Invasive species could destroy ‘Dow’s Great Swamp’

It was once a sprawling wetland that stretched from the Rideau River near Hog’s Back Falls about four kilometres north to the Ottawa River — a marshy expanse that would have encompassed today’s Carleton University campus and Dow’s Lake, as well as much of Little Italy and LeBreton Flats.

Dow’s Great Swamp was a wonder of biodiversity for millennia — and surely a prime source of duck, beaver, muskrat and other resources for the region’s Indigenous people — before Lt.-Col. John By and his work crews arrived in the Ottawa Valley nearly 200 years ago to build the northernmost leg of the Rideau Canal.

Now, an aggressive, alien reed-grass that is threatening aquatic environments almost everywhere in the Great Lakes region — phragmites australis, “one of Canada’s worst invasive species” for the past 20 years, according to Parks Canada — has a strangling grip on the last, lingering bit of the once-mighty marshland encountered by By and his men.

One Carleton researcher specializing in Canada’s fight against invasive phragmites says the university should quickly begin battling the incursion of the monopolistic marsh plant on its campus.

“It’s important … to try to contain it in some way,” said Courtney Robichaud, a postdoctoral fellow in Carleton’s biology department. “Those populations on campus are going to provide the rhizomes (subterranean stems) or the seeds that can cause more spread in the area.”

The area abuts Colonel By Drive and the canal, directly across from the Central Experimental Farm’s renowned Arboretum, the national tree garden.

Though pretty with its purplish or silvery seed plumes swaying in the breeze, invasive phragmites can devastate shoreline ecosystems as its dense, matted growth crowds out native cattails and other vegetation. These impenetrable thickets can also be impossible for nesting turtles and other species to penetrate, dramatically reducing biodiversity as phragmites monopolizes growing space, water, nutrients and sunlight.

“With those ditches and ponds on campus, if you have any big storms, the runoff and the (phragmites) rhizomes and stuff can go right into the canal where they can establish in the wetlands and shores and Arboretum,” said Robichaud, a keynote speaker at the first National Conference on Phragmites in 2022.

Tall, dense masses of the non-native reed have also taken over a long drainage ditch near the Bronson Avenue entrance to the university. But the larger swath of the destructive species — and presumed source of the ditch infestation — dominates the wet, low-lying scrubland next to Carleton’s student residences.

“That swampy area – such as it is – is indeed the last, sad remnant of what was once a fabulous wetland,” said Ottawa ecologist Daniel Brunton, one of the city’s leading naturalists and an expert on the national capital’s green spaces, past and present.

“The low forest area by the canal where the phragmites has come in — yes, it’s a really nasty invasive — was a beat-up wetland-edge elm and ash forest with little wetland quality left by the early 1970s,” added Brunton. “It likely has been made wetter in recent years by the alteration of drainage patterns, however — hence the phragmites.”

The mosquito-infested swamp was once a wellspring of plant and animal life, but also of death; malaria claimed the lives of hundreds of canal workers during the 1827-1832 construction of the future UNESCO World Heritage Site.

Even after they dammed and dredged Dow’s Great Swamp to complete the canal — creating Dow’s Lake in the process — surviving parts of the transformed wetland remained a prime habitat for birds, orchids and countless other species of flora and fauna, while performing the vital filtering and overflow functions of any marshland ecosystem connecting two major rivers.

Over time, though, Ottawa’s southward and westward growth ravaged the great swamp — the roads, neighbourhoods, government buildings and commercial strips we know today erasing nearly every trace of pre-settlement nature.

A few wild areas near the Rideau were still a birdwatching paradise in the 1950s. But by the year 2000 — when Carleton launched controversial plans to build its Leeds House residence along the edge of the small, remaining pocket of the original swamp — some professors and local environmentalists objected that the project would destroy the last, distressed vestige of what had been one of Ottawa’s original ecological treasures.

Facing intense pressure at the time to create more student housing, Carleton proceeded with the project.

