Greater short-horned lizard COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

Natural limiting factors for Greater Short-horned Lizards in Canada are probably a combination of physiological and behavioural restrictions acting in conjunction with climatic barriers (Powell and Russell 1998). Late fall and over-wintering survivorship, reproductive biology, and perhaps habitat requirements, may all contribute to their limited range in Canada (Powell and Russell 1996; 1998).

The relative significance of natural limiting factors for the Alberta populations differs from those in Saskatchewan. For the Saskatchewan populations, natural limiting factors are expected to be their most significant population-level constraint, as the province’s populations are confined within Grasslands National Park, an area of limited human impact (Powell and Russell 1996; 1998). Powell et al. (1998) suggested that a combination of climate and geography limits the range of Short-horned Lizards in Saskatchewan, as its range closely corresponds with that of the Prairie Rattlesnake (Crotalus viridus viridus) in that province, whereas in Alberta, where amenable habitat is less confined by uplands than it is in Saskatchewan, rattlesnakes range much further north and west than Short-horned Lizards. 

For populations in Alberta, the effects of increased levels of human activities on the landscape present additional threats. Powell and Russell (1991b) suggested that Alberta populations of Short-horned Lizards are “probably extremely sensitive” to human-caused stresses (p 2764). Not all populations in Alberta are equally impacted by anthropogenic disturbances. For example, the sites along the Milk River and tributaries exhibit almost entirely intact habitats and should offer relatively uncompromised sanctuary for lizard populations in Alberta, although as noted earlier recent surveys failed to find lizards at many of these sites (see Fig. 3). Of great concern is that the largest set of sub-populations (about one third of all the Alberta lizards) is in the Manyberries Sensitive Area, which is under considerable pressure from oil and gas exploration, and, potentially, from new surface mining for humate and ammonite. These disturbances could threaten the lizard populations severely (ASRD 2004; Kissner 2005). The following discussion of anthropogenic threats will therefore be most relevant to the more northern three of the four main populations in Alberta. A discussion of the populations most affected by each threat type is also incorporated.

Industrial development/roads

The development of roads in southeastern Alberta continues to increase, especially with the expansion of petrochemical exploration and development. Roads have been implicated as distribution corridors for harvester ant populations (DeMers 1993). Ironically then, it is possible that roads may improve access to food sources for horned lizards while concurrently increasing the probability of road mortality. The seasonal movements of male Texas Horned Lizards, related to the spring mating season (Henke and Montemayor, 1998), rendered males as much as five times more likely than females to be found on roads and therefore considerably more vulnerable to road-kill (Sherbrooke 2002). Henke and Montemayor (1998) hypothesized that male horned lizards employ the improved line of sight available on road surfaces to locate females. Sherbrooke (2002) also estimated that the number of lizards killed on “sparsely traveled” roads in Arizona and New Mexico to be between 25 – 50% of the live population collected from along those same roads. He further suggested that, in areas affected by dense networks of roads, road-kill might contribute to local extinctions.

The impact of road density on Short-horned Lizard populations in Canada is not well documented. Powell and Russell (1993a; 1998) found Short-horned Lizards used vehicular tracks as travel routes through grassy areas. In such situations, there is an increased probability that lizards would be run over by vehicles following the same trail (Powell and Russell 1998). G.L. Powell (pers. comm.) has also found Short-horned Lizards run over on oil access roadways.

The Alberta sub-population that is most evidently impacted by industrial road development is that located in the Manyberries Hills (Fig. 7; Powell and Russell 1998; ASRD 2004; Kissner 2005) although those along the South Saskatchewan River also face this factor to a somewhat lesser degree. In the Manyberries Hills location, 564 new wells have been drilled since 1980 (Kissner 2005). A recent cumulative effects assessment estimated that approximately 15% of available lizard habitat was potentially influenced by development features, such as wells, roads, trails and pipelines. The “available habitat” measure is not equivalent to the area of occupation, but a more encompassing calculation including all parts of the area with potential habitat, but not necessarily with known records for Short-horned Lizards (Kissner 2005). Therefore, the effect of development within the areas of known lizard occupation may be even higher, if all of the remaining potential, but apparently unoccupied area is removed from consideration.

Agricultural development/cultivation and irrigation

Agriculture is the primary human land use in southeastern Alberta. Generally, the steep slopes of the habitats that Short-horned Lizards occupy are unsuitable for cultivation. The Alberta sub-populations most affected by irrigation would include the area surrounding the Forty-Mile coulee, southeast of Grassy Lake, and those along the South Saskatchewan River. Forty-Mile Coulee has at least three dams along its length. The intensity of cultivation surrounding Forty-Mile coulee and the level of water within the coulee itself have probably contributed to the extirpation of the sub-population previously recorded here. Short-horned Lizards have been relatively recently documented along the Chin Coulee near Foremost (James 2002), and near the confluence of Chin and Forty-Mile coulees (Powell and Russell 1991a). A 1923 Grassy Lake specimen was located in a tributary coulee near the upper tip of the central reservoir; however, habitat in this area is now considered extremely limited.  There is also a 1981 record from near the last dam along Forty-Mile coulee (FWMIS 2006). Powell and Russell (1992a) were unable to confirm the presence of this sub-population. Recent survey efforts were also unable to confirm the presence of the species in this location (James 2002).

Urban development

The City of Medicine Hat and the nearby centre of Redcliff are the only locations for which urbanization is of much concern for this species in Canada (Powell and Russell 1992a; James 2002; ASRD 2004). Fragmentation and loss of remaining habitats, collection by curious humans, or mortality due to interaction with pets, are all factors that may contribute to reduced survivorship within this location (Powell and Russell 1992a; James 2002; ASRD 2004). These factors likely combine with the increased risk of road and other vehicle-related mortalities, expected to be associated with generally higher levels of traffic, to reduce the overall long-term viability of this population.

Recreational activities

The use of off-road vehicles, such as all-terrain vehicles (ATVs), motorcycles, or trucks, is anticipated to have harmful effects on Short-horned Lizard populations in Canada, both by way of habitat disturbance and direct mortality of individuals (James 1997; ASRD 2004). Off-road vehicle use in dune habitat has been recorded to negatively impact horned lizards in California (Beauchamp et al. 1998). The use of dirt bikes and ATVs in locations occupied by Short-horned Lizards has been observed to cause significant localized habitat damage (J. James pers. obs.).