Greater sage-grouse (Centrocercus urophasianus) COSEWIC assessment and status report: chapter 8
Limiting factors and threats
Current population declines are likely due to an accumulation of causes (Crawford et al. 2004). Loss and degradation of habitat was thought to be the most important factor influencing declines (Connelly et al. 2000a; Aldridge and Brigham 2002). Reseeding practices, alien plant invasion, water diversion, energy extraction and industrial development, insecticides, grazing and climate change, are also factors in habitat loss, degradation and fragmentation (Braun 1998; Miller and Eddleman 2000; Lyon and Anderson 2003; Adams et al. 2004; Aldridge 2007). Predation pressure and fire intensity may have increased as a function of habitat changes (Storch and Willebrand 1991; Crawford et al. 2004). West Nile Virus and loss of genetic diversity may pose additional threats to small populations.
Agricultural practices
Although many of the specific relationships between Greater Sage-Grouse demographics and habitat quality are unclear, the overall relationship is illustrated by the fact that remaining populations are associated with intact habitats (Crawford et al. 2004). Vast tracts of sagebrush have been removed in the USA since settlement for western rangeland improvement (Beck and Mitchell 2000). Methods of sagebrush removal have included herbicide applications (Klebenow 1970; Martin 1970; Wallestad 1975), fire (Fischer et al. 1996b; Nelle et al. 2000), and mechanical means (Swenson et al. 1987). There are few examples of similar management objectives to remove silver sagebrush on public rangelands in Canada (Adams et al. 2004). Cultivation of rangeland in Alberta likely caused the desertion of 2 leks (Aldridge and Brigham 2003). Cultivation did not appear to be associated with lek abandonment in Saskatchewan (McAdam 2003; Thorpe et al. 2005). Although cultivation has eliminated 60% of native vegetation in the Mixed Grassland Ecoregion of Saskatchewan (Hammermeister et al. 2001), the crash in provincial Greater Sage-Grouse cannot be linked to land use changes (Thorpe et al. 2005).
Historically, wild fires occurred every 30-50 years in arid sagebrush habitat and 100-200 years in low sagebrush types (Braun 1998; Miller and Eddleman 2000). Fires produced a mosaic of burned patches with enhanced herbaceous plant production (Pyle and Crawford 1996; Miller and Eddleman 2000), and reduced woody plant abundance (Miller and Eddleman 2000). In Canada, fires ended as an ecological force on the landscape in the early 1900s, during a period of extensive grazing by domestic livestock (Adams et al. 2004).
Regeneration of big sagebrush after fire is by reseeding, and prescribed burning has had negative short and long-term impacts on Greater Sage-Grouse food supplies, nesting habitat and abundance (Fischer et al. 1996b; Connelly et al. 2000c; Nelle et al. 2000). The impact of fire may be less severe on the silver sagebrush community as it is fairly resistant to fire and has strong regeneration potential (Aldridge and Brigham 2002; Thorpe 2002; Adams et al. 2004). Although a shortage of information exists on the impacts of natural or prescribed burning in the silver sagebrush community (Connelly et al. 2000a), at this time it is expected fire would further reduce habitat quality and availability in Canada (Adams et al. 2004).
Livestock grazing
Livestock grazing can have a strong influence on the productivity of Greater Sage-Grouse populations (Beck and Mitchell 2000). Grazing may directly affect the composition, density and structure of vegetation. Additionally, large areas of sagebrush-grassland have been converted to exotic crested wheat grass (Agropyron desertorum) for cattle forage (McAdoo et al. 1989). These introduced stands have limited potential in terms of winter forage, shrub and forb cover (Miller and Eddleman 2000). A reduction in shrub and herbaceous cover negatively affects nesting success (Gregg et al. 1994; Delong et al. 1995, Aldridge and Brigham 2002; Holloran et al. 2005) and forage and cover available for brood-rearing (Call and Maser 1985; Aldridge 2000). In southeastern Alberta, trampling on livestock wintering sites may be the principal reason for silver sagebrush decline (Adams et al. 2004). The loss of cover can negatively influence grouse by exposing them to weather and predators (Aldridge 2000) and reducing the amount of available winter range.
