One answer for now, is the western point of Lake Ontario, with its own micro-climate and unique physical aspects, concerning water current and position close to the escarpment. It has been under threat of industry and residential use and all their side effects like polluted water from the steel industry, silt and erosion from the former brick factory, etc...
Another answer is the Oak Ridges Moraine
"The Oak Ridges Moraine is one of the most significant landforms in southern Ontario. The moraine gets its name from its rolling hills and river valleys extending 160 kms from the Niagara Escarpment to Rice Lake and was formed 12,000 years ago by advancing and retreating glaciers.
The moraine contains the headwaters of 65 river systems (35 in the GTA alone) and has a wide diversity of streams, woodlands, wetlands, kettle lakes, kettle bogs and significant flora and fauna. It is one of the last remaining continuous green corridors in southern Ontario: it is still 30 per cent forested and is one of the last refuges for forest birds in all of southern Ontario."
The Oak Ridges Moraine area north of Toronto was under threat of "urban sprawl", the relentless growth of new residential areas and commercial/industrial land use, as well as continued extraction of sand and gravel and wasting the life that was on top of that for many thousands of years. Its also the source of much of the water that feeds the rivers and replenishes (in part) Lake Ontario with fresh clean water. Developers were competing with environmentalists for that land and thankfully it will be preserved as much as possible, with opportunities for people to appreciate nature in some parts of it.
Another area is in south-western Ontario, the Ojibway Prairie
Most of the land is developed for industry or agriculture or residential use, but this is a small pocket that contains rare and endangered indigenous species.
Also, on the topic of Prairies, 95% of the Canadian prairies were taken over for agricultural use, but there are still some pockets that have native species of plants and a place for animals to live without competition from farmers. http://canadianbiodiversity.mcgill.ca/en…