Greater short-horned lizard COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

Detailed accounts of the 1995-1996 survey by Powell et al. in Saskatchewan are available in Powell et al. (1998). The 2001 and 2002 surveys in Alberta are available in public format (James 2002; 2003) and in extensive detail as additional internal Alberta government versions of the same.

Search effort

Greater Short-horned Lizards are notoriously difficult to survey, due to their low densities, cryptic colouration and ability to remain motionless, even when approached quite closely. As they are not that mobile, and leave few indications of their presence, such as tracks or identifiable scat, trapping them or using some other index of presence is not viable. Short-horned Lizards in Alberta and Saskatchewan have always been censused on foot (e.g. Powell 1982; Powell and Russell 1996; Powell et al. 1998; James 2002; 2003).

Searchers look at the ground directly in front of themselves, and walk at a relaxed pace, usually with a walking stick to probe vegetation, back and forth across suitable habitat, with each pass being approximately 2 m from the last path taken. In this manner, each searcher can effectively investigate a strip of vegetation approximately 2 m wide. When more than one person is searching at a time, walking more or less next to each other, or with one person trailing slightly, makes the task less arduous, and may offer the chance for the trailing person to spot any lizards that might flush following the passage of the first person. The low density of lizards within these populations can result in very few captures, if any, per day, and this can lead to searcher fatigue. The level of movement of some lizards can be so slight as to be almost negligible, such as the tilting of the head or a simple shift of posture, so watching carefully for any movement is important.

There will always be a degree of uncertainty associated with the determination of the presence and true abundance of these exceedingly well-camouflaged animals. Capture rates are thought to be highest following the parturition period, in late July and early August (Powell and Russell 1992a; James 2002). Captures may be higher because of the increased total number of individuals, and because neonates seem more easily flushed than experienced animals (James 2002).

In both Alberta and Saskatchewan populations, surveys were concentrated in locations with preceding records, which have generally been documented at the level of the Quarter-Section-Township-Range format (Powell et al. 1998; James 2002, 2003). Some older, historical records are simply at the level of the township, which is less useful, or even by townsite. Since the vast improvement of the ability to record locations using GIS technology, the precision of records has increased substantially.

Searches carried out in 1991 resulted in the verified presence of lizards at 16 of the 28 (57%) areas where they had been previously recorded (Powell and Russell 1992a). At that time, Powell and Russell (1992a) suggested that populations appeared to have declined overall since the time of Powell’s 1978-1982 field work (Powell 1982). There are 74 sections (each section = one mile²) of land that have confirmed locations for Short-horned Lizards in Alberta, with 68 of these having intact habitat (ASRD 2004). Short-horned Lizard populations were recently surveyed again in Alberta (James 2002; 2003). The field surveys of 2001-2002 resulted in capture of 130 individuals. A total of 59 sections were searched with 48 of these sections having previous records of the species. Lizards were found on 19 sections, with three of these sections representing new records. The success rate of validating the presence of the species where it had previously been recorded, on a per section basis, was approximately 33% (16/48). The total area over which Short-horned Lizards are documented to occur in Alberta is within 68 individual sections. This includes all sections with historical records that were searched, as well as those that were not searched, but excluding locations where the habitat is currently unsuitable or lost.

An attempt to evaluate population parameters, on a very crude basis, was made by James (2002; 2003; ASRD 2004) and is reviewed in the following two paragraphs. The 2002 Alberta survey effort was largely unsuccessful, due to poor search conditions throughout the survey period (James 2003). As such, only the 2001 data were considered in the following calculations.

The 2001 Short-horned Lizard survey in Alberta resulted in a cumulative total of 330 hours logged searching for lizards (James 2002). After subtracting 10 minutes per capture to account for the time spent documenting each capture, the total adjusted time spent searching was 291 hours, 34 minutes. This total adjusted time was then divided by the total number of captures (125) to achieve the average time actually spent searching per lizard capture of 2 hours, 20 minutes.

