Greater short-horned lizard COSEWIC assessment and status report: chapter 4

Distribution

Global range

Horned lizards (Phrynosoma) are endemic to the North American west. Of the 17 species within the genus, Greater Short-horned Lizards (Phrynosoma hernandesi) are the most widely distributed, both in terms of altitude and latitude, and in terms of overall range. They are found from central Mexico, to the southernmost portions of the Canadian prairies (Fig. 2; Russell and Bauer 1993; 2000; Sherbrooke 2003; Stebbins 2003).

Figure 2. Map of North American distribution of Phrynosoma hernandesi. From Stebbins 2003.

Figure 2.  Map of North American distribution of Phrynosoma hernandesi. From Stebbins 2003.

Canadian range

The proportion of the global distribution for this species that falls within Canada is less than 5% (ASRD 2004). In Canada, Greater Short-horned Lizards are found in a limited number of widely scattered and isolated populations across the southeastern corner of Alberta and extreme southern Saskatchewan (Powell and Russell 1991a; 1992a; 1993a; 1998; Powell et al. 1998; James 2002; 2003; ASRD 2004). The species reaches the periphery of its global range in Canada, with its most northern locations along the eastern banks of the South Saskatchewan River, north of the city of Medicine Hat in Alberta, at approximately 50°10’N (Powell and Russell 1993a). Milner (1979) listed the southeastern portions of the Red Deer River as part of the range for this species in Alberta, but Laird and Leech (1980) disregarded this suggestion, as there are no confirmed records from this far north. The most westerly record is near the village of Grassy Lake, in Alberta, and the most easterly record is to the west of the West Poplar river, within the East Block of Grasslands National Park in Saskatchewan (Powell and Russell 1993a). In the following discussion of the range of this species in Canada, the Alberta populations will be considered first, followed by the Saskatchewan population.

The isolated nature of these populations in Alberta has been questioned, as patches of apparently suitable habitat occur in other parts of the region. In some cases, such as along the South Saskatchewan River, it seems odd that the populations are not more continuous, as there appear to be patches of potentially suitable habitat, with relatively short distances between them. It is feasible that the cryptic nature of this species, the low densities at which they occur, and the generally limited human presence in the region could have prevented the detection of other sub-populations or even other locations within the region. No rigorous effort to investigate all areas with potential habitats has been attempted, probably due to sheer scale of the undertaking and the resultant expense.

However, several factors suggest that the lizards are confined to relatively small areas. A much higher level of public awareness of this species, due to the considerable study over the past 25 years, as well as some limited public education efforts, should have contributed to new location records being turned in for this species. Employees of the petrochemical industry are often aware of the concern for this species; perhaps more than the general public, and since the industry covers almost the entire area, one would have expected records from such sources. Ranchers and farmers in the region have wide social networks and are well aware of most of the species on their lands. As they are often excellent sources of such information, it would be anticipated they would know if the species were present. Although no concerted efforts have been made to query landowners throughout the entire area, there have not been additional suggestions put forward by long-time stakeholders either. Many landowners and stakeholders may not readily provide the information they would have at one time. This is often due to stronger laws for at-risk species and the worry of limitations on land use. It would seem likely that records for the species would be highest in the City of Medicine Hat, where higher levels of human presence would be presumed to offer an increased probability of discovery. There have been only five locations collected from Medicine Hat and area, with three of those being recorded in the past six years (FWMIS 2006). The earliest capture in Alberta (1918) is from this area, with only one other record in the interim (FWMIS 2006).

Alberta

The total Alberta population is spread out within four widely distributed locations: 1) sub-populations along the South Saskatchewan River; 2) those within the Manyberries Hills (or Badlands); 3) those along the Chin / Forty-Mile Coulee complex; and 4) those along the Milk River and its associated tributaries (Fig. 3; James 2002; ASRD 2004). These four main locations are considered to be isolated from each other by distance, low lizard densities, and lost habitat connectivity. Furthermore, sub-populations within each locality may be isolated from others within the same site. For example, the sub-populations found along the South Saskatchewan River are on the northern banks in the western section of the site, and on the opposite side in the eastern portion of that same site (Powell and Russell 1992a; James 2002).

