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Climate change is forcing wildlife to move north — and they're bringing diseases with them

Climate change is forcing invasive species to interact in new ways and new spaces, and as they interact with humans, there's the risk of spreading disease. Read more in this first article by scientist Darius Mahdavi, as part of CBC's Great Lakes Climate Change Project examining climate issues affecting people across Ontario.

More interaction between humans and wildlife like ticks mean rise in risk of spreading disease

A woman standing in a forest examines a piece of white cloth.
Catherine Bouchard, a veterinary epidemiologist with the Public Health Agency of Canada, checks for ticks during drag sampling. (Submitted by Catherine Bouchard)

CBC's Great Lakes Climate Change Project is a joint initiative between CBC's Ontario stations to explore climate change from a provincial lens. Darius Mahdavi, a scientist with a degree in conservation biology and immunology and a minor in environmental biology from the University of Toronto, explains how issues related to climate change affect people across the province and explores solutions, especially in smaller cities and communities.


COVID-19 has shown us how quickly a new disease can spread, upending our lives. Even if it doesn't happen within our lifetimes, research suggests there will be another pandemic and it will likely happen through a disease that reaches humans from animals. 

In Canada, the risk of diseases being passed from animals to humans is relatively low — but not zero. Based on existing trends, some scientists expect the rate of emergence of new diseases to triple over the next several decades due to increased interaction between humans and animals. 

Invasive species — those that enter a new habitat and out-compete native wildlife — may also bring new diseases, which can be devastating. 

With both native and invasive species often having no choice but to move through densely populated areas when searching for new habitats, there is a higher risk of those diseases being passed from animals to humans. 

This is known as zoonosis. 

Zoonosis events can lead to outbreaks of novel diseases, such as SARS-CoV-2, the virus that causes COVID-19. 

Scientists have estimated there are over 10,000 viruses with the potential to infect humans and that are currently residing in animal hosts — and that doesn't include bacteria or other pathogens. 

A recent paper published in the journal Nature shows climate change is increasing the risk of those viruses crossing the species barrier and infecting humans. 

In other cases, known carriers of existing human diseases are being given the opportunity to move into new areas, increasing the risk of transmission.

Here in Canada, many native and invasive species can host and transmit diseases — one of many reasons scientists are wary of species expanding into new areas. 

Enter the blacklegged tick, Ixodes scapularis, found across the eastern provinces, and its cousin the western blacklegged tick, Ixodes pacificus, found on the Pacific Coast. 

Blacklegged ticks are smaller than the common brown dog tick and can be vectors for Lyme disease. (Ben Garver/The Berkshire Eagle via The Associated Press)

Though not likely to cause a pandemic, in Canada, they are the only known carriers of Borrelia burgdorferi, the bacteria that causes Lyme disease, which has been on the rise over the past decade.

They can carry a variety of other pathogens as well. 

Blacklegged ticks, also known as deer ticks, were once a rare sight across Canada. Today, they are found across large parts of Ontario and other provinces, and more during the year than ever before. 

Catherine Bouchard, a veterinary epidemiologist with the Public Health Agency of Canada and adjunct professor at the University of Montreal, has seen this first hand. 

"Fifteen years ago ... sampling for over six months per year, I would find maybe 1,000 ticks over a two-year period," said Bouchard, who works primarily in the Estrie region of Quebec. "Nowadays, when we go out there in the same region ... within two months, we are getting 1,000 ticks."

This is the same trend seen across much of Canada, including Ontario, and Bouchard said it is expected to continue. 

This has also led to a major increase in the number of cases of Lyme disease — an inflammatory disease that can start as a rash, headache, fever and chills, and develop into more serious issues like arthritis, long-lasting fatigue, and neurological and cardiac problems.

In order to transmit Lyme disease, a tick must remain attached for at least 24 hours, with the chance of transmission increasing significantly the longer it feeds, said Bouchard. 

Ticks are typically found in brushy, forested areas where there is leaf litter on the ground. Staying on trails and wearing long clothing is the easiest and most effective way to avoid being bitten.

She added that although only about 20 per cent of ticks carry the disease-causing bacteria on average, it can be as high as 50 per cent in some areas. 

Line graph that shows the number of reported cases of Lyme disease have been increasing in both Ontario and Canada as a whole since 2009, with some fluctuations.
Reported cases of Lyme disease across Ontario and Canada have been rising over the last decade. The data is from Public Health Ontario and the Public Health Agency of Canada. (CBC News)

Moving to new areas

These ticks are one of many species undergoing a range shift — moving north because of climate change. 

"With key climate change drivers such as temperature but also precipitation … the weather that we are experiencing, that is changing, of course it has a direct impact on vector ticks," said Bouchard. 

A range shift occurs when species are forced to move out of their typical homes and into new areas that can support them. 

You can see an example of that in Ontario, where blacklegged tick populations have spread since 2016.

A comparison of two maps of Ontario labelled 2016 and 2022. The 2022 map has considerably more yellow, especially in eastern coastal areas.
Estimated Lyme disease risk areas in Ontario have been growing since 2016. These risk areas are based on where blacklegged ticks are found during active sampling by researchers. (CBC News)

Every species has a niche — a specific set of environmental constraints that must be met for survival and to reproduce. These include temperature, humidity, precipitation and the presence or absence of certain other species. 

