Prairies Ecozone+ Evidence for Key Findings Summary
National key finding
At a national level, the extent of forests has changed little since 1990; at a regional level, loss of forest extent is significant in some places. The structure of some Canadian forests, including species composition, age classes, and size of intact patches of forest, has changed over longer time frames.
Woodlands make up a small percentage of the land cover in the Prairies Ecozone+, mainly in the Aspen Parkland Ecoregion and other moister ecoregions. Woody cover has increased within areas of natural vegetation, but has decreased overall. Compared to other ecozone+ in Canada, the Prairies have only a small amount of forest cover. The Canadian National Forest Inventory found that forests comprised approximately 195 km2 (4.2%) of the area of the Prairies Ecozone+ in 2001. Based on 2005 remote sensing data, Ahern et al.Footnote7 estimated forest cover in the Prairies Ecozone+ at approximately 0.9%. Differences between the two estimates reflect different methodologies and definitions of forest rather than a change in the area of forest in the ecozone+ (The Canadian National Forest InventoryFootnote10 Footnote11 used inventory data from provincial, territorial, and other forest management agencies as well as remote sensing data to estimate forest cover. In contrast, Ahern et al.Footnote7 (Figure 2) is based solely on remote sensing data (and defines Forest as areas with tree crown density >10%)).
Tree cover has expanded into grasslands in many parts of the Aspen Parkland Ecoregion between settlement and the 1960s.Footnote12 Footnote13 Footnote14 Footnote15 This expansion has usually been attributed to the reduction in fire frequency since European settlement,Footnote16 Footnote17 although some authors have linked it to the extirpation of bisonFootnote14 (Bison bison) and to nitrogen deposition.Footnote18 Tree cover expansion, primarily stands of trembling aspen (Populus tremuloides), has sometimes been interpreted as indicating southward shifts in vegetation zones since European settlement (e.g., the shift of Aspen Parkland into areas that were formerly continuous grassland).Footnote19 Thorpe,Footnote20 however, reviewed historical sources from the 19th century and found that many of these sources clearly described parkland vegetation in places that now fall within the Aspen Parkland Ecoregion. For example, a map produced by the Palliser Expedition (1857–1860) shows the transition from the partially wooded "fertile belt" to the treeless "true prairie" near the same position as the southern edge of the Aspen Parkland on modern maps.Footnote21 ZoltaiFootnote22 also concluded that the boundary between Boreal Forest and Aspen Parkland ecoregions has not shifted, based on range limits and ages of boreal conifers and peatlands. The key difference is that before European settlement, recurrent fires kept the aspen groves smaller in area and shorter in stature than at present.
While expansion of tree cover has been documented within areas of natural vegetation, tree cover has undoubtedly been lost in areas converted to cropland. Based on Census of Agriculture data for a portion of the Aspen Parkland, CouplandFootnote23 showed that the percentage of woodland decreased from 10% in 1941 to 3% in 1981. Watmough and SchmollFootnote24 found a 6% decline in naturally treed habitats from 1985-2001 on 153 transects widely distributed across the ecozone+ (although more weighted towards more settled parts of the Prairies). Declines ranging from 1 to 12% were found in five ecoregions (Boreal Transition, Cypress Upland, Lake Manitoba Plain, Southwest Manitoba Uplands, and Interlake Plain) while increases were found in the other three ecoregions ranging from 13 to 18% (Southwest Manitoba Uplands, Cypress Upland, and Fescue Grassland). Tall shrub habitat increased by 3% overall, however, this was the result of regrowth of woody cover in wetland-upland transition areas and in cut blocks in the Aspen Parkland.Footnote24
HoggFootnote25 Footnote26 showed that most of the variability in aspen growth between 1951 and 2002 could be attributed to climate variation (occurrence of drought years) and outbreaks of the forest tent caterpillar (Malacosoma disstria).
Climate change is predicted to result in increasing aridity that will cause expansion of the grasslands and a reduction in extent of Aspen Parkland vegetation. If the aridity results in increased fire frequency, this will also result in reductions to aspen grove area and extent (see Climate change key finding on page 52).