Figure 3. Recorded locations for Phrynosoma hernandesi in Alberta. The population distribution in Alberta is comprised of four widely distributed main sites each with widespread sub-populations. The most northern population location is along the South Saskatchewan River. A second location is associated with the Chin Coulee/Forty-Mile Coulee complex. The third population is east of the town of Manyberries. The last is along the Milk River and tributaries. Historical records were taken as those from prior to 1991, since this is when the initial survey by Powell occurred (Powell and Russell 1992a). Data for map provided by Alberta Sustainable Resource Development database (FWMIS 2006).

Figure 3. Recorded locations for Phrynosoma hernandesi in Alberta.The population distribution in Alberta is comprised of four widely distributed main sites each with widespread sub-populations.

The overall historical range in Alberta has likely been reduced primarily by agricultural practices in the region. For example, a 1923 record of a Short-horned Lizard, collected near the village of Grassy Lake, is in an area where the remaining suitable habitat for the species is currently extremely limited. It is the most western of all records for the species in Alberta (Powell and Russell 1991). The existence of this record strongly suggests that Short-horned Lizards extended the full length of Forty-Mile Coulee, perhaps even connecting, at one time, with the South Saskatchewan River populations. Therefore, this implies that there was an upper Forty-Mile coulee population that has become extirpated since 1923, probably related to the cultivation, damming, and irrigation along the watercourse.

Currently, none of the remaining four major Alberta locations can be considered entirely extirpated within the last 10 years or three generations. Surveys for Short-horned Lizards were conducted over the summers of 2001-2002 (James 2002; 2003) and presence of the species was verified within all of the four major locations. However, these surveys could not verify presence in some of the sub-populations that were previously considered numerous (James 2002; 2003; ASRD 2004). The lack of captures, particularly for some of those sub-populations within the Milk River location, were puzzling, as conditions seemed ideal, and a respectable search effort was expended (James 2002).

The estimate of the Extent of Occurrence (EO) in Alberta is about 8110 km², based upon the intersection of observation points with 2 X 2 km grid cells. From this calculation, an Area of Occupancy (AO) for Greater Short-horned Lizards in Alberta of 144 km² was derived.

Saskatchewan

The Saskatchewan total population is concentrated within each of the two parcels of Grasslands National Park (GNP) and is considered separate from the Alberta total population by distance and natural topographic features (Fig. 4; Powell and Russell 1992b; 1993a, Powell et al. 1998). Like one of the main Alberta populations, the total population in Saskatchewan is also affiliated with the Milk River (Missouri) system (Powell and Russell 1992b; 1993a; Powell et al. 1998). They extend no further north than 49° 30’N, or within 30 km of the U.S. border, in Saskatchewan (Powell and Russell 1993a, Powell et al. 1998). Range expansion in Saskatchewan is apparently confined by the Cypress Hills to the northwest, the Boundary Hills to the west, and by Wood Mountain and Pinto Butte to the east and northeast (Powell and Russell 1993a). The sub-population in the East Block is also likely to be isolated from that of the West Block by distance (Powell et al. 1998). Surveys in 1995-1996 confirmed presence of sub-populations in both Blocks of GNP (Powell et al. 1998).

The range of Short-horned Lizards in Montana is considered to be contiguous with that of the Saskatchewan populations by some (Thompson 1982), although they are not considered to be north of the Milk River proper, by others (MNHP 2006, Reichel and Flath 1995; Werner et al. 2004).  However, a relatively recent record from a watershed north of the Milk River, and directly south of the West Block of Grasslands National Park, is reported in Cooper et al. (2001). This record suggests there is the potential for some level of continuity between the Saskatchewan and Montana populations in that area.

Figure 4. Records of the Greater Short-horned Lizard inSaskatchewan. Lizards have been recorded in both the West and East Blocks of Grasslands National Park in Saskatchewan. Record locations derived from Powell et al. (1998) and Saskatchewan Conservation Data Centre (2006) database records.

Figure 4. Records of the Greater Short-horned Lizard inSaskatchewan. Lizards have been recorded in both the West and East Blocks of Grasslands National Park in Saskatchewan. Record locations derived from Powell et al. (1998) and Saskatchewan Conservation Data Centre (2006) database records.

The estimate of the Extent of Occurrence (EO) in Saskatchewan is about 720 km². This was calculated based on the intersection of observation points with 2 X 2 km grid cells. From this estimate, the Area of Occupancy (AO) of Greater Short-horned Lizards was calculated to be 76 km² in Saskatchewan.

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