Flat-tailed horned lizards (Phrynosoma mcallii): Population size estimation, effects of off-highway vehicles, and natural history

Publication Type:
 Thesis
Year of Publication:
 2005
Authors:
 Tyler J. Grant
Publication/Journal:
 Department of Fishery and Wildlife Biology
Publisher:
 Colorado State University
Keywords:
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Abstract:

Considerable conservation concern and controversy has developed over the status and management of the flat-tailed horned lizard (Phrynosoma mcallii), a rare, difficult-to-detect species with a limited distribution. Monitoring the abundance of this species, as well as investigating factors that may be affecting demographic parameters, has proven problematic. A current conservation concern is the increasing use of off-highway vehicles on public lands and the potential effects of these vehicles on the lizard. I addressed these concerns in my research.

I incorporated detection probability into abundance monitoring of the flat-tailed horned lizard using closed mark-recapture and distance sampling methods. These are improvements over previous methods which relied upon unvalidated indices. Adult lizard density estimates from mark-recapture analyses averaged 1.97/ha (SE = 0.27). Estimates of capture probability ranged from 0.03 to 0.65 in different areas and years. Population estimates were 25,514 (95% CI 14,444 – 38,970) and 42,619 (95% CI 23,161 – 67,639) for the Yuha Basin Management Area in 2002 and the East Mesa Management Area in 2003, respectively. Estimates of 10,849 (95% CI 5,022 – 23,486) and 73,017 (95% CI 10,738 – 163,635) were obtained for West Mesa Management Area in 2003 and iii the Yuha Basin Management Area in 2004, respectively, but these estimates should be viewed with caution as they were based on sparse data. I tested for relationships between habitat characteristics of mark-recapture plots and lizard abundance using a variance components analysis. Number of ant colonies and percent sand coverage explained the most variance in lizard abundance. Perennial density and percent vehicle track coverage explained little variance in lizard abundance. I found closed mark-recapture methods were compromised from a lack of geographic closure. I addressed lack of geographical closure with an adjustment factor based on the amount of time lizards spent off of plots. I also suggest additional remedies for the future, such as enlarging or fencing plots.

One of the primary assumptions of distance sampling, namely that detection probability of animals on the line [g(0)] is equal to one, was not supported in this species [two estimates by different methods were g(0) = 0.65 and g(0) = 0.45]. After adjusting for this low detection on the line, adult density estimates derived from distance analyses for three East Mesa MA plots and the Yuha Basin MA were 1.55/ha (95% CI 0.64 – 3.76) and 0.41/ha (95% CI 0.22 – 0.7), respectively. Future distance sampling efforts must make greater efforts to estimate detection probability on the line.

A manipulative study was performed on lizards during the hibernation season, which coincides with the off-highway vehicle recreation season. Lizards were equipped with radio-telemeters and followed into hibernation. A 100 m x 100 m plot was then randomly selected around each lizard. Thirty-six lizards were divided into three treatment groups: high impact, low impact and control. Treatments consisted of timed riding of off-highway vehicles. In all treatment groups survival was 100%, despite iv hibernation of lizards at very shallow depths. Indirect effects of off-highway vehicles deserve increased attention.

Throughout my studies I was opportunistically able to describe some flat-tailed horned lizard natural history of interest in this little-known species. I document the first known occurrence of ‘rain-harvesting’ in this species as well as describe the variation in hibernation in this species and informing some hypotheses of interest on the effect of rainfall on growth and reproduction.

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