Long-term Monitoring of Texas Tortoises on National Park Lands: Use and Suitability of Available Habitats

Carlson, Jane; Bracewell, Jeff; Speyrer, Fabiane; Finney, William; Granger, Whitney; Segura, Martha

Background/Questions/Methods 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 seven core units of scrublands on low relief ridges, or lomas, which are partially isolated from each other within a grassland matrix and have slightly different elevations and 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 loma study units and whether the tortoises that inhabited them had different behaviors or physical attributes. To address these questions, the seven loma units were characterized using lidar data and aerial imagery going back to the 1930’s. To describe tortoise subpopulations, data were collected twice a year during standardized visual ground searches for tortoises in each of the seven loma units. Each encountered tortoise was weighed, measured for carapace dimensions, had its GPS location recorded and was uniquely marked. Home ranges were estimated in two units (#1 and #3) for 18 tortoises total over a single year of study, with one encounter per month. Results/Conclusions We found that although the seven lomas were superficially similar, there were key differences in the relative elevation, land use history, and likely as a consequence, in the total area of shrubland versus grassland on the loma. The characteristics of tortoise subpopulations also differed among loma study units. Specifically, the most tortoises were found on the lomas with little to no historical agricultural use, and tortoises weighed more on the two least-disturbed lomas (#2 and #3), relative to more heavily and recently-disturbed lomas (loma #6 for females and lomas #1 and #7 for males). Even so, tortoises on the most intensively disturbed loma (#4) were not less often seen or in worse condition than those on lomas with intermediate disturbance levels. The characteristics of tortoise home ranges also differed between the two lomas where radio-tracking occurred, based on the vegetation heights encompassed within kernel density estimates of home ranges. Specifically, tortoises from the smaller, more grassland-ecotone loma (#1) had home ranges that extended further from the loma into the grassland, relative to tortoises from the higher elevation and more shrub-dominated loma (#3). Together, these results suggest that management for tortoises should consider shrublands and adjacent grasslands, because tortoises do not restrict their activities to shrublands when those habitats are relatively limiting. Land use history also matters, but tortoises nevertheless occupy sites recovering from a range of disturbance intensities. Although habitat quality may be among the drivers of cross-unit differences in tortoise behavior and condition, additional data are needed. Because monitoring is ongoing (including population size estimates), we can assess whether apparent differences among lomas have additional consequences that will be revealed over the long-term.

Long-term Monitoring of Texas Tortoises on National Park Lands: Use and Suitability of Available Habitats

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