Without urgent action against environmental degradation, the forest ravines covering 20% of the city could be reduced to sterile valleys within decades

It can be difficult to keep up with Lawrence Warriner while walking; running, it’s next to impossible. Many of the trails that weave through the ravines near his house in Toronto are well groomed, but for Warriner – a decorated trail runner and coach – the more exciting ones are off the beaten path, tracks only faintly visible to the eye.

He has come to these forests, which rise along the sides of the river valleys that snake through the city, ever since he was a child. He has discovered a secret communal stone grill next to a sandy beach, hidden by trees; he has watched awestruck as a dozen white-tail deer crossed a bridge. He has also seen things he can’t explain, such as a parade of men, women and children, clad in period clothing, walking the woods at dusk with antique rifles slung over their shoulders.

“You wouldn’t believe what you’ll find here,” he says.

Facebook Twitter Pinterest Lawrence Warriner, a trail runner, has been coming to the ravines since he was a child. Photograph: Leyland Cecco for the Guardian

The thickly forested valleys of Toronto’s ravine system are peerless: no major urban centre in the world comes close to its steep, voluminous corridors of woodlands. In total, the dramatic formations measure more than 30 times New York’s Central Park, earning Toronto the nickname “the city within a park”.

But the valleys and forests – which cover nearly 20% of the city – are disappearing with worrying speed, according to a study released earlier this year by the University of Toronto. Invasive species are making short work of the ecosystem. Without dramatic action, the ravines could be reduced to sterile valleys in the coming decades, the authors warn.

The ravines are to Toronto what canals are to Venice, hills are to San Francisco and the Thames is to London Robert Fulford

“No matter where you go in the ravines, you’ll quite quickly see something about the ecosystem that is really troubling,” says Eric Davies, an ecologist and one of the co-authors of the report, which re-examines the ecological health of the ravines 40 years after a pair of amateur naturalists collected extensive data on biodiversity in the area.

Today, along the undulating forest floor, the extent of invasion is evident: clumps of Japanese knotweed, garlic mustard and dog-strangling vines smother the native grasses, trout lilies and trilliums. Norway maples – a popular street tree sold at garden centres around the city – have reproduced so rapidly they have seized control of nearly half the forest, killing off local undergrowth and saplings with their roots and poisonous sap. The disappearing ground cover has caused erosion, stripping soil away from the base of maple and oak trees and exposing their naked roots. The death of native plants is also destroying nesting areas for birds and rodents.

“Ravines are the mothership of biological diversity,” said Paul Scrivener, a naturalist who was one the authors of the 1977 study and consulted on last month’s follow-up report. “They have everything from hardwoods to wetlands. They offer food and shelter to animals and birds.” The last time Scrivener took stock of the forests, invasive trees made up 10% of the woods. Now, it’s 40%.

Facebook Twitter Pinterest Toronto’s ravines are off-limits to the development that has conquered much of the rest of the city. Photograph: Leyland Cecco for the Guardian

“The ravines are to Toronto what canals are to Venice, hills are to San Francisco and the Thames River is to London,” wrote the journalist Robert Fulford in his 1995 history of Toronto, Accidental City. “They are the heart of the city’s emotional geography, and understanding Toronto requires an understanding of the ravines.”

They are also a vestige of the region’s dramatic geologic transformations. Glaciers once covered much of Toronto; when they retreated they left sandy ground, which was gradually eroded into deep channels that would eventually become winding narrow valleys, heavily populated with trees, flowers, skunks and songbirds.

“If you had to imagine putting that into a city after the city has been built, it is inconceivable,” said Todd Irvine, an arborist and ecology lecturer at the University of Toronto.

Yet for generations, the steep hills and wetlands were a dumping ground. Companies used the creeks as a waste disposal system, and piles of garbage built up along the forest floor, with rusted vehicles lying fallow alongside oil drums. They also have a darker legacy: during the second world war, a brick factory here housed German prisoners of war, and in the last year police have found human remains in the ravines attributed to the alleged Toronto serial killer Bruce McArthur.