Climate change has affected these factors in habitats around the world. As a result, many species' niches are less common or no longer exist within their historical range. 

"Because of climate change, the conditions are changing throughout the range of all the species, and what was the region where they were having their optimum of abundance is shifting," said Marie-Josée Fortin, a professor of ecology and evolutionary biology at the University of Toronto. 

"[Species] are finding that the location where they are is too hot, or too dry or too humid for them, so they have to move."

Movement can be unpredictable, but generally, species tend to head toward cooler climates: toward the poles, to higher elevations or in the case of aquatic species, to lower depths. 

These range shifts pose many challenges for individual species, ecosystems and even human communities.

Movement isn't always easy

For some species, like caribou or migratory birds, movement is a natural part of their lives. 

"They are used to moving through large regions," Fortin said. "But other species, they cannot move that fast, right? So they need to slowly acquire some new habitat along the way."

These discrepancies between species' abilities to move to new habitats can make range shifts difficult even for those that can move with relative ease. 

Not only do species need to have the right climate — they also need their resources, food and other members of their species to survive, Fortin explained. 

New areas also mean new competition with new species, including humans. 

"If you think of species from southern Ontario that are at the northern range of their limit in North America, to move north, what they are faced with is an agricultural landscape, so there's not much habitat to colonize," said Fortin.

"They are competing with humans for the best habitat that they could use."

Even when trying to return land to its natural state by rehabilitating areas with native species, climate change is a big part of the conversation, said Tys Theysmeyer, head of natural lands at the Royal Botanical Gardens (RBG) in Hamilton. 

That's why more northern species, like hemlock trees, may not be part of planting efforts at the RBG in the future, even though they're native to the area. 

Yellow and brown bird with black eyestripe that has several ticks attached around its eye.
Many animals are capable of carrying and spreading invasive species to new areas. They include this common yellowthroat with ticks around its eyes. In the past, invasive species may have died in colder habitats, but climate change may allow them to establish themselves in new areas as they warm. (Submitted by Catherine Bouchard)

"We're starting to think a little more about those slightly more southern plants as part of site restoration projects," said Theysmeyer. 

"And at the same time, you're looking at, well, what are the trees that will march north into this area and be the foundation of the future forest."

Range shifts can entail contractions or expansions

For many species, the term "range shift" is a bit of a misnomer. 

Species at risk often face what is better described as a range contraction — where their southern range border moves faster than the northern one, causing its range to shrink. 

This is also common for species that live on mountains. As the climate warms, their range shifts to a higher elevation, but eventually there is no mountain left to ascend. 

Meanwhile, range shifts may drive other species to become invasive, harming other ecosystems they enter. 

Indeed, invasive species, like blacklegged ticks, often enjoy range expansion as a result of climate change, as they gain more suitable habitat than they lose. 

Adult and young caribou walking through the snow in front of evergreen trees.
Even for migratory species like caribou, it can be difficult to travel through human-dominated landscapes. (Danita Delmont/Shutterstock)

For many protected areas, removal of invasive species is a top priority. 

Climate change is threatening to make that even more difficult. 

"In the climate we have, it's usually the cold that [keeps invasive species away]," said Theysmeyer. "If the climate doesn't get as cold, then what limits your survival in the winter no longer limits your survival and you start to move in.

"The impacts that we worry about more than anything are the Eurasian insects, or bacteria or plants that have made it to North America and are invasive species, but are held at bay by the fact that it gets too cold here in the winter," he said.

"It's definitely a thing we're watching, everybody's watching."

Monitoring and solutions

Work is underway through research networks to track and monitor these shifts. 

"By having these big networks of research, trying to track these emerging diseases, and I think that's how we have a chance — just by doing that collective work, collective effort of all these different partners," said Bouchard.

Disease emergence and pandemic vigilance have become a key focus of many jurisdictions in the wake of COVID-19, with the public, scientists and policymakers alike recognizing the dangers of being caught unaware. 

A man stands on a wooden lookout in front of a large body of water.
Tys Theysmeyer, head of natural lands at the Royal Botanical Gardens in Hamilton, says the RBG has to make hard decisions about which species to use in habitat restoration efforts based on whether they can survive in the RBG long term in the face of climate change. (Darius Mahdavi/CBC)

As for those working on the ground to help species affected by range shifts, it really comes down to helping nature do its thing. 

One of the best ways to do this is to provide corridors or stepping stones of natural habitat so species can move across human-dominated landscapes to new habitats. 

"Parks Canada has an initiative to create some corridors throughout the country," Fortin said. "But it's a work in progress."

When restoring these areas, diversity is key.

"Rebuilding ecosystems is mostly about building resilience," said Theysmeyer of the RBG.  "The greater diversity you can have, the better." 

ABOUT THE AUTHOR

Darius Mahdavi

Science communicator

Darius Mahdavi is a CBC science specialist covering the impacts of climate change on the people and ecosystems of Ontario. He's worked as a researcher and graduated from the University of Toronto, where he earned a degree in conservation biology and immunology with a minor in environmental biology. If you have a science or climate question, reach out at darius.mahdavi@cbc.ca.

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