The final patch of Dow’s Great Swamp became a mere wedge of fragile wetland clinging to existence between a campus parking lot and Colonel By Drive, a sliver so vanishingly small and cut off from natural corridors that some observers have declared the original, epic ecosystem destroyed well before the 1950s. But when a battle briefly raged over the construction of Leeds House in 2000, the expected impact on the adjacent wetland was decried by opponents hoping to protect “the only part of Dow’s Great Swamp . . . that remains,” according to a Citizen report from that era.

Bill Royds of the city’s Greenspace Alliance environmental group argued at the time that even in its isolated and distressed state, the campus marshland was not only historically significant but still “biologically important” and “home to a number of significant plants and the rare ‘Bladdernut’ — a tree with a fruit full of air,” the Citizen reported.

Whatever was left of the area’s biological importance after the residence was built is now being ransacked by phragmites. And the invasive enemy of biodiversity creates a vegetative monoculture wherever it spreads.

And the relatively isolated invasion zone at Carleton highlights a much bigger fight going on throughout parts of Central Canada to combat the rampant spread of phragmites — from roadside ditches and the edges of ponds to some of the country’s most significant aquatic ecosystems, including Georgian Bay Islands National Park, Thousand Islands National Park and Lake Erie’s Point Pelee National Park, the southernmost tip of mainland Canada.

“Invasive Phragmites is able to grow up to five metres in height and have a density of 200 stems per square metre,” notes a recent Parks Canada report on Point Pelee, a Ramsar Wetland of International Significance and home to 19 endangered species. “By growing densely, quickly, building up dead biomass, and releasing toxins from its roots, it can outcompete other plants through shading and resource competition. As plant biodiversity decreases, so does animal diversity, creating a more uniform ecosystem.”

The Ontario government recently announced a three-year, $16-million fund in the latest phase of the province’s fight against phragmites and other invasive species. The fund, to be co-administered by Ontario’s Invasive Species Council and the Nature Conservancy of Canada, aims to address “the potential impacts of invasive species to agricultural, fisheries, forestry, health care, tourism and the recreation industry,” which “may be as high as $3.6 billion per year in Ontario,” according to the June 20 announcement by provincial Natural Resources Minister Graydon Smith.

In May, the Ottawa Riverkeeper released its annual report card on the health of the watershed — which encompasses the tributary Rideau River drainage basin — and noted that invasive species “can wreak havoc on the habitats of vulnerable species. For example, invasive phragmites . . . reduce endangered turtle habitats in our watershed.”

Many other parts of Ottawa are also under assault by phragmites. For example, the invasive species has overrun lengthy stretches of the Green’s Creek shoreline in the city’s east end.

Robichaud noted that Humber College led an effective phragmites eradication effort in Toronto in which volunteers used spades to sever the plant’s stalk from its root systems and significantly reduce its impact on wetlands.

“It’s something that students and people on campus can get involved in to restore these ecosystems,” she said, adding that there is research indicating the “phrag invasion” will only become worse with climate change.

Matt Bolding, invasive species lead with the conservation group Ducks Unlimited Canada, echoed Robichaud’s call for action: “They say: ‘Early detection, rapid response.’ It is best when you see invasive species in general to take action. Managing those populations … removes that source in the area and prevents other sorts of establishments nearby.”

Bolding is leading a major project with his conservation group in which phragmites-eating moths are being used as a biocontrol measure to weaken and curb the spread of the plant in sensitive ecosystems where endangered wildlife species are further threatened by dense infestations of the invasive reed.

Carleton University, meanwhile, conducted an initial biodiversity assessment to evaluate the current state of biodiversity across the campus. “This assessment will inform the development of future programs aimed at enhancing and protecting the campus’s ecological health,” Carleton spokesman Steven Reid stated in response to an inquiry about the phragmites infestation. “Any identified invasive species will be treated and removed as necessary to preserve native ecosystems and maintain ecological balance. Through these efforts, the university is committed to fostering a thriving, sustainable campus environment.”