Human disturbance
Oil and gas exploration has had detrimental effects on Greater Sage-Grouse breeding behaviour, seasonal habitat selection and population demographics (Aldridge 2000; Braun et al. 2002; Lyon and Anderson 2003; Holloran 2005; Aldridge and Boyce 2007). Across the species range, oil and gas wells and associated pipelines influence 28% of the sagebrush habitats (Connelly et al. 2004). Oil and gas exploration began in Alberta in the 1940s and development intensified in the 1980’s (Braun et al. 2002). Density of wells within Greater Sage-Grouse range in southern Alberta in 2002 was 1 active and 2 inactive wells per km² (Braun et al. 2002).
During an exploration boom in the 1980s the number of males displaying on leks in southern Alberta decreased by approximately 50% (Braun et al. 2002). Leks < 200 m of a road or well sites were rendered inactive, and leks within view of a development were either reduced or inactivated (Aldridge 2000). Similarly, drilling within 5.0 km of leks in western Wyoming resulted in displacement of adult males and low recruitment of juveniles (Holloran 2005). Impacts continued even after drilling and construction activities ceased; the number of breeding males failed to recover within 3.0 km of producing wells (which typically remain on site from 30 to 50 years) even after drilling was completed. Light traffic disturbance from natural gas development (1-12 vehicles/day) during the breeding season in western Wyoming resulted in reduced nest initiation rates and increased lek to nest distances (Lyon and Anderson 2003). Females avoided gas field infrastructure when selecting nest and brood-rearing habitat (Holloran 2005; Aldridge and Boyce 2007), and chick survival decreased with increasing well site density (Aldridge and Boyce 2007).
Development has also fragmented sagebrush habitat through the addition of buildings, highways, trails, fences and electrical poles. These structures provide raptor perching sites (Connelly et al. 2000a) and can cause injury or death to Greater Sage-Grouse that fly into human-made objects or traffic (Patterson 1952; Crowley and Connelly 1996, ASGRAG 2005).
Silver sagebrush occurs on mesic sites with deep fertile soils requiring periodic flooding (Thorpe 2002; Jones et al. 2005). Drainage impediments such as dams, dugouts, berms and reservoirs have increased four-fold from 1951-2001 in southeastern Alberta, altering hydrological regimes and degrading silver sagebrush communities (McNeil and Sawyer 2003). More than 80% of current Greater Sage-Grouse range in Alberta is altered by impediments (ASGRAG 2005).
Diseases
Historically, disease was not considered a major factor in Greater Sage-Grouse population change until the discovery of the first West Nile Virus (WNv) caused mortality in 2003. A widespread, cross-border study reported dramatic impacts of WNv on survival of radio-marked populations (Naugle et al. 2004). Associated deaths with WNv decreased female survival in 2003 by 25% in 4 populations in Wyoming, Montana and Alberta (Naugle et al. 2004). In 2004 and 2005, disease prevalence decreased range-wide and there was a single WNv-associated mortality in Alberta (Naugle et al. 2005; Nicholson pers. comm. 2006). This was likely a result of unseasonably cool temperatures in summer which reduced mosquito (Culex tarsus) production, the vector for WNv (Naugle et al. 2005). To date, the species has not developed any resistance to WNv and low survival rates could devastate small populations (Naugle et al. 2004, 2005).
Genetics
The decline and extinction of natural populations has been linked to the loss of heterozygosity and inbreeding, thus small populations are prone to a reduction in genetic diversity (Bush pers. comm. 2006). Genetic flow among and between populations is essential in maintaining population viability. Loss of genetic diversity in Greater Sage-Grouse populations may result in enhanced susceptibility to parasitic agents or disease (Oyler McCance et al. 2005).
Preliminary genetic analysis in Alberta indicates inbreeding may be resulting in a skewed sex ratio favoring females at hatch, and reproductive morphological deformities (Bush 2004). Analysis of Greater Sage-Grouse embryos indicated 13% (n = 48) exhibited external abnormalities (anophthalmia [no eyes], microphthalmia [small eyes], deformed beaks, and dwarfism) and 52% of abnormal and apparently “normal” embryos exhibited a variety of cranial abnormalities (anophthalmia, microphthalmia, deformed beaks, shortened beaks, malformed cranial cartilage, and deviated nasal septa) (Bush pers. comm. 2006). Inbreeding and selenium toxicity are being examined as potential causative factors.
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