistant professor of biology at Trent University, they are especially abundant in the stretch of river leading up to Lakefield and will likely be in Stoney Lake within a few years. They are an aggressive competitor to native fish and are also connected to botulism in birds.

Invasive fungi can also be killers. Bretziella fagacearum, which probably emerged in Mexico,causes a fatal disease called oak wilt.  As of this summer, the disease has been spotted north of Toronto and in Niagara-on-the-Lake. Butternut canker is another invasive fungus. It infects and kills butternut trees which are now endangered in Ontario.  

Finally, there is an invasive fungus that is killing frogs. Known as Batrachochytrim or chytrid fungus, it has been linked to dramatic frog population declines or extinctions in western North America and much of Latin America. Although the fungus is present in Ontario, its impact so far is minimal. That, however, may change as the climate warms.

Government irresponsibility

In yet another failure of the Ford government to protect the environment, Ontario’s response to invasive species is weak and ineffective. In a 2022 report, Ontario Auditor General Bonnie Lysyk wrote, “With the global COVID-19 pandemic, people now understand the urgent need to act when a dangerous new virus or other biological threat emerges. The same is the case for responding to invasive species.” She went on to say that, “Overall, our audit found that the Natural Resources Ministry is not effectively monitoring and managing the introduction and spread of harmful invasive species in Ontario.” There is no unified framework in the province to identify and respond to invasive species. 

You can report invasive species at or by calling the Invading Species Hotline at 1-800-563-7711. Remember that the iNaturalist app is an excellent tool for identification purposes.

Categories: Columns

Drew Monkman

I am a retired teacher, naturalist and writer with a love for all aspects of the natural world, especially as they relate to seasonal change.