Biodiversity Portrait of the St. Lawrence

Story of a river and its inhabitants (Source: Bigras, 1992)

The formation of the St. Lawrence: The geological history of Quebec began more than three billion years ago, with the formation of a thin crust constituting the primitive core of the North American continent. This event occurred some 11 billion years after the universe was created and nearly two billion years after life first arose on earth. Several times in earth's early history, all the landmasses that were above water joined to form a supercontinent, which subsequently split apart, creating a new ocean. As a result of successive collisions of landmasses at the southeastern edge of the North American continent over a period of 300 million years, the chain of mountains known as the Laurentians was born. A billion years ago, when this mountain building process was complete, the Laurentians looked very much like the present-day Himalayas. Back then, on the site where the St. Lawrence Valley now lies, there was a very high plateau that could be likened to the Tibetan Plateau. Rapid erosion of this plateau resulted in the present-day Laurentians, which are all that remains of the deep root (about 35 km) of this major chain of mountains. After the Laurentians formed, heat began building up under the continental mass of North America. Some 600 million years before the present, large trenches opened up along deep cracks in the crust, and the supercontinent broke apart, creating a huge ocean, called Iapetus. Some of these trenches formed in the area along the Ottawa, Richelieu and Saguenay rivers, while other trenches opened up along an axis that would later become the St. Lawrence River. Meanwhile, along the edge of the continent, opposite Montreal, a huge gulf began to form.

Since the portion of the continental landmass of North America that existed at this time lay in the tropical latitudes, the climate was hot, resembling Florida's present-day climate. This is when biological life originated in the oceans and the first lines of marine invertebrates were established. The fossil-rich rocks and sediments that can be found in the St. Lawrence Lowlands date back to this period and resulted from the accumulation of materials near the Iapetus Ocean, on the continental shelf.

Roughly 500 million years ago, the tectonic plates reversed direction and the Iapetus Ocean closed partially. This movement triggered two successive orogenies, leading to the creation of the Appalachian Mountains 350 million years BP.

Dispersal of Life forms in the sea: About 290 million years ago, the final collision of the continents marked the end of the repeated cycles whereby the Iapetus Ocean would expand and then close up. Africa, South America and Eurasia united with North America, creating a supercontinent called Pangaea. This vast continental landmass was surrounded by the Panthalassa Ocean, which contained many orders of fishes. Around 180 million years before the present, the southern part of Pangaea began to break apart, and the Atlantic Ocean was born. This ocean basin is still expanding by the addition of ocean floor along the Mid-Atlantic Ridge.

Life forms emerge on Earth: Quebec had a hot and humid climate owing to its location near the Equator. This period was marked by the appearance of the earliest life forms, including the first woody plant species and the earliest species of amphibians, which presaged the transition that some aquatic vertebrates would go through to become terrestrial tetrapods. More than 20 000 fossil specimens representative of this period, primarily terrestrial arthropods, plants and fishes have been collected from the fossil-rich cliffs at Miguasha, near Chaleur Bay. Some 125 million years ago, during the Jurassic and the Cretaceous periods, the Montegregian hills were formed by volcanic eruptions. Quebec's climate was still quite hot and humid. The dinosaurs, which had appeared about 250 million years ago, dominated the existing animal communities; they disappeared roughly 50 million years later but not before giving rise to mammals (200 million years ago) and birds (150 million years ago).

With the advent of the Quaternary, 1.6 million years ago, the Earth's climate underwent many changes. This was also a time of great diversification in terrestrial invertebrates and flowering plants. As well, the early Quaternary marks the point when humans first appeared on earth, probably in Africa.

Roughly 125 000 years ago, the climate went through a major cooling period, leading to the formation of ice fields that eventually gave rise to the glaciers. A mere 20 000 years ago, a huge icecap, the Laurentide Ice Sheet, covered nearly all of Quebec, with a layer of ice more than 2500 metres thick overlying Hudson Sea. The majority of species migrated to refugia situated at lower latitudes, remaining there until a warming trend began 18 000 years ago. The Laurentide Ice Sheet finally disappeared completely about 6500 years ago. Its meltwater filled low-lying areas, forming the lakes and rivers that comprise the current drainage system of the St. Lawrence.

Biodiversity after the glacial phases: As the ice retreated, salt water from the Atlantic Ocean flowed in to take its place, flooding low-lying regions. An immense inland sea, called the Champlain Sea, formed some 12 000 years ago; however, it disappeared 10 000 before the present as a result of the continental uplift associated with the retreat of the ice sheet. The riparian landscapes of the St. Lawrence, initially dominated by vegetation with an affinity for boreal and tundra regions, were gradually transformed as the glaciers receded (Richard, 1977). The plants and animals which had previously moved south to escape the cold reinvaded their former territories. A new species appeared with them, however, and began reshaping the landscapes of the St. Lawrence. This species was man. The first humans were Amerindians, who originated in Asia and arrived in Quebec 9000 years ago; they were followed by the first white men, about 500 years ago. Today, Quebec's human population, which is more than 7 million strong, undoubtedly represents the most "invasive" species living along the St. Lawrence in post-glacial times.