Boundaries

Encompassing portions of southern Ontario and eight American states, this ecoregion is comprised of the watersheds of the five Great Lakes (Superior, Michigan, Huron, Ontario, and Erie).

Topography

Until approximately 10,000 to 15,000 years ago the entire region was covered by glaciers associated with the Wisconsinan Age, and the basins of the Great Lakes were created by the movements and the erosional forces of these glaciers. This has resulted in gently rolling topography with elevations below 250 m across much of the landscape, although a few ranges along the northern and western border reach upwards of 600 m (McNab & Avers 1994).

Much of this region is underlain by the acidic, Archean bedrock of the Canadian Shield. Bedrock outcroppings are common, and may be covered with sandy to loamy till in the north, and a thin, acidic sandy till in the south. The glaciers that once covered this ecoregion left areas now mantled with thick deposits of glacial drift. Limestone and dolomite cliffs of the Niagara Escarpment extend from Lake Michigan’s northern shoreline, northeast to Manitoulin Island, and southward along the Bruce Peninsula to Niagara Falls. Lakes Erie and Ontario are underlain by Palaeozoic bedrock and have low relief with poorly drained depressions, morainic hills, drumlins, eskers, and outwash plains as a result of glaciation.

Freshwater habitats

The five Laurentian Great Lakes comprise the largest freshwater ecosystem in the world, holding over 20% of world’s surface freshwater. In addition to numerous streams, rivers, lakes (including pothole and kettle lakes), springs, spring ponds, and wetlands, over 35,000 islands are found within the Great lakes. There are also unique freshwater features,  such as Manitoulin Island, the largest freshwater island in the world; St. Clair River Delta, the largest freshwater river delta in the world; and the most sand dunes of freshwater origin in the world (TNC 2000).  

The extensive interior wetlands and sand dune systems, such as Lake Ontario’s Presqu’ile, Lake Erie’s Long Point, Rondeau, and Point Pelee, support unique plant communities on large sand pits. Some of these wetlands are also recognized internationally for their outstanding biological significance, including Long Point and Point Pelee on the north shore of Lake Erie, and the National Wildlife Area on Lake St. Clair. Long Point is designated a UNESCO Biosphere Reserve.

Terrestrial habitats

The northwestern part of the ecoregion is dominated by mixed forest characterized by white and black spruce (Picea mariana), balsam fir (Abies balsamea), jack pine (Pinus banksiana), trembling aspen (Populus tremuloides), and paper birch (Betula papyrifera). Here, forest fires are an important natural disturbance (ESWG 1995). To the south deciduous forest dominates, with species  such as sugar maple (Acer saccharum), red maple (Acer rubrum), American beech (Fagus grandifolia), hop hornbeam (Ostrya virginiana), basswood (Tilia americana), yellow birch (Betula alleghaniensis), and eastern hemlock (Tsuga canadensis). In the eastern edge of the ecoregion rare alvar communities support prairie species that are at their eastern extremity, and are globally endangered. Ancient eastern white cedars on the limestone cliffs of the Niagara Escarpment have been aged at 700 to 800 years, making them some of the oldest in eastern North America (Ricketts et al. 1999).

Description of endemic fishes

An endemic cisco species flock consisting of bloater (Coregonus hoyi), blackfin cisco (C. nigripinnis), deepwater cisco (C. johannae), and shortnose cisco (C. reighardi) has been described in the Great Lakes. All of these species are extirpated in one or more of the Great Lakes in which they originally occurred, and the deepwater and shortnose ciscoes are considered to be extinct. All of these species are thought to have evolved from a common ancestor, lake cisco (C. artedi), within the Great Lakes since their most recent formation c. 14,000 years ago. Siskiwit lake cisco (Coregonus bartlettii) and Ives lake cisco (Coregonus hubbsi) are two other endemics, although experts disagree whether they are separate species from C. artedi. A subspecies of walleye (Sander vitreus vitreus) known as the blue pike (Sander vitreus glaucus) was once endemic to lakes Erie and Ontario, but is now considered extinct.

Other noteworthy fishes

Many of the over 150 native fish species in the Great Lakes ecoregion are considered to be at risk. Habitat alteration, invasive species, and overexploitation are considered to be the greatest threats to fishes in the Great Lakes Basin. In addition to the endemic fishes listed above, notable fish species at risk in this ecoregion include the extirpated Atlantic salmon (Salmo salar), American eel (Anguilla rostrata), which is virtually extirpated, and the lake sturgeon (Acipenser fulvescens), which has exhibited a precipitous decline (>95% population decline) since the late 18^th century. In addition to the many native species at risk, many introduced species have become established in the Great Lakes Basin, including common carp (Cyprinus carpio), goldfish (Carassius auratus), brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss), Pacific salmons (Oncorhynchus spp.), sea lamprey (Petromyzon marinus), rainbow smelt (Osmerus mordax), alewife (Alosa pseudoharengus), ruffe (Gymnocephalus cernuus), and round goby (Neogobius melanostomus). Many of these species have had substantial negative impacts on the ecosystems of the Great Lakes Basin.

Ecological phenomena

Historically, the lake sturgeon and American eel undertook long spawning migrations, and the lake cisco and lake whitefish (Coregonus clupeaformis) likely formed large spawning schools in the larger lakes. The wetlands of the lower Great Lakes as well as Lake Superior and Lake Huron are crucial for migrating birds and serve as stopovers and major breeding areas.

Justification for delineation

Ecoregion boundaries are taken from Abell et al. (2000) and are based on subregions defined by Maxwell et al. (1995). The boundaries were then modified based on the faunal similarity of 166 major watersheds based on a cluster analysis of freshwater fish occurrences in these watersheds. The Laurentian Great Lakes ecoregion includes the watersheds that drain into the Great Lakes and the Great Lakes themselves. This ecoregion has the greatest fish species richness of any ecoregion in Canada. This is a result of proximity and multiple connections to the Mississippian and Atlantic Coastal refugia, relatively moderate climate, and diversity of habitats.

Level of taxonomic exploration

Good