National key finding
Native grasslands have been reduced to a fraction of their original extent. Although at a slower pace, declines continue in some areas. The health of many existing grasslands has also been compromised by a variety of stressors.
Grasslands covered most of the Prairies Ecozone+ under natural conditions, and have decreased greatly since European settlement.
Based on an analysis of remote sensing data from Riley et al.,Footnote27 an estimated 70% of the natural vegetation on the Canadian Prairies was lost prior to the 1990sFootnote27 (see Ecosystem conversion key finding on page 39). Much of this would have been native grasslands and most of the loss occurred prior to the 1980s.Footnote27 In the mid-1990s, a study of land cover by Agriculture and Agri-food CanadaFootnote28 based on satellite imagery found that native grasslands comprised 23% of the landscape while Riley et al.Footnote27 found that native grasslands covered 25% of the ecozone+.Footnote27 While the data from both studies have sources of error, there is strong agreement between their results
Figure 5 shows the conversion of rangeland to cultivated land for selected areas of farmland in Alberta and Saskatchewan from 1941 to 2006 (excluding protected areas). Most of the native grassland in the Prairies Ecozone+ is "mixed prairie" with communities a mixture of mid-sized grasses and short grasses. The second broad type of grassland is "fescue prairie" which is much more restricted in area and of relatively greater conservation concern. Conversions were highest from the 1940s to 1970s. Losses were relatively more severe for fescue prairie compared to mixed prairie, and in Saskatchewan compared to Alberta.
Source: Coupland, 1987Footnote23 for 1941–1981 data, Statistics Canada, 2003Footnote29 for 2001 data, and Statistics Canada, 2008Footnote30 for 2006 data.
Long description for Figure 5
This line graph shows the following information:
|Year||Mixed prairie(AB)||Fescue prairie(AB)||Mixed prairie(SK)||Fescue prairie(SK)|
No value indicates not significant
For the Prairies Ecozone+ as a whole, the loss of rangeland has levelled off in recent decades (Figure 6), with the percentage of rangeland declining from 27 to 24% from 1971 to 1986, and changing only slightly after that.
Source: adapted from Statistics Canada, 2008Footnote30
Long description for Figure 6
This line graph shows the following information:
However, in some parts of the Prairies Ecozone+, losses have continued. Along 153 sampling transects that were weighted to the more settled parts of the Prairies, Watmough and SchmollFootnote24 found an overall 10% loss of native grasslands from 1985 to 2001. Area lost was greatest in the Aspen Parkland (15%), Fescue Grassland (13%), and Boreal Transition (13%) ecoregions (Figure 7). Most losses were remnant fragments on field margins (mean size was 2 ha; largest fragment lost was 64 ha). Forty-eight percent of the losses were to tame grass, 37% to annual crops, 10% to built cover (roads, houses, etc.), 4% to tree or shrub, and 1% to artificial water developments.Footnote24
Source: Watmough and Schmoll, 2007Footnote24
Long description for Figure 7
This bar graph shows the following information:
|Moist mixed grasslands||-8|
|Lake Manitoba Plain||-6|
|Southwest Manitoba Uplands||-12|
Large areas of intact grasslands remain in some agricultural areas that are used for grazing. Agriculture and Agri-Food Canada's Prairie Farm Rehabilitation Administration, through its Community Pasture Program, has managed 9,390 km2 of community pastures across the three Prairie provinces, 7,920 km2 (84%) of which are in native vegetation. Originally created in the 1930s to reclaim land that was badly eroded by drought, the program has returned over 145,000 ha of poor-quality cultivated lands to grass cover.Footnote31 Saskatchewan also has 2,260 km2 of existing provincial community pastures and Manitoba has 640 km2 of pasture conservation lands.Footnote32 Alberta has provincial grazing reserves totalling 1,260 km2.Footnote33 While the primary use of these areas is to provide livestock grazing, many of them are located in the remaining areas of native vegetation and their management practices emphasize conservation. Starting in 2012, the federal Community Pasture Program is being phased out with management of pastures in the program returned to the provinces over a six-year period.Footnote31
A small but unique area of grassland is the tallgrass prairie, which occurs mostly in the United States (U.S.) but extends into Canada in the Lake Manitoba Plain Ecoregion (Figure 3). It is North America's most endangered grassland type. Tallgrass prairie provides habitat for a number of distinctive animals and plants including two endangered orchids--western prairie fringed orchid (Platanthera praeclara) and small white lady's slipper (Cypripedium candidum)--and a threatened butterfly--Poweshiek skipperling (Oarisma poweshiek).Footnote34 Footnote35 Tallgrass prairie had been reduced to less than 1% of its original range in Manitoba by 1989. Footnote36 Koper et al.Footnote37 surveyed 65 remnant tallgrass prairie patches in 2007 and 2008 that had been previously surveyed in 1997 and 1998. They found that most patches, especially smaller ones, declined in quality. Furthermore, richness of native plants was negatively corrected with the cover and richness of non-native species. Also, 14% of prairie patches were so severely degraded by non-native species that they could no longer be recognized as tallgrass prairie. Footnote37 Most remnant patches are unlikely to be sustained without active management.
