The Texas tortoise (Gopherus berlandieri) population in Palo Alto Battlefield National Historical Park is one of the park’s key ecological indicators and is the focus of long-term monitoring. Although the park land is protected, its tortoise population is at risk due to isolation and rapid landscape change. Additionally, some of the tortoise’s preferred scrubland habitat is under consideration by the park for replacement with historical, Spartina-dominated grasslands. Population monitoring began in 2008 in several core units of higher-elevation scrublands, which are isolated from each other within a grassland matrix and have slightly different vegetation assemblages. All contain prickly pear, however, which is a preferred food source. A key preliminary goal of the monitoring was to determine the relative suitability of the different study units and whether the tortoises that inhabited them had different behaviors or physical attributes. To address these questions, the study units were characterized using lidar data and other sources. To describe tortoise subpopulations, data were collected twice a year during standardized visual ground searches for tortoises in each unit. Each encountered tortoise was weighed, measured for carapace dimensions, had its GPS location recorded and was uniquely marked.
Results/Conclusions
We found that although the units were superficially similar in vegetation structure, there were key differences in vegetation height, composition, relative elevation and the unit’s isolation from other, similar habitat. The characteristics of tortoise subpopulations also differed among units. Specifically, we found significant differences in mean home ranges of tortoises (based on kernel density estimates), their body size and sex ratios. Tortoises from smaller, isolated units had smaller home ranges, confirming the importance of higher-elevation scrublands for food and shelter. Additionally, tortoises in one of the smaller units weighed less for their body dimensions, potentially indicating poorer health. Although habitat quality may be among the drivers of cross-unit differences in tortoise behavior and condition, additional data are needed. To this end, we are now conducting a radio-tracking study to determine tortoise locations and habitat uses throughout the year.