Change in extent of wetlands
Wetland loss in the Prairies
The millions of small wetlands of the Canadian and U.S. prairies are the most productive waterfowl habitat in the world, supporting 50 to 88% of the North American breeding populations of several species.20-22 Availability and condition of wetlands are primary factors determining the number and diversity of these waterfowl. Although these factors are influenced greatly by climate variation,22 land use change is also important.
As land was settled and converted to agriculture, extensive areas of wetlands were drained. No comprehensive data on historical loss exist, however analysis of localized studies in the Canadian Prairies shows high variability12, 23-25 with loss estimates between settlement and the 1990s of 40 to 71%.12, 24, 26, 27 Despite conservation efforts over the past several decades, wetland loss and degradation continue, largely as a result of intensification of agriculture.25, 28 Between 1985 and 2001, 6% of wetland basins were lost, representing 5% of the total estimated wetland area. In addition, estimates of wetland area suffering a loss of function due to factors such as partial drainage were about 6% annually.12 An analysis of agricultural impact and recovery of wetlands between 1985 and 2005 found the edges of wetlands were impacted more than wetland basins. Although the rate of impact for edges declined over the period, the rate of recovery was slower, indicating an increasing overall impact. The percent of edges impacted ranged between 82 and 97% in 1985, depending upon location, and stabilized in the early 1990s at between 90 and 95%.28
Up to 90% of prairie wetlands are estimated to be smaller than 1 ha.12 Research indicates that, overall, smaller wetlands support a greater number of waterfowl than larger ones.29 These small wetlands are also suffering the greatest losses. From 1985 to 2001, the average size of wetland basins lost was 0.2 ha, with 77% smaller than 2.6 ha.12 Between 1985 and 2005, shallow seasonal wetlands in agricultural fields had the highest rate of impact and slowest recovery rates relative to other wetland types.28
Wetland loss in southern Ontario
Prior to European settlement, southern Ontario had approximately 20,266 km2 of wetlands. By 2002, 72% had been converted to other uses. This represents a decrease in the proportion of wetland cover on the landscape from 25 to 7%.11 Historically, the highest concentrations of wetlands were found in southwestern and eastern Ontario. These areas are also where the most severe losses have occurred. For example, prior to settlement, 83% of Essex County, at the tip of southwestern Ontario, was wetland but by 2002 this was reduced to less than 2%.11, 30 From 1967 to 1982, conversion of wetlands for agriculture accounted for 85% of the losses.30 Urban development and associated transportation infrastructure were significant factors in the areas surrounding southeastern Lake Ontario.11
Most wetland conversion happened in the 19th and early 20th centuries (68% of wetlands were converted prior to 1967).30 Nevertheless, despite wetland gains in some areas, overall net loss continues. While the estimated extent of wetlands larger than 10 ha remained relatively stable between 1967 and 1982, from 1982 to 2002 an additional 3.5% of pre–settlement wetlands were lost – an average of 3.5 km2 per year. These estimates are conservative since Great Lakes coastal wetlands and wetlands smaller than 10 ha were not included in the analyses.11
Change in extent of wetlands along the St. Lawrence River
Over 60 km2 of riparian habitat along the St. Lawrence River was modified from 1945 to 1984.51 Most changes occurred prior to the mid–1970s and were a result of draining and filling of open waters and wetlands for housing, roads, and agriculture. Losses near major urban centres were the greatest,49, 51 for example, 83% of Montreal's wetlands were lost by 1976.52 Construction of water control structures, including dams and the St. Lawrence Seaway (1954–1958), was also responsible for change in the late 1950s,49 while urbanization was more important after that time.52
Since the 1970s, the overall extent of wetlands has increased, although there is variability depending upon the type and location of the wetland.51 While wetland loss continues due to urbanization, particularly in the Montreal and Lac Saint–Pierre areas, restoration efforts and reduced water levels have resulted in a 2.7% net gain of marshes and swamps between 1990 and 2002.51 Gains were mainly in the fluvial, upper, and lower estuaries and occurred mainly at the expense of open water. Declining water levels in the 1990s may have accelerated the drying trend in some areas,51, 53 transforming low marshes to high marshes and swamps that are dominated by invasive plant species. Water levels are influenced by a number of factors, including water control structures, flow from the Great Lakes and the Ottawa River, and climate change, particularly in the estuary and Gulf of St. Lawrence.49, 51
Exotic wetland plants now comprise 14% of vascular plants in St. Lawrence River wetlands.54 Their expansion can be attributed to shoreline alteration, excavation of the navigation channel, and water level regulation, which have reduced the magnitude of floods, decreased circulation in shallow littoral areas, and reduced the efficiency of the river to flush nutrients from sediments and to uproot robust emergent vegetation.55
Old Crow Flats
Designated as a wetland of international importance,4 Old Crow Flats is a large, undeveloped complex (over 6,000 km2) of more than 2,000 lakes and wetlands formed by thawed permafrost. It provides continentally significant habitat for up to half a million breeding and moulting waterbirds.57, 58 The overall surface area of water decreased by 13 km2 (3.5%) from 1951 to 2001, with greatest overall decreases found in large and very large lakes. Ponds increased in extent by 7% from 1951 to 1972, and decreased by 8.5% between 1972 and 2001. Changes are attributed to a mix of interacting processes with some lakes forming or expanding, and some suddenly draining due to collapse of permafrost – along with an overall drying trend due to increased evaporation from hotter summers in recent years.56
Change in extent of wetlands in the south Okanagan and lower Similkameen valleys, B.C.
Wetlands occupy a small portion of the Western Interior Basin due to the region's climate, soil, and topographic features.2, 88 Nevertheless, they play a crucial ecological role particularly because wetlands in arid areas support more species than other ecosystems.88, 89 Wetlands of the southern interior of B.C. support many species at risk. Most wetlands in this area are located in valley bottoms where development is also concentrated and wetland loss has been extensive since European settlement mainly due to conversion for agriculture and more recently for urban development.87, 90 Between 1800 and 2005, specific wetland communities suffered different degrees of loss, including, 92% of shrubby water birch/red–osier dogwood riparian wetlands, 63% of black cottonwood–red osier dogwood riparian wetlands, and 41% of cattail marshes from the south Okanagan and lower Similkameen valleys.87 Wetlands continue to be lost and degraded by urbanization, intensive agriculture, and, in some areas, heavy recreational use.87, 91, 92 In addition, invasive species and climate change pose serious threats.
St. Lawrence River wetland
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