Natural disturbance regimes that historically maintained grasslands are highly altered in the Prairies Ecozone+ (see Natural disturbance key finding on page 82), particularly from the suppression of fire and replacement of free-ranging bison with confined cattle. Historically, the Prairies were part of the plains bison's range,Footnote38 and bison grazing was a significant driver of grassland composition and structure, maintaining short-grass prairie in areas climatically suitable for taller growth.Footnote39 Bison impact was spatially and temporally patchy, with intense damage in some areas and years, and less in others, creating a mosaic of habitats.Footnote39 Wild bison were extirpated by about 1870.Footnote14 Bison grazing has been replaced by domestic livestock grazing, mainly by beef cattle (Bos taurus).. Although bison have certain advantages over cattle for open-range grazing,Footnote38 Plumb and DoddFootnote40 argued that the biggest difference between bison and cattle impacts is that bison were free-roaming but cattle are confined in pastures and moved throughout the year to achieve a uniform level of grazing across the landscape. In confined pastures, both animals have similar impacts. Truett et al.Footnote39 argued that the patchy impact that occurred under historic bison grazing was better for prairie biodiversity than the uniform utilization sought by modern range managers and recommended recreating the earlier type of grazing regime for conservation purposes.
Grazing affects grassland biodiversity, but the relationship between the two is complex. Plant species diversity is usually higher in grazed grasslands compared to ungrazed grasslands, and may be highest at intermediate levels of range condition.Footnote41 Footnote42 Footnote43 McCanny et al.Footnote44 examined diversity of plants, songbirds, and large insects at Grasslands National Park and found that some species occur in grazed habitats and others in ungrazed ones. Similarly, in a review of Great Plains grasslands in the U.S., Bock et al.Footnote45 found that nine species of birds respond positively to grazing and eight respond negatively. These results suggest that maximizing regional biodiversity requires the presence of a wide range of grazing intensities, while the least desirable situation is uniform grazing management.
Using data on species composition to indicate change, ThorpeFootnote46 found that most Saskatchewan rangelands were similar to their potential composition or only moderately altered (Figure 8). However, 12% of plots overall were significantly or severely altered, mainly due to overgrazing and invasion by non-native plants. Rangelands were more altered in the Aspen Parkland Ecoregion than in the Mixed Grassland Ecoregion, because of higher levels of non-native invasion in the Aspen Parkland, and more conservative grazing management in the Mixed Grassland.
Health assessments for native rangelands and tame pastures in SaskatchewanFootnote47 and AlbertaFootnote33 showed similar results. About 8% of rangelands were "unhealthy", while another 43% were "healthy with problems", indicating early warning signs of a negative trend (Figure 9). Native rangelands had similar results to tame pasture. The results for Saskatchewan were similar for all ecoregions, whereas the results for Alberta indicated a lower proportion of healthy scores in the Aspen Parkland and Foothill Fescue ecoregions. Factors affecting range health include grazing intensity and invasion of non-native species.
