Patch Occupancy and Population Density of the Crevice Spiny Lizard (Sceloporus poinsettii) in the Central Mineral Region of Texas
Date
2016-05
Authors
Jenkerson, Jeffrey T.
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Abstract
Occupancy is a state variable that can be used as a response variable to assess the suitability of habitats (Larson 2014). In this project, I compared crevice spiny lizard (Sceloporus poinsettii) distributions of occurrence to variables relevant to a local rock habitat (patch) and a landscape (extent) scale. In addition, I investigated the influence of habitat size on crevice spiny lizard density. When detection of an animal is imperfect, occupancy modeling becomes critical, providing a detection estimate that will serve as a correction factor for a less biased estimation of true occupancy (Royle et al. 2005).
Herpetofaunal species within a landscape are strongly associated with the amount and availability of suitable habitat as well as numerous characteristics of the habitat. The goals of my research were to (1) determine the relative influences of local habitat and extent landscape variables on crevice spiny lizard occupancy of monadnock features present within the Llano Uplift area of the Central Mineral Region of Texas and to (2) determine population density using mark-recapture based population estimates. I estimated site occupancy of crevice spiny lizards using presence-absence surveys conducted from June to September 2015. I recorded survey-specific covariates, which are known or are suspected to influence the probability of detection. In addition, I measured habitat variables and calculated indices which are known or suspected to influence lizard occupancy at local rock habitat scale (habitat size, percent cover vegetation, ground cover complexity, number of refuges, and refuge quality) and at extent landscape scale (percent cover vegetation, ground cover complexity, aspect, roughness, and burn interval). For the estimation of lizard density, I conducted a series of mark-recapture surveys at 3 sites that differed in size but were similar with respect to other habitat characteristics. To estimate lizard density, I captured and marked 46 adult crevice spiny lizards using ®Floytag T-bar anchor tags. Average lizard density across sites was 1.41/100 m2 (SE = 0.023, n = 46).
For the assessment of occupancy, 350 lizards were detected on 69 surveys (n = 19 sites) over one field season. I used single-season occupancy models in the program PRESENCE to estimate effects of environmental and habitat variables on probability of detection (p) and occupancy (ψ). Multimodel information-theoretic approach suggests that at a local rock scale, crevice spiny lizard occupancy may be more closely related to refuge quality. At an extent landscape scale, most parsimonious models suggest that geographic aspect is most influential to occupancy. These results indicate that patterns of occurrence may be tied closely to characteristics most immediately affecting the ability to thermo-regulate and find cover. In addition, extent landscape may also play a role in microhabitat suitability and thus quantification of suitable habitat without consideration of landscape context is unlikely to be sufficient for the assessment of species distribution. Lizard density decreased with increasing site size, indicating that habitat size may not be positively correlated with habitat suitability for the crevice spiny lizard.
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Keywords
Sceloporus, Occupancy
Citation
Jenkerson, J. T. (2016). <i>Patch occupancy and population density of the crevice spiny lizard (Sceloporus poinsettii) in the central mineral region of Texas</i> (Unpublished thesis). Texas State University, San Marcos, Texas.