The total area searched was estimated by assuming that each searcher covered a swath approximately 2 m wide at an estimated walking pace of 2.1 km/h. From this, (width of area covered, rate area was covered, and total time walked), the total area searched per person per search hour, was estimated (2100 m/hr * 2 m wide) to be about 4200 m²/hr. In turn, this was converted into a rate per minute (70 m²/min) and that multiplied by the cumulative number of minutes searched by all individual searchers. By multiplying the total adjusted time spent searching (291:34 hrs) by the area covered per minute (70 m²), an estimation of the total area searched, 1,224,545 m² (or 122.45 ha) was derived. When this value (122.45 ha) was divided by the total number of captures (125) the result (9796 m²) was approximately one hectare (10,000 m²) searched per lizard captured, which was interpreted as a loose, and perhaps minimal, estimation of overall density. Since Short-horned Lizards were not encountered at all locations, a calculation was also derived using strictly those locations where captures had been made. In that instance, the approximate density of lizards jumped to two per hectare searched. Potential for error within these calculations includes missed observations, inappropriately recorded time spent searching and variance in the walking speeds of individual searchers. The number of searchers varied, from one to nine people, by day and location in the 2001 surveys. Suffice to say that all estimates of density, abundance and presence could be underestimates unless one assumes presence or higher density than the capture data indicate. These uncertainties are considered in the section on abundance.

All Saskatchewan population information currently available is based upon the work of Powell et al. (1998). In Saskatchewan, approximately 200 person hours were expended to capture 8 lizards in a portion of Grasslands National Park in 1995. There may also be considerable variation between years and seasons; the previous year a single researcher made 4 captures over approximately 50 hours of searching (Powell et al. 1998). To compare, it is worth considering the number of hours searching per capture; this varied from 12.5 hours (West Block) to 25 hours/capture (East Block) for the 1995-1996 Grassland National Park surveys (Powell et al. 1998). This is notably higher than the average capture rate for Short-horned Lizards in the 2001 Alberta surveys suggesting Saskatchewan populations may occur at lower densities than in Alberta (Powell et al. 1998).

Abundance

Across their distribution, abundance is low in most horned lizard species and they are regarded as “rather uncommon” even where they do occur (Pianka and Parker 1975). Turner (1977) found a geometric mean density of 51 individuals per ha for studies of small iguanid lizard species. Tanner and Krough (1973) found P. platyrhinos densities of 5 per ha in Nevada. In Alberta, densities are considered generally low (Powell and Russell, 1991a). A range between 2 and 4 individuals per ha might be considered reasonable for Alberta populations (James 2003, ASRD 2004).

A very rough estimate of the total population of Short-horned Lizards in Alberta was attempted previously (James 2003; ASRD 2004). The basis upon which this was calculated is reviewed in this and the following two paragraphs. To begin, the remaining area of potentially viable habitat in Alberta appears to be confined within 68 sections of land (176.12 km²). Suitable habitat generally comprises far less than the full amount of each section potentially occupied, and so it is reasonable, and perhaps inflated, to assume that the total available potential habitat where lizards are known to be found is approximately half of that – or 88.06 km².

The minimum Alberta population was further estimated by an extrapolation of this logic. Since captures were only accomplished on approximately one-third of the searched sections, the population can only be verified as present on approximately one third of the known area, or 29.35 km². At a density of two lizards per hectare, or 200 per km², the resulting minimum population estimation is 5871 individuals.

To estimate a maximum population, it was assumed that searchers missed a proportion of lizards actually present. If it is assumed that only half of potential captures were made, a density of four lizards per hectare may be inferred. Further, if it is presumed they occupy all of the available suitable remaining habitat (88.06 km²), then the outcome could be as high as 35 224 individual animals in Alberta.

