Flat tailed horned lizard in handDesert sceneryFlat tailed horned lizardDesert indigenous plant
The effects of off-highway vehicles on the flat-tailed horned lizard and improvements in lizard density estimation.

Principal Investigator: Paul Doherty

Graduate Student: Tyler Grant

Project Start Date: 09/03

Expected Completion Date: 12/05

Funding Agency: USGS, RWO 73

Introduction:
The flat-tailed horned lizard (Phrynosoma mcallii; FTHL) has the most restricted range of any species of horned lizard (genus Phrynosoma, 14 species). Much of the FTHL’s habitat has been converted to agriculture, golf courses, and urban areas or covered by the Salton Sea. Much of the remaining habitat is being impacted by geothermal development, immigration foot traffic, border patrols, power lines, roads and off-highway vehicle traffic (OHV). The FTHL has been proposed for listing under the Endangered Species Act. Methods for estimating lizard density are needed to address this listing proposal. Information on the effect of OHV on the lizard is also needed. This project focuses on these two aspects. Estimation methods will be developed and applied to FTHL. A manipulative experiment focusing on the effects of OHV on the lizard will also be conducted.

Objectives: 1) Evaluate the effectiveness of distance sampling and mark-recapture in estimating flat-tailed horned lizard density. To date estimation of flat-tailed horned lizards has been based on indices. In a pilot study we demonstrated that estimation based upon mark-recapture was a viable alternative. We will expand the number of mark-recapture plots as well as incorporate a distance sampling procedure to further refine our methodology.

2) Improve the estimation of lizard density by incorporating model averaging and mixture models into the estimation procedures. To date we have focused on the use of Program Capture and the basic closed-capture models in Program Mark. We will investigate the use of mixture models to account for heterogeneity in our abundance estimates. We will also utilize a model averaging approach to obtain better abundance estimates. We will attempt to combine information from our distance sampling data and mark-recapture data using a model averaging approach.

3) Evaluate the effects of off-highway vehicles on the density of flat-tailed horned lizards. We will evaluate the effects of off-highway vehicles in two ways. First we will conduct a correlative study by correlating the amount of off-highway vehicle tracks per plot with lizard abundance. Secondly, we will conduct a manipulative experiment in which we estimate the number of lizards in treatment and control plots, apply an off-highway vehicle treatment, and then assess any differences in lizard abundance due to the treatment.

Progress: Tyler Grant finished his coursework and passed his Comprehensive Exams last spring. He completed field work in February, 2005. He is now writing his thesis.