The CBC Great Lakes Climate Change Project is a collaborative initiative between CBC stations across Ontario to research climate change from a provincial perspective. Darius Mahdavi, a scientist with degrees in conservation biology and immunology and a minor in environmental biology from the University of Toronto, explains how climate change issues 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, turning our lives upside down. Even if it doesn’t happen in our lifetime, research shows that there will be another pandemic and it is likely to happen through a disease that reaches humans from animals.
In Canada, the risk of disease transmission 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 few decades due to increased interaction between humans and animals.
Invasive species—those that enter new habitat and overtake native wildlife—can also bring new diseases that can be devastating.
Because both native and invasive species often have no choice but to move through densely populated areas when seeking new habitats, there is a higher risk of these diseases being transmitted from animals to humans.
This is known as zoonosis.
Zoonoses can lead to outbreaks of new diseases, such as SARS-CoV-2, the virus that causes COVID-19.
Scientists have estimated that there are more than 10,000 viruses with the potential to infect humans that are currently found in animal hosts — and that doesn’t include bacteria or other pathogens.
A recent paper published in the journal Nature shows that climate change increases the risk that these viruses will cross the species barrier and infect humans.
In other cases, known carriers of existing human diseases are 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 disease—one of the many reasons scientists worry about the spread of species into new areas.
Enter the black-legged tick, Ixodes scapularis, found in the eastern provinces, and its cousin, the western black-legged tick, Ixodes pacificus, found on the Pacific coast.
Black-legged ticks are smaller than common brown dog ticks and can be vectors for Lyme disease. (Ben Garver/The Berkshire Eagle via The Associated Press)
Although unlikely to cause a pandemic, in Canada they are the only known carriers of Borrelia burgdorferi, the bacterium that causes Lyme disease, which has been on the rise over the past decade.
They can also carry various other pathogens.
Black-legged ticks, also known as deer ticks, were once a rare sight in Canada. Today, they are found in large parts of Ontario and other provinces, and more throughout the year than ever before.
Catherine Bouchard, a veterinary epidemiologist with the Public Health Agency of Canada and assistant professor at the University of Montreal, has seen this firsthand.
“Fifteen years ago … sampling for more than six months of the year, I would have found maybe 1,000 ticks over a two-year period,” said Bouchard, who works primarily in the Estrey region of Quebec. “Nowadays, when we go there in the same region … within two months, we get 1,000 ticks.”
It’s the same trend seen across much of Canada, including Ontario, and Bouchard said it’s expected to continue.
It has also led to a large 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 problems such as arthritis, long-term fatigue and neurological and heart problems.
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, Bouchard said.
She added that while on average only about 20 percent of ticks carry the disease-causing bacteria, in some areas it can be as high as 50 percent.
Reported cases of Lyme disease in Ontario and Canada have been increasing over the past decade. Data are 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 due to climate change.
“With key drivers of climate change like temperature, but also precipitation … the weather we’re experiencing that is changing, of course, has a direct impact on vector ticks,” Bouchard said.
Range shifting occurs when species are forced to leave their typical homes and move to new areas that can support them.
You can see an example of this in Ontario, where black-legged tick populations have been expanding since 2016.
Estimated Lyme disease risk areas in Ontario have been increasing since 2016. These risk zones are based on where blacklegged ticks are found during active sampling by researchers. (CBC News)
Each species has a niche – a specific set of ecological constraints that must be met to survive and reproduce. These include temperature, humidity, rainfall and the presence or absence of certain other species.
Climate change has affected these factors in habitats around the world. As a result, the niches of many species are less common or no longer exist in their historical range.
“Because of climate change, conditions are changing throughout the range of all species, and what is the region where they had their optimum abundance is changing,” said Marie-José Fortin, a professor of ecology and evolutionary biology at the University of Toronto.
“[Species] they find that where they are is too hot, or too dry, or too wet for them, so they have to move.”
Movement can be unpredictable, but in general, species tend to head for cooler climates: toward the poles, toward higher elevations, or in the case of aquatic species, toward shallower depths.
These range changes pose many challenges for individual species, ecosystems and even human communities.
Movement is not always easy
For some species, such as caribou or migratory birds, movement is a natural part of their lives.
“They’re used to moving across large regions,” Fortin said. “But other species, they can’t move that fast, right? So they have to slowly acquire new habitats 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 about species from southern Ontario that are in the northern part of their limit in North America to move north, what they’re facing is an agricultural landscape, so there’s not a lot of habitat to colonize,” Fortin said.
“They compete with humans for the best habitat they could use.”
Even as we try to return the land to its natural state by restoring areas with native species, climate change is a big part of the conversation, said Tice Theismeier, Head of Natural Lands at the Royal Botanic Gardens (RBG) in Hamilton.
Therefore, more northern species such as hemlock trees may not be part of the RBG planting efforts in the future, even though they are native to the area.
Many animals are capable of carrying and spreading invasive species to new areas. These include this common yellowthroat with ticks around the eyes. In the past, invasive species may have died out in colder habitats, but climate change may allow them to establish in new areas with warming. (Submitted by Catherine Bouchard)
“We’re starting to think a little bit more about these slightly more southern plants as part of site restoration projects,” Theismeier said.
“And at the same time, you’re looking at which are the trees that are going to march north into that area and be the foundation of the future forest.”
Shifting the range can cause it to shrink or expand
For many species, the term “range shift” is a bit of a misnomer.
Species at risk often face what is better described as range contraction—where their southern range limit moves faster than their northern range, causing their range to shrink.
This is also common for species that live in the mountains. As the climate warms, their range shifts to higher elevations, but eventually there is no mountain left to climb.
Meanwhile, range changes can cause other species to become invasive, harming other ecosystems they enter.
Indeed, invasive species such as black-legged ticks often enjoy range expansion as a result of climate change, as they gain more suitable habitat than they lose.
Even for migratory species like caribou, it can be difficult to travel through human-dominated landscapes. (Danita Delmont/Shutterstock)
For many protected areas, the removal of invasive species is a top priority.
Climate change threatens to make this even more difficult.
“In the climate we have, it’s usually the cold [keeps invasive species away]” Teismeier said. “If the climate doesn’t get that cold, then what limits your survival in the winter no longer limits your survival, and you start moving in.
“The impacts we’re most worried about are Eurasian insects, or bacteria, or plants that have come to North America and are invasive species, but are held back by the fact that it gets too cold here in the winter,” he said.
“It’s definitely something we’re watching, everybody’s watching.”
Monitoring and solutions
It works through research networks to track and monitor these changes.
“By having these large networks of research, trying to track these emerging diseases, and I think that’s how we have a chance — just by doing…
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