Determinants of Habitat Selection by Desert Horned Lizards (Phrynosoma platyrhinos): The Importance of Abiotic Factors Associated with Vegetation Structure
The degree to which microhabitat elements such as shade from shrubs are used by lizards and the relative importance of such factors compared to others (e.g., prey availability) remains poorly understood. We examined habitat associations of Desert Horned Lizards (Phrynosoma platyrhinos) to evaluate the determinants of lizard occurrence using surveys along a shrub-steppe bajada in northwestern Utah. We then examined microhabitat preferences of lizards maintained in an experimental arena to explore further the potentially influential factors explaining habitat choice. Greater shrub cover, open understory, and greater cover by cryptobiotic soil crusts were the best predictors of lizard occurrence. While the role of soil crusts is unclear, these other factors may benefit lizards by providing open space for unhindered locomotion and opportunities for temperature regulation through sun/shade shuttling and burrowing. In contrast ants, a primary food source, did not appear to be a significant determinant of P. platyrhinos occurrence. Within the experimental arena, lizards preferred high-shade microhabitats during the day (82% of day selections) and demonstrated strong preference for microhabitats with soil mounds overall (85% of selections). Soil mounds, which form naturally beneath shrubs, tended to warm more quickly in the morning when lizards basked to elevate body temperature, remained cooler in the early evening when high temperatures persisted, and were used as midday and overnight burrow sites. Findings from these studies highlight the important role of abiotic factors associated with vegetation as strong determinants of habitat selection by P. platyrhinos.Abstract
Photographs illustrating model shrubs and their arrangement within the arena used in the microhabitat selection study. (A) Example of a model shrub (low shade with no soil mound) with a temperature data logger positioned adjacent to the shrub “stem.” (B) One row of 4-model arrays with a single array highlighted by the white dotted lines. (C) Whole-plot photograph showing the drift fence used to enclose the arena and, in the foreground, vegetation characteristic of the study plot before shrub removal. For scale, note the person in the far left corner.
Activity of Desert Horned Lizards as a function of (A) time of day and (B) estimated air temperature for all lizard locations (N = 147) and microhabitat selection of Desert Horned Lizards as a function of (C) time of day and (D) estimated air temperature for lizards that selected model-shrub microhabitats (N = 41). Numbers at the top of bars indicate number of lizards per time or temperature interval. For panels C and D, numbers in parentheses within bars indicate number of lizards buried per microhabitat type. Microhabitat abbreviations are lsnm, low shade with no soil mound; lsm, low shade with soil mound; hsnm, high shade with no soil mound; hsm, high shade with soil mound.
(A) Maximum and (B) minimum air temperatures (mean ± SE) associated with microhabitats beneath four model shrubs, a natural shrub, and on the bare substrate. Measurements were recorded at the soil surface at 15-min intervals over a 4-day period using temperature data loggers. Microhabitat abbreviations are lsnm, low shade with no soil mound; lsm, low shade with soil mound; hsnm, high shade with no soil mound; hsm, high shade with soil mound. Microhabitats with the same letter are not significantly different based on analysis of variance.
Average air temperatures (mean ± SE) associated with microhabitats beneath four model shrubs and on the bare substrate for (A, B) morning, 0700–1000 h, and (C, D) evening, 1700–2045 h. Microhabitat abbreviations are lsnm, low shade with no soil mound; lsm, low shade with soil mound; hsnm, high shade with no soil mound; hsm, high shade with soil mound. Asterisks indicate significant differences in temperature between mound and no soil mound microhabitats within a shade cover category (low or high) during a specific time interval. Air temperature on the bare substrate (dotted lines) was included for visual comparison only.
Abundance (mean ± SE) of (A–E) five ant species commonly found in Phrynosoma platyrhinos diet (see Newbold and MacMahon, 2009, Fig. 2b) and (F) P. platyrhinos scats summarized by soil type along the elevational gradient. Soil map unit numbers correspond to the upper bajada (Soil 92), mid-elevation slopes (Soils 78 and 79), and the playa at the lowest portion of the bajada (Soil 74). (A) Pogonomyrmex salinus. (B) Formica obtusopilosa. (C) Crematogaster mormonum. (D) Forelius pruinosus. (E) Camponotus vicinus and Camponotus hyatti combined.
Photograph at sunrise of a partially-buried adult Desert Horned Lizard beneath a shadscale (Atriplex confertifolia) shrub. Note the remnant of the collapsed burrow, which is supported structurally by surface soil crusts.
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