Source: adapted from Alberta Sustainable Resource Development, 2008Footnote33(Alberta) and Saskatchewan Watershed Authority, 2008Footnote47 (Saskatchewan)
Long description for Figure 9
This stacked bar graph shows the following information:
|-||Healthy||Healthy with problems||Unhealthy|
|Tame pasture (AB)||53||39||8|
|Tame pasture (SK)||38||53||9|
As discussed under the Species of special economic, cultural, or ecological interest key finding on page 70, elk and moose populations are expanding as grassland areas experience woody vegetation expansion and reduction in hunter numbers.
Loss of native grasslands affects grassland birds. Current landbird populations are also affected by habitat degradation caused by the intensification of grazing, expansion of woody cover due to fire suppression, continued fragmentation, and invasion of invasive non-native plants (see page 43).Footnote48 There was an overall loss of 35% of grassland bird populations from the 1970s to 2000s.Footnote49 (Figure 10) with several species showing declines of greater than 60%. Some species, however, also increased (Table 3). Some of the birds showing long-term declines--horned lark (Eremophila alpestris), McCown's longspur (Rhynchophanes mccownii), and upland sandpiper (Bartramia longicauda)--did not decline on recent (1996 to 2006) survey routes that have more than 50% grassland, but did decline on routes with less grassland. Habitat loss or fragmentation may be a major factor for these species as they are still doing well where habitat is common and in large blocks. Other species (e.g., Sprague's pipit) are showing greater declines where grassland is common, which may reflect decreased habitat quality.Footnote49
Long description for Figure 10
This line graph shows the following information:
|Grassland birds||Population |
|P||BBS abundance index |
|BBS abundance index |
|BBS abundance index |
|BBS abundance index |
|McCown's longspur (Rhynchophanes mccownii)||-11.0%||*||6.10||2.05||0.77||0.24||-96%|
|Chestnut-collared longspur (Calcarius ornatus)||-5.4%||*||18.87||14.80||7.97||2.58||-86%|
|Short-eared owl (Asio flammeus)||-5.0%||n||0.47||0.21||0.09||0.10||-78%|
|Sharp-tailed grouse (Tympanuchus phasianellus)||-4.0%||*||1.49||1.73||0.47||0.53||-64%|
|Sprague's pipit (Anthus spragueii)||-3.8%||*||6.68||5.35||2.09||2.04||-69%|
|Horned lark (Eremophila alpestris)||-3.3%||*||81.15||77.03||48.81||31.38||-61%|
|Northern harrier (Circus cyaneus)||-3.0%||*||2.07||1.70||1.14||0.92||-55%|
|Western meadowlark (Sturnella neglecta)||-1.3%||*||60.21||49.25||43.23||43.67||-27%|
|Baird's sparrow (Ammodramus bairdii)||-1.1%||-||3.53||2.88||3.10||1.39||-61%|
|Vesper sparrow (Pooecetes gramineus)||0.8%||-||22.00||26.88||27.03||28.41||29%|
|Savannah sparrow (Passerculus sandwichensis)||1.0%||*||27.77||29.32||35.10||33.92||22%|
|Le Conte's sparrow (Ammodramus leconteii)||1.6%||-||1.14||1.22||2.01||1.26||11%|
|Sedge wren (Cistothorus platensis)||5.7%||*||0.31||0.23||0.70||0.94||199%|
P is the Statistical significance: * indicates P< 0.05; n indicates 0.05< P< 0.1; no value indicates not significant.
Species are listed in order from those showing most severe declines to those showing the most positive increases.
