Using Road Surveys and N-Mixture Models to Estimate the Abundance of a Cryptic Lizard Species

Abstract

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.