The Effects of Winter Burning and Grazing on Resources and Survival of Texas Horned Lizards in a Thornscrub Ecosystem

The ecological effects of land-use practices on reptiles, especially endangered or threatened species, are of conservation and scientific interest. We describe the effects of rotational livestock grazing and prescribed winter burning on resources and survival of the Texas horned lizard (Phrynosoma cornutum) during the summers of 1998 to 2001 in southern Texas, USA. We evaluated survival rates of Texas horned lizards (n = 111) on 6 study sites encompassing 5 different burning and grazing treatments. We also measured indices of cover (i.e., vegetation) and food abundance (i.e., harvester ants [Pogonomyrmex rugosus]). We telemetered and relocated adult lizards daily. We divided the study into 2 seasons, spring (15 Apr-30 Jun) and summer (1 Jul-15 Aug), corresponding to the relative activity of horned lizards. Winter burning provided an increase in food resources and led to increased survival of Texas horned lizards in the second growing season after fire, but grazing-induced changes in vegetation cover reduced survival, likely by increasing lizard vulnerability. Fire and grazing reduced litter and increased bare ground and forb cover but did not affect woody vegetation. Ant activity was greater in burned sites and varied with grazing level, season, and year. Summer survival functions of horned lizards varied by burning treatment, with higher survival observed on burned sites in the second year after burning. Survival rates were ordered from highest in ungrazed sites to lowest in heavily grazed sites. We recognize the limitations of our work resulting from a lack of spatial replication of treatments. However, our mensurative study provides fertile ground for future hypothesis testing regarding the effects of land management on shrubland and grassland reptiles. We propose that future studies focus on the population consequences of variation in burn frequency, burn timing, and grazing intensity.