"Change" is the percent change in the average index of abundance between the first decade for which there are results (1970s) and the 2000s (2000-2006)
The relative stability of the grassland guild as a whole in the past decade (Figure 10) reflects the strong influence of some common (vesper sparrow, savannah sparrow) or wet meadow-associated (LeConte's sparrow, sedge wren) grassland birds. These species are more widely distributed and may be tolerant of, or even helped by, tall non-native plant species associated with linear development and farm programs that plant tall non-native grasses on crop fields.Footnote51 Footnote52 Footnote53 Declining species (e.g., Sprague's pipit, McCown's longspur, chestnut-collared longspur, Baird's sparrow) are those needing moderate or short, preferably native, cover and make little or no use of planted cover.Footnote51 Although some grassland birds will use hay-fields, 50–60% of ground nests, eggs, young, and fledglings are typically lost during a haying operation.Footnote54 Footnote55 One large study found 100% nest failure from haying operations as the remaining nests were abandoned.Footnote56
National key finding
High loss of wetlands has occurred in southern Canada; loss and degradation continue due to a wide range of stressors. Some wetlands have been or are being restored.
Although wetlands only cover 3% of the Prairies Ecozone+, they contribute disproportionately to prairie biodiversity. The majority of wetlands in this ecozone+ are known as "potholes", small, shallow seasonal wetlands that form every spring. Numbering in the millions, these wetlands are a result of the unique glacial history of the region combined with its cold semi-arid climate. Thus, conditions in these wetlands each year are determined by year-to-year variation in precipitation and snowmelt. Prairie potholes have the greatest capacity of all the wetland types to return water back to the soil and atmosphere, significantly reducing the impacts of floods.Footnote57
The potholes and their surrounding uplands in both the U.S. and Canada form what is known as the Prairie Pothole Region, an area that is the most productive waterfowl breeding habitat in the world. The region supports 50% of annual continental duck production,Footnote58 Footnote59 and between 50 to 88% of the North American breeding populations of several species.Footnote60 Footnote61 Footnote62 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,62 land use change is also important.
Extensive areas and numbers of wetlands in the Prairies Ecozone+ have been drained although there are no comprehensive data on historic loss. There are many small studies that have examined loss but they are very localized, most are over 30 years old, and they vary in scale.Footnote24 Footnote63 Footnote64 Footnote65 As an example of a newer study, Ducks Unlimited Canada analyzed the watershed of Broughton's Creek, a tributary of the Assiniboine River located northwest of Brandon, MB. From 1968 to 2005, 5,921 wetland basins or 70% of the total number in the watershed, were degraded or totally lost due to drainage (Figure 11). This resulted in the loss of 21% of the watershed's wetland area and the loss of the various ecological functions played by those wetlands.Footnote66
A review of these localized studies shows the high variability of wetland loss across the landscape and across time. Estimates for overall wetland loss since European settlement range from 40 to 71%.Footnote24 Footnote67 Footnote68 However, percent loss varies considerably between locations.Footnote24 Footnote64 Some of the greatest losses were near major urban areas, with 76 to 96% lost by 1966 and a further 17% lost between 1966 and 1981.Footnote69
Watmough and SchmollFootnote24 provided the best estimate of the recent rate of wetland loss on a larger scale. Between 1985 and 2001, 6% of wetland basins were lost, representing 5% of the total estimated wetland area. Although all ecoregions showed declining trends, losses were not uniform (Figure 12). The Aspen Parkland Ecoregion accounted for 45% of wetland area lost and half the total number of basins lost. The Mixed Grassland Ecoregion had the highest relative area lost, however, at 7%. The average size of lost wetland basins was 0.2 ha, with 77% being less than 0.26 ha in size. Fifty percent of the total area of wetland lost was in the grass/sedge cover type, and 40% was within the cultivated landscape. Sixty-two percent of the area drained was used for cultivation, 21% for perennial grass, 6% for development, and 8% was in transition.Footnote24
Source: adapted from Watmough and Schmoll, 2007Footnote24
Long description for Figure 12
This bar graph shows the following information:
|-||Net change in wetland area||Net change in number of wetland basins|
|Moist Mixed Grassland||-4%||-3%|
|Lake Manitoba Plain||-2%||-5%|
When examined on a municipality-by-municipality basis, the highest rates of wetland loss between 1985 and 2011 were in parts of southeastern Saskatchewan (Figure 13). Areas of highest loss were generally correlated with areas of high wetland density.Footnote70
Watmough and Schmoll'sFootnote24 study also recorded changes to wetlands that did not result in total loss, but that may have caused a loss of wetland function, such as partial drainage or limited filling. The percent of the wetland area affected by these factors was similar in 1985 (6%) and 2001 (7%), although results show a decline in wetlands affected in the Fescue Grassland Ecoregion and an increase in the Lake Manitoba Plain Ecoregion (Figure 14). The analysis also found that 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%.Footnote71
Source: adapted from Watmough and Schmoll, 2007Footnote24
Long description for Figure 14
This bar graph presents the following information:
|Moist Mixed Grassland||4%||6%|
|Lake Manitoba Plain||8%||11%|
Through an analysis of other studies, Watmough and SchmollFootnote24 found that their results were consistent with estimates of the rate of loss from other localized studies from previous time periods. They concluded that wetland loss is variable across the landscape but that there has been a continuous slow decline overall with large losses in localized areas.
