Using Road Surveys and N-Mixture Models to Estimate the Abundance of a Cryptic Lizard Species
The cryptic behavior and appearance of some reptile species lead to difficulty in conducting effective surveys to estimate abundance. As an example, Horned Lizards (Phrynosoma spp.) often have a pattern of camouflage on the dorsum and they sometimes become motionless upon the approach of a human observer. However, they also venture onto roads to thermoregulate. When basking on a road surface, they are much easier to visually detect; hence, road-cruising surveys are an effective technique for surveying. We designed and implemented a survey protocol for a population of Texas Horned Lizards (Phrynosoma cornutum) at the Chaparral Wildlife Management Area (CWMA) in south Texas, USA. The protocol involved repeatedly driving a continuous 64-km survey route while visually scanning for lizards. Over the course of 17 survey days in summer 2015, we obtained 167 observations of lizards. To estimate lizard density, we used N-mixture models on datasets in which the route was divided into either 39, 77, or 117 segments (survey units) with lengths of 1,651, 836, and 550 m, respectively. Density estimates ranged from one to six lizards per 10 ha, depending on how the route was subdivided into segments and assumptions about the amount of roadside area surveyed. The density estimates were extrapolated to an estimated population size of 1,000–2,800 Texas Horned Lizards at the CWMA. Because this is a species of conservation concern, populations should be monitored within protected areas and perhaps elsewhere in landscapes where road surveying is possible.ABSTRACT
Photograph of Phrynosoma mcallii, demonstrating the crypsis of Horned Lizards. Photo by K. V. Young and reprinted with permission of Ecological Society of America.
Historic range of Texas Horned Lizards (Phrynosoma cornutum) and estimated eastern edge (blue line) of current range. The range has been contracting westward for several decades. Map for historic range derived from Hammerson et al. (2007). Current eastern edge of range estimated from iNaturalist data from 2008 to present. Study site (CWMA) is shown in south Texas, USA. Inset map shows boundaries of the CWMA, survey route (blue lines), and locations of lizard observations (open circles).
Secondary road on the CWMA and typical surrounding vegetation. The roads were used for surveying Texas Horned Lizards (Phrynosoma cornutum) that readily bask on the pavement. Road is ∼4 m in width. Photograph taken on 23 June 2015.
Distribution of λ values from the top 10 sets of N-mixture models applied to surveys for Texas Horned Lizards (Phrynosoma cornutum) at the CWMA, south Texas, USA. Results are shown for each of three datasets having either 39, 77, or 117 survey units. Horizontal bars denote maximum and minimum values, filled rectangles show the 25–75% quartiles, and “X” indicates the mean λ value of the 10 models. For a given model and dataset, λ is the mean abundance among the survey units.
Estimated density of Texas Horned Lizards (Phrynosoma cornutum) at the CWMA, south Texas, USA. Densities are based upon λ values from the top 10 sets of N-mixture models for each of three datasets having either 39, 77, or 117 survey units. Estimates are shown for presumed survey distances of 30, 50, and 100 m from the road. Horizontal bars denote maximum and minimum values, filled rectangles show the 25–75% quartiles, and “X” indicates the mean density estimate from the 10 models.
Range of estimated population sizes for Texas Horned Lizards (Phrynosoma cornutum) at the CWMA in south Texas, USA. Assumed survey distances represent 30 m (short), 50 m (intermediate), and 100 m (long) on either side of the road (see text for details). At each survey distance, estimates of population size are based on mean densities over the three datasets (N = 39, 77, and 117). Black shading indicates population sizes as derived from densities ranging between the mean minimum to the mean maximum density, where minimum and maximum values are obtained from the λ values of the top 10 model sets. Gray shading indicates population sizes as derived from densities ranging over the mean 95% CIs as based on the model-specific CIs for the λ values of the models in the top 10 set. Dashed line shows population sizes that would obtain if densities of lizards (one every 550, 836, and 1,651 m) were based on λ = 1 for all sampling units in all datasets.
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