Biodiversity Loss in Southern Ontario: Scale and Stakes
Healthy Landscapes Ontario | December 8, 2025
Southern Ontario has lost more than 70 percent of its wetlands, 98 percent of its grasslands, and 80 percent of its forests since European settlement. Those numbers come from provincial and federal habitat assessments, and they are worth sitting with for a moment. In the Carolinian zone, the narrow band of land stretching from Toronto to Windsor that holds more biological diversity than anywhere else in Canada, over 90 percent of the original forest cover is gone. What remains is fragmented into patches too small and too isolated for many of the species that once depended on them.
This is not a slow trend that might reverse on its own. A 2025 study by researchers at the University of British Columbia and World Wildlife Fund Canada, published in Ecological Solutions and Evidence, found that 130 species in the Lake Simcoe-Rideau ecoregion of southern Ontario are at risk of local extinction by 2050 if no new conservation actions are taken. Of the region's 133 at-risk species, 98 percent could be functionally gone within 25 years under a business-as-usual scenario.
What Is Actually Disappearing
The Carolinian zone supports over 2,200 plant species and 400 bird species. It is home to more than half of Canada's species at risk, including the Blanding's turtle, the king rail, the spiny softshell turtle, and the redside dace. Every one of Canada's ten native freshwater turtle species is now listed as at risk in at least part of its range, and for several of them, southern Ontario is the last viable habitat.
Aerial insectivores, the birds that feed on flying insects, tell a particularly stark story. Ontario populations of barn swallows have declined by 66 percent since 1970, losing about 2.5 percent per year in a slow but relentless slide. Bobolink populations fell 65 percent over four decades, with the rate of decline accelerating to over seven percent per year in the most recent decade of measurement. Chimney swifts, once common in Ontario's towns, have shown significant decreases tied to both habitat loss and insect population collapse.
These are not obscure species. Barn swallows were a common sight on Ontario farms within living memory. Their decline reflects cascading changes in agricultural land use, pesticide application, and the loss of the grassland and wetland margins where insects breed.
Wetlands Under Pressure
Riparian corridors and stream buffers are among the most cost-effective habitat conservation investments in southern Ontario.
Between 2015 and 2020, the Mixedwood Plains ecozone, which includes most of southern Ontario's settled areas, lost another 4,562 hectares of wetland. That figure is actually lower than the 7,300 hectares lost between 2011 and 2015, suggesting that some policy measures have slowed the rate of loss. But the baseline is already so depleted that continued losses, even at a reduced pace, push remaining wetlands closer to the functional thresholds below which they can no longer support their characteristic species.
In the Carolinian zone specifically, researchers have identified 79 percent of Ontario's non-fish species at risk as dependent on this small geographic area. More than half of those species rely on wetland or semi-aquatic habitats, and those habitats are now contending not only with development pressure but with invasive species. A 2025 bioRxiv preprint catalogued 33 invasive plant species posing current or imminent threats to Carolinian wetlands, many of which directly compete with the habitat needs of at-risk wildlife.
The Cost of Recovery
The UBC and WWF Canada study offered something that biodiversity research often lacks: a price tag. The researchers identified eight conservation strategies, including habitat protection, wildlife-safe road crossings, invasive species management, landowner stewardship, and targeted restoration, and estimated their combined cost. The finding was that an investment of roughly seven dollars per Ontarian would be enough to help 75 percent of the region's at-risk species recover.
Seven dollars. For context, Ontario spends more than that per capita on road salt each winter. The barrier to action is not financial. It is political will and public awareness.
Some of that investment is already happening at smaller scales. The federal Habitat Stewardship Program allocated up to $4.5 million in 2024-2025 for non-profit and Indigenous community-led species recovery projects. The provincial Species at Risk Partnerships on Agricultural Lands (SARPAL) program offers cost-share funding to farmers for habitat restoration targeting 12 specific species. And organizations working on pollinator pathways and backyard biodiversity are building awareness at the neighbourhood level.
But the scale of the problem requires a corresponding scale of response. Southern Ontario is one of the most biologically significant regions in Canada, and it is also one of the most heavily modified. The remaining fragments of wetland, grassland, and forest are not just habitats. They are the infrastructure that filters water, controls flooding, pollinates crops, and supports the native plant communities that hold the landscape together. Losing them is not an abstraction. It is a measurable decline in the ecological services that Ontario communities depend on, whether they recognize that dependence or not.