Trends in wetland distribution and abundance directly affect continental waterfowl populations and research indicates that, overall, smaller wetlands support a greater number of waterfowl than larger ones on a per area basis.Footnote57 For example, data on waterfowl use of wetlands indicate that ten 1-ha wetlands will support approximately three times as many waterfowl as one 10-ha wetland.Footnote57 A study of a representative sample of prairie wetlands found 91% were 1 ha or smallerFootnote24 and these small wetlands also suffer 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.Footnote24 Research also found that, between 1985 and 2005, shallow ephemeral wetlands located in agricultural fields had the highest rate of impact and slowest recovery rates relative to other wetland types.Footnote71 Wetland habitat, together with changes in the agricultural landscape, drive waterfowl populations. These trends are discussed in the Birds section of the Species of special economic, cultural, or ecological interest key finding on page Footnote75.
As an example of the impacts of wetland loss on waterfowl populations and indirect impacts from upland habitat changes, a recent analysis estimated the deficit in waterfowl productivity for 1971 to 2006 relative to the 1970s. Results showed that while wetland loss resulted in an estimated decrease in waterfowl carrying capacity of just under 100,000 pairs from 2001 to 2006, upland changes substantially reduced the hatched nest "deficit" from ~150,000 to ~113,000 hatched nests (Figure 15).
Source: updated from Devries, 2004Footnote72 with Ducks Unlimited Canada, unpublished dataFootnote73
Note: Decline in pair population over time is based on model relating habitat changes to carrying capacity for waterfowl.
Long description for Figure 15
This bar graph presents the following information:
|Estimated pair population||4,248,904||3,920,602||3,662,897||3,564,018|
|Estimated total nest hatch||1,160,897||1,034,522||1,009,426||1,047,874|
|Nest "deficit" (relative to 1971)||0||-126,376||-151,472||-113,024|
Lakes and rivers
National key finding
Trends over the past 40 years influencing biodiversity in lakes and rivers include seasonal changes in magnitude of stream flows, increases in river and lake temperatures, decreases in lake levels, and habitat loss and fragmentation.
Water availability is an important driver and issue in the Prairies Ecozone+ and the anticipated changes in moisture regimes as a result of climate change will exacerbate the challenges.
It is not possible to determine trends in streamflow for the Prairies Ecozone+ from the national hydrometric data because there are too few stations that are suitable. This is largely because many of the stations are only monitored seasonally.Footnote74 Footnote75 As discussed below, there are also a large number of Prairie streams that are dammed and channelized, altering their normal flow pattern making them unsuitable for the trend analysis on natural streamflow.
There are data from other analyses focused on the Prairie provinces. GanFootnote76 analyzed streamflow trends on streams in the Prairies that were unaffected by dams from the late 1940s or early 1950s to 1993 and found that significant decreases were much more common than increases (overall, 61 significant negative trends vs. 16 positive trends). Fifty-six percent of the positive trends were in March and were related to the earlier onset of spring melt. All other months showed greater declining trends, particularly May and June flow. Burn et al.Footnote77 looked at 25 streams across the Prairies and also found decreasing trends in the spring snowmelt runoff volume and peak flow from 1966 to 2005. They found an earlier spring snowmelt peak and decreasing trends in seasonal (March–October) runoff volume.