Facebook Twitter Pinterest Sherwood Park ravine in Toronto. Photograph: Rene Johnston/Toronto Star via Getty Images

The brick factory has since been transformed into the Evergreen Brick Works, an environmentally focused community and cultural centre. But otherwise the ravines are largely free of human intervention. Their steep slopes have long frustrated city planners: “They were these valleys you had to build bridges to cross or expensive roads to go down one slope and up the other,” said Scrivener.

In a city otherwise taken over by rampant property development, the 11,000 acres of ravines have been largely left alone since 1954, when they were flooded due to Hurricane Hazel, which destroyed an entire neighbourhood and killed 81 people. It remains Canada’s worst natural disaster in the postwar period. The natural flood-prevention system of the ravines has been kept free from roads and buildings ever since.

We take it for granted that in one of the most populated places in North America you can be beside a babbling creek Todd Irvine, arborist

They are also incredibly valuable in their natural state. Toronto-Dominion Bank economists have estimated the worth of Toronto’s tree cover at more than C$7bn (£4.1bn): among other benefits, they filter polluted air, provide a cooling effect in the summer and absorb vast amounts of rainwater. Today the ravines remain strictly off-limits to developers, although development projects – mainly condominiums – are pushing as close to the forest edge as legally allowed.

“We take it for granted that you can be in one of the most populated places in North America and in less than 10 minutes, you can be beside a babbling creek surrounded by trees and birds,” said Irvine. Canoeists can paddle more than 10km through the centre of the city on the ravine rivers, which eventually empty into Lake Ontario. “There’s no other major city like that. It’s remarkable.”

Facebook Twitter Pinterest Japanese knotweed is an invasive species that kills native grasses, trout lilies and trilliums. Photograph: Andrew Francis Wallace/Toronto Star/Getty

While conservationists have won a key battle against urban development, the “hands off” philosophy of leaving nature untouched may have permitted invasive species to spread unabated for decades.

Irvine argues that the city should treat it like an emergency, issuing public awareness campaigns of the dangers of Norway maple and triaging pockets of forest that haven’t yet succumbed. As for the rest: “In areas that are all Norway maple and dog-strangling vine, you need to cut them flush to the ground and start over.”

That might be a difficult sell. The ravines are managed by the Toronto Region Conservation Authority, which is dependent on city hall for funding. Although Toronto residents boast about the superiority of the ravines to New York’s Central Park, it’s hard to ignore the success of the Central Park model: the park is maintained under a public conservancy, overseen by an independent group, with donation boxes throughout the park and permanent staff working to maintain its health.

Since 1980, the Central Park Conservancy says it has invested more than $1bn in the park, with much of the money raised through philanthropic donations.

In Toronto, however, the current needs of the ravines cost much more than the city can afford, said Davies. He argues that a Central Park-style conservancy model, staffed with scientists, could help save them.

Facebook Twitter Pinterest The Prince Edward Viaduct takes Bloor Street over the Don Valley Parkway. Toronto’s ravines have long frustrated city planners. Photograph: Tom Szczerbowski/Getty

Ecologists will soon get the chance to prove that this “hands-on” approach to conservation works: the Toronto Botanical Gardens, already on the site of a ravine valley, is set to inherit more than 30 acres of forest ecosystem. Oversight of the land will come with the opportunity to rip out invasive species and begin replanting native plants.

For many, the fight to save the ravines is about more than nursing a damaged ecosystem. Despite – or perhaps because of – their location, the trees and creeks offer an escape from city life.

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“It’s really, really important to have urban natural areas for our citizens to be able to get out and about and away from the city, even for an hour or two,” says Scrivener. “It does something to your soul.”

“Everybody who comes out here inevitably comes back,” says Warriner, scampering over deadfall and keenly scanning the woods ahead. “It’s one of the few places in the city that makes you feel like a kid again.”

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