Upon re-evaluating the 2001 data to account for an SVL limit for mature adults of > 46 mm, as was suggested by Powell and Russell (1985a), the 2001 data may be interpreted somewhat differently than it was in James (2002). Given this parameter, the population structure of the 2001 captures was as follows: Adult males; n=9, Juvenile males; n = 27; Adult females; n = 48; Juvenile females; n = 1, Young-of-year; n = 40. In this instance, the proportion of adults was 45.6% of the total captures. By then applying this ratio to the maximum and minimum total populations, the total population of mature adults in Alberta may be estimated to fall into the range of 2677 – 16379 individuals.

Of the adult captures, adult males comprised only 15.8% and adult females made up 84.2%. If these proportions are assumed to be representative of the population at large, then an estimate of the range of effective population size may also be put forth. If effective population size is defined as:

where N_m and N_f are the number of breeding males and females (Smith and Smith 1998), as calculated above, then the effective population, in Alberta only, ranges from 1421 – 8716 adult individuals. Table 1 lists the range of population sizes by location in Alberta. All such calculations should be viewed as extremely tentative, owing to the many untested assumptions that have been incorporated.

Fluctuations and trends

Climate is thought to provide the primary constraint to the range of Greater Short-horned Lizards in Alberta (Powell and Russell 1991a; 1991b; 1996). Annual weather fluctuations may have pronounced effects on populations, especially if an extreme weather event occurs during sensitive periods, such as mating or parturition (Powell and Russell 1991b). Drastic weather events, suitably timed, could possibly eliminate recruitment in some seasons. Prolonged drought in the 1980s may have diminished arthropod populations, and led to the apparently reduced lizard population levels recorded by Powell and Russell (1992a).

Earlier estimates of population trends and fluctuation were extrapolations based upon the personal observations of G.L. Powell in combination with anecdotal evidence from local residents (Powell and Russell 1993a). Overall, population trends appear to be stable or downward in Alberta. The overall capture rate in 2001-2002 was two hours, twenty minutes per capture which is approximately similar to G.L. Powell’s previous experiences in the early 1990s (2-3 hours; pers. comm.), suggesting population levels are roughly stable. However, some degree of overall decline in populations may have occurred based upon the overall reduced success at confirming the persistence of sub-populations in 2001-2002. Although some of the failures to find lizards at previously occupied sites may simply reflect the difficulties in detecting them, certainly, some populations at Forty-Mile Coulee have been lost because their habitat has been lost. Failure to find historic populations along the Milk River is mysterious because there is still good habitat at these sites.

For the Saskatchewan population, no reliable evidence of population trends or fluctuation is available; however, the species has been suggested to be even less numerous than at Alberta locations (Powell et al. 1998). Populations are expected to be most stable in those habitats that receive the least disturbance, and are predicted to be declining in areas more affected by human disruption.

Rescue effect

If one of the isolated populations of Short-horned Lizards in Alberta were to become extirpated, the likelihood of a natural re-population of the previously occupied habitat is thought to be extremely low to nil. Even if connectivity between the main locations was completely intact, it is improbable that the population levels within occupied areas are adequate to act as sufficient sources. In addition, the estimated distance between the populations is likely prohibitive to natural dispersal. The closest two populations, the Manyberries Hills location and the eastern portion of the Milk River location, are estimated to be 15-20 km apart (ASRD 2004). The distance between the Manyberries Hills location and the Chin coulee population is approximately 48-50 km, and between the Chin coulee and nearest segment of the South-Saskatchewan river populations, it is approximately 35-38 km. This is probably applicable within populations as well; for example, the South Saskatchewan River sites are spread out along > 90 km of channel length. As well as being widely separated, some of the major drainages that populations are associated with are not directly connected, further reducing the probability of interaction between populations through any existing habitat corridors.

For the Alberta populations there appear to be very limited prospects for contiguous populations south of the border due to the considerable effects of cultivation along the Alberta – Montana border. However, the landscape south of the border in Saskatchewan appears primarily intact with much less cultivation. The nearest Montana record (Cooper et al. 2001) to those populations in Saskatchewan is approximately 52 km directly south of Grasslands’ East block records.