Bradwell National Wildlife Area Management Plan: chapter 3
3 Management challenges and threats
The following four management challenges and threats affect the ecological integrity of Bradwell National Wildlife Area (NWA). Ecological integrity of Bradwell NWA is considered fair despite these challenges and threats because it provides important habitat for a variety of nesting and migrating birds, including several species at risk.
3.1 Artificial water control structures
Historically, infrastructure on Bradwell NWA has required heavy equipment access, excavation and dumping of soil and subsoil, and installation of metal and cement structures. Subsequent operation of this infrastructure requires accessing the site by vehicle several times each year between April and October to monitor water levels and make decisions regarding placement or removal of stop-logs. In this region, natural closed basin wetlands fill early in the year with spring runoff from snowmelt and lose water from evaporation or seepage thereafter (Stewart and Kantrud 1971). However, several wetland basins on Bradwell NWA have artificial inlet and outlet connections that support supplementary inflow of fresh river water for several days or weeks in late spring each year to help fill downstream basins. Occasionally beavers and muskrats must be trapped, trees and shrubs cut, grassland mowed, and ditch bottoms dredged in order to keep the engineered structures operating. Redevelopment of structures every 20-30 years requires repetition of the same heavy equipment access, excavation, fill, and installation process.
The activities necessary to maintain or redevelop the water control structures fall under six to seven of the prohibited activities described in section 3 of the Wildlife Area Regulations. The primary mitigating factor of this activity on Bradwell NWA is the positive outcome for sustaining wetland habitat and waterbird populations, particularly duck broods, through periods of drought for 5 out of every 20 years. However, it becomes challenging to implement other forms of active management for fear of causing cumulative environmental effects and additional threats to the NWA.
For example, cattle trails in the uplands converge at a few crossings around wetlands and over ditches, and cattle preferentially use slopes of deeply cut ditches as mineral licks or wallows (Figure 4). This has reduced vegetation cover along the slopes and likely contributes to increased siltation or eutrophication of wetlands during periods of precipitation and flow between basins (Kauffman and Krueger 1984).
In the aquatic habitats, submergent or floating vegetation that might interfere with operation of the Saskatoon Southeast Water Supply System (SSWSS) are controlled upstream with bi-weekly injections of the aquatic herbicide acrolein, which has the potential to destroy Pondweed, Water-milfoil, and other aquatic plants found at Bradwell NWA that serve as important food or cover for migratory waterbirds (Cota-Sánchez and Remarchuk 2007). Authorities at the Saskatchewan Water Supply Board were of the opinion that by the time this short-lived, light-sensitive herbicide reached Bradwell NWA and the Baldwin Project, the acrolein would break down and the herbicidal effects would be negligible. They also close the water intake for Bradwell NWA for 24 hours after applying the herbicide approximately 1.6 km upstream (Franz Environmental Inc. 2006). No monitoring has occurred in the Bradwell NWA basins to detect or evaluate the effect of acrolein additions.
The presence of Fathead Minnow could be a conservation concern, even though this species is native. Elsewhere in the region where artificial connections have been made between wetlands, Fathead Minnow have been able to swim upstream and colonize wetlands that do not otherwise support fish. Once they arrive, they compete directly with waterfowl for food by consuming aquatic invertebrates (Hanson et al. 2005). The effect can be substantial and may decrease the value of the wetlands as waterfowl brood habitat in mid-summer, thus negating any benefit accrued by the water control infrastructure. Sampling to monitor the presence and abundance of Fathead Minnow has not taken place since the species was detected in 1983 (Canadian Wildlife Service 1985).
Other cumulative effects of the water control structures include the spread of invasive alien aquatic and emergent plants, or the addition of other deleterious substances to wetland habitats through the canal system. Efforts to combat those threats with the use of chemical or mechanical tools may only exacerbate cumulative effects. Thus far, livestock have not damaged or interfered with the metal infrastructure, but that conflict between the two tools to manage wetland or upland habitats, including conflicting needs for water during the summer and fall seasons, could become problematic in future.
3.2 Alien invasive plant species
As mentioned in section 2.4, there is extensive Canada Thistle, Perennial Sow-thistle, Absinthe, and Sweet Clover invasion of the uplands. Most tame uplands were seeded to Smooth Brome and Alfalfa, and Smooth Brome has now invaded downslope into most wetland margins. The canal along the southern edge of the NWA contains substantial populations of Reed Canary Grass and Giant Reed, though it is unknown if the latter is the exotic or native species.
The management decision to reseed cropland to tame forage grasses and legumes in the 1970s, and to then allow much of that vegetation to go unmanaged for years, has been the greatest source of seed for the spread of invasive species observed at Bradwell NWA. Habitat and landscape requirements for waterfowl are not always suitable for other bird species, particularly endemic grassland songbirds, many of which are species at risk. Perennial cover is still important to grassland songbirds (McMaster and Davis 2001), but responses to the structure of that cover and the surrounding landscape is what differs between songbirds and ducks (McMaster et al. 2005). Some songbird species depend on grazed native grasslands with shorter cover or may avoid shrubby or wetland margin habitats (Koper and Schmiegelow 2006, 2007; Skinner and Clark 2008).
Since 2008, deferred grazing with cattle has been used to reduce fire hazards from litter build-up and create heterogeneity in nesting habitat. However, the combination of leaving this seeded grassland idle for several decades, followed by deferred grazing, has resulted in significant seed production and widespread invasion by various perennial and biennial alien plant species. While this vegetative community appears to be good nesting cover for ducks and preferred habitat for pocket gophers and mice, it is unsuitable habitat for most other upland migratory bird species or species at risk.
Restoration and enhancement of this habitat for a broader range of migratory birds would require more targeted livestock grazing and possibly conversion of some fields to annual cropland for several years before attempting to re-seed a more diverse native grass mix.
3.3 Gravel extraction
Several gravel pits have been operated in the vicinity of Bradwell NWA in the past, but most have been exhausted. Currently, there is a large gravel pit 1.3 km east of the NWA. The working of a gravel pit may disturb wildlife in the immediate vicinity, alter local water tables and drainage patterns, and introduce new alien invasive species to the area. Although gravel extraction and demand is increasing in the region due to economic development in Saskatoon, it is not believed to be a serious threat given the conservation easements on most surrounding property. Socio-economic considerations may change in future and increase the likelihood of this particular threat.
3.4 Multi-agency land management
In the vicinity of Bradwell NWA, numerous agencies manage and administer land, including Ducks Unlimited Canada, Saskatchewan Ministry of Agriculture (provincial crown lands) and Saskatchewan Ministry of Environment (Wildlife Habitat Protection Lands) (Figure 4). This multiple-agency approach has ensured numerous parcels of land are available for wildlife habitat, covering approximately 2,000 ha (not including Ducks Unlimited Canada (DUC) wetland projects) in the immediate vicinity around Bradwell NWA. Multi-agency coordination could further improve efforts by sharing resources and designating a single management authority.