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

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Limiting Factors and Threats

Pygmy Short-horned Lizard populations tend to be localized and are therefore susceptible to local extinction if the area is developed or some other lethal threat occurs. The restricted mobility of horned lizards also curbs their ability to cross unsuitable habitat, such as developments, and canals can create impassable barriers or ongoing sources of mortality.

The first large-scale threat that may have significantly reduced Pygmy Short-horned Lizard populations was large cattle drives that went through the Okanagan Valley from the 1860s up to the first World War (R. Manuel pers. comm.). During this period, tens of thousands of cattle were herded from Oregon to the Cariboo to sustain miners working in the gold rush (Cox 2004). The destructive impact from trampling would have been concentrated in the Okanagan due to the narrowness of the valley. It is possible that these numerous herds of livestock decimated local populations that lived on the benchlands of the valley. Normal grazing practices appear to have no effect or a positive effect on Pygmy Short-horned Lizard habitat (Reynolds 1979), although overgrazing reduces horned lizard abundance by affecting the structural richness of the plant community (Fair and Henke 1998) and may also reduce harvester ant populations, the lizards’ main food source (Rogers et al. 1972).

Horned lizards require an uncluttered environment to navigate through and to thermoregulate. Invasive plants, such as cheatgrass (Bromus tectorum), are tenacious and widespread, effectively choking interstitial spaces between native vegetation. This may make much of the habitat unusable by restricting movement. Some weeds, such as puncture vine (Tribulus terrestris) and longspine sandbur (Cenchrus longispinus) could cause physical injuries.

Unlike most other temperate-zone reptiles, Pygmy Short-horned Lizards seek only shallow subterranean refuge during winter. In years of low snowfall, extreme or prolonged cold could cause widespread mortality.

Predation may be increased through enhanced natural predator populations (e.g., ravens, crows, kestrels) and the introduction of domestic predators (e.g., dogs and cats).

Road and off-road traffic has been implicated as a source of mortality for horned lizards (Nicolai and Lovich 2000, Flat-tailed Horned Lizard Interagency Coordinating Committee 2003). Two of the anecdotal observations made in Canada were of road-killed individuals on highways (Sarell unpubl. data).

Fire suppression has changed much of the dry landscape that could be used by Pygmy Short-horned Lizards. Lack of fires has allowed trees to encroach into grasslands and fuels to accumulate. The Texas Horned Lizard has been found to have better survivorship in areas that have been burned (Moeller et al. 2005).