Master's Theses



Degree Name

Master of Science (MS)


Texas horned lizard (Phrynosoma cornutum) populations have been in steady decline over the past twenty years due to habitat loss, pesticide use, the pet trade, and invasion of the red imported fire ant (Solenopsis invicta). At Aransas National Wildlife Refuge the P. cornutum population has declined, except for on Matagorda Island. A prescribed burn regime was initiated to maintain the coastal prairie habitat. This closed environment was unique in its composition of arid adapted and wetland species. There was minimal human disturbance, and a single road bisects the length of the island. The island was invaded by S. invicta around 1993. I conducted an extensive survey of the island herpetofauna to determine the distribution of P. cornutum, as well as to quantify the influence of the burn regime on P. cornutum. I also investigated the effect of road-side habitat on both P. cornutum and habitat structure, as well as the microhabitat associations of P. cornutum. Herpetofaunal populations were investigated in six distinct habitat types on Matagorda Island by using drift-fence arrays with funnel traps, random 50-m walks, and systematic road cruising. In 2009, individual P. cornutum were captured along the road and individuals were tracked by using a fluorescent powder method. I also sampled habitat structure and ant mounds along a 60-m transect perpendicular to Middle Road. Habitat among burn treatments exhibited significant differences in percent ground cover, standing stem density, and soil compaction in 2008 and 2009. Eleven herpetofaunal species were captured in the drift fence arrays. The most abundant species were 55 six-lined racerunners (Aspidoscelis sexlineata) and 16 slender glass lizards (Ophisaurus attenuatus); however, only three P. cornutum were captured with drift-fence arrays, of which I observed high mortality rates by S. invicta. I did not observe any P. cornutum on the random 50-m searches. I captured 41 individuals in 2008 and 128 individuals in 2009 along the road, and estimated the population size at 138 individuals by using the Schnabel index. Two distinct distributions in the population were isolated by a recent burn in both 2008 and 2009. A majority of P. cornutum activity occurs within 5 m of the roadside, which accounts for only 2% of the grassland habitat. Ant species also were not randomly distributed on the island. Solenopsis invicta was observed throughout the island, along with cone ants (Dorymyrmex sp.). Acrobat ants (Crematogaster sp.) were concentrated on the north and south portions of the island. No Pogonomyrmex sp. were observed. Phrynosoma cornutum were associated positively with older burn treatments and the road-side habitat, and were negatively associated with recent burns. Habitat variation was not the only factor that influenced distribution. Solenopsis invicta occurred in high abundance in Burn 1 treatments. The distribution of Crematogaster sp. was congruent with the distribution of P. cornutum. In areas with the most heterogeneous habitat, I observed high diversity of ant genera. Phrynosoma cornutum were observed in alarmingly low abundance and warrant monitoring and augmented management strategies.


Dr. William Stark

Date of Award

Spring 2010

Document Type



© The Author(s)


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