- a 20% reduction from 1958 to 2003 for the Athabasca River at Fort McMurray, Alberta;
- a 42% reduction from 1915 to 2003 for the Peace River near Peace River, Alberta;
- a 57% reduction from 1912 to 2003 for the Oldman River at Lethbridge, Alberta; and
- an 84% reduction from 1912 to 2003 for the South Saskatchewan River at Saskatoon, Saskatchewan.
In the Prairies, the combination of glaciation and the dry climate has resulted in numerous closed-basin lakes (that is, lakes with no outlet). They are very sensitive to climate, with water levels and salinity driven by precipitation on the lake, local runoff to the lake, and evaporation off the lake. Changes in land use, such as water control structures that change the amount of water being delivered to the lake, also have significant effects on lake levels. Aquatic communities within these closed-basin lakes are sensitive to chemical changes that result from changes in water levels. For example, water levels affect salinity and the diversity of aquatic species declines as salinity increases. When salinities reach very high values, species diversity becomes very low.Footnote79
To provide a regional overview of lake level changes, van der Kamp et al.Footnote80 analyzed long-term water level changes in 16 closed-basin lakes with little or no groundwater interactions and no strong influence by large water control structures. Their results show an overall decrease in most lake levels of approximately 4--10 m from ca. 1920 to 2006, with more rapid declines since the late 1970s (Figure 17). There was, however, a rise in the level of four lakes in the east-central area since the 1960s (three other lakes in this area had declining levels). These increases have been linked to either higher precipitation or lower evaporation, in addition to sensitivity to agricultural drainage and changing land use due to low-lying relief. No lakes were included in the study from the south-central part of the ecozone+ because all lakes with long-term data records were highly affected by water control structures and diversions. Oro Lake was included as an attempt to fill the gap and results show that water levels in this region may not have declined.
The declines observed can be explained, at least in part, by climate. The early part of the 20th century was wet, contributing to the high lake levels.Footnote80 Significant increases in spring temperatures from 1950 to 2007Footnote81 could have led to increased evaporation rates and declining stream runoffFootnote82 could also have contributed. Nevertheless, the declines in lake levels were not completely consistent with climate. For example, with the exception of declines at two stations in south-central Saskatchewan, there was no significant change in precipitation from 1950 to 2007.Footnote81 Also, Zhang et al.Footnote81 found no significant change in Palmer Drought Severity Index from 1950 to 2007 to explain the recent declines, and Bonsal and RegierFootnote83 found that most droughts over the past century were between 1915 and 1930, a time when lake levels were higher.
Other contributing factors that reduce surface runoff to the lakes include land use changes such as dams, ditches, wetland drainage and dugouts,Footnote80 as well as changes in agricultural use and practices, such as the decline in summer fallow,Footnote84 increased conservation tillage (see Agricultural landscapes as habitat key finding on page 64),Footnote85 and increased continuous cropping.Footnote80
Alteration of hydrology through water control structures
Water control structures are one of the greatest threats to freshwater ecosystems. In addition to being barriers to movement of fish and wildlife, they alter natural hydrological regimes, changing water depths and flows, and resulting in changes to the availability and distribution of habitat for in-stream communities.Footnote86 Footnote87 Disruption of the natural regime can occur as a result of both lateral barriers (such as dams, weirs, roads) or riparian barriers (such as gaps in riparian buffers).Footnote88 Footnote89
Most major prairie streams are dammed. As of 2008, there were 83 large dams (greater than 10 m in height) in the ecozone+Footnote90 – at least 73% of them with reservoirs less than 1,000,000 m3.Footnote91 Only two dams (those forming Lake Diefenbaker on the South Saskatchewan River) have reservoirs greater than 10,000,000 m3.The number of large dams is highest in the Mixed Grassland (34), Moist Mixed Grassland (22), and Aspen Parkland (18) ecoregions, with only one in the Cypress Uplands.Footnote90 Footnote91 In the Prairies Ecozone+, where agricultural production is limited by low rainfall, irrigation was the most common reason that dams were constructed. The majority of these large dams were built in the 1950s and 1960s (Figure 18).
