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

Abstract

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.

Variation in vital‐rate sensitivity between populations of Texas horned lizards

“…Work on WR3 occurred from 2003 to 2011 (Endriss et al 2007;Wolf et al 2013), whereas study on CWMA occurred from 1998 to 2005 (Hellgren et al 2010). We captured lizards during April-August through intensive visual searching, road cruising, and fortuitous encounters, and recorded basic morphometric information for each lizard, including snout-vent length (SVL), total length (TL), mass, and sex.…”

“…We implanted lizards [5.0 g with a passive integrated transponder (PIT) tag (0.5 g), or clipped a unique combination of toes for smaller lizards. Lizards had a single toe clipped as a secondary mark (Hellgren et al 2010). We attached a 0.95-1.95 g radio transmitter (BD-2, Holohil Systems Ltd., Carp, Ontario, Canada or SOPR-2038, Wildlife Materials Inc., Murphysboro, IL, USA) to each lizard on WR3 if the transmitter was \10 % of the lizard's body mass.…”

“…We attached a 0.95-1.95 g radio transmitter (BD-2, Holohil Systems Ltd., Carp, Ontario, Canada or SOPR-2038, Wildlife Materials Inc., Murphysboro, IL, USA) to each lizard on WR3 if the transmitter was \10 % of the lizard's body mass. On CWMA, we attached a transmitter-backpack bundle totaling 3 g (\8 % of lizard body mass) to adult lizards (Hellgren et al 2010). The difference in transmitter mass resulted from larger lizards on CWMA (R. T. Kazmaier, unpublished data).…”

“…The southern site, Chaparral Wildlife Management Area (hereafter CWMA; Dimmit and La Salle counties, Texas, 28°19 0 40 00 N, 99°24 0 39 00 W), was a 6,150-ha area managed by the Texas Parks and Wildlife Department (Hellgren et al 2010). Mean annual temperature at nearby Catarina, TX during 1971-2000 was 22.1°C, with mean January and July temperatures of 11.9 and 30.5°C, respectively.…”

“…Mean annual precipitation was 51.8 cm (National Climatic Data Center 2002). The site, which was predominantly honeymesquite woodlands or parklands, experiences managed grazing and prescribed burns (Flanders et al 2006;Hellgren et al 2010). A detailed site description is available in Flanders et al (2006).…”