While these large dams are the most visible, there are also a large number of smaller dams and water control structures. For example, the Prairie Farm Rehabilitation Administration constructed approximately 12,000 dams in the Prairie provinces although few new ones are being built due to the lack of suitable sites and because of environmental concerns.Footnote92
Damming to create a reservoir also impacts terrestrial habitats in the flood zone. This impact is particularly important because it is concentrated on riparian zones, which contribute disproportionately to biodiversity. For example, the creation of Lake Diefenbaker flooded a significant area of riparian cottonwood (Populus deltoides) forest, an ecosystem that is relatively restricted in range in the Prairies Ecozone+. Damming can also have downstream effects on riparian ecosystems by eliminating flooding events that are important to these ecosystems. For example, it is thought that riparian cottonwoods are failing to regenerate on some dammed Prairie rivers because they require flood-deposited silt.Footnote93
Another important cause of hydrological alteration and corresponding impacts on biodiversity is channelization, which is very common on the Prairies. However, data on the number of streams and the kilometres of channelization are not available.
Ice across biomes
National key finding
Declining extent and thickness of sea ice, warming and thawing of permafrost, accelerating loss of glacier mass, and shortening of lake-ice seasons are detected across Canada's biomes. Impacts, apparent now in some areas and likely to spread, include effects on species and food webs.
There is very little long term trend data on freeze-up and break-up of lake and river ice in the Prairies Ecozone+. What data there is shows mixed trends for freeze-up.Footnote75 Some significant changes to earlier break-up were found--Duguay et al.Footnote94 found that ice on Lake Diefenbaker broke up ten days earlier between 1971 and 2000, and RannieFootnote95 found that, from 1815 to 1981, ice on the Red River at Winnipeg broke-up 12 days earlier. RannieFootnote95 also found that the Red River froze ten days later.
National key finding
Dunes are a unique biome with a very limited distribution in Canada. As a result, information on dunes was not identified as a nationally recurring key finding nor was it included in one of the other key findings in the national report.Footnote3 However, because of their significance to biodiversity in the Prairies Ecozone+, information on dunes is included as a separate ecozone+-specific key finding in this report.
Sand dunes are important habitats in the Prairies Ecozone+, and in recent decades many of the active dunes have shifted to stabilized dunes as a result of vegetation growth. In southeastern Alberta's Middle Sand Hills, the total area of active sand dunes declined at a rate of 40% per decade and the number of active dunes declined by seven per decade since 1950; all dunes could become stabilized by 2014.Footnote96 In southwestern Saskatchewan's Seward Sand Hills, a 70% decline in active dune area between 1944 and 1991 was documented;Footnote97 however, while active sand area declined from 1944 to 1979 in part of Saskatchewan's Great Sand Hills, it increased from 1988 to 1991 to the extent that in 1991, the active area was equivalent to that of 1944.Footnote97The authors attributed this increase to the drier and hotter conditions of the mid- to late 1980s. In general, climate drives dune activity, which increases in dry periods (low ratio of precipitation to potential evapotranspiration) and decreases in moist periods.Footnote98 Fire suppression may also contribute to sand-dune stabilizationFootnote99 (see Natural disturbance key finding on page 82).
The decline in active sand dunes poses a threat to the nationally Endangered Ord's kangaroo rat (Dipodomys ordii)Footnote96, in addition to several other species that depend on dune activity, including western spiderwort (Tradescantia occidentalis) (Threatened),Footnote100 small-flowered sand-verbena (Tripterocalyx micranthus) (Endangered),Footnote101 dusky dune moth (Copablepharon longipenne) (Endangered),Footnote102 and pale yellow dune moth (Copablepharon grandis) (Special Concern).Footnote103
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