Texas Tortoise Monitoring at Palo Alto Battlefield

The Texas tortoise is one of the park's most notable inhabitants.

NPS photo

Cause for Concern

The Texas tortoise (Gopherus berlandieri) is the smallest of four North American tortoise species. Its distribution in the United States is limited to the lower Texas plain from Brownsville to south of San Antonio and westerly to the east side of the Big Bend area.

Historically, this tortoise was more abundant and associated with drier scrub and thorn-brush habitats throughout its range. By the mid-20th century, populations were in decline in many areas (Rose and Judd). The combination of increasing habitat-loss and documented declines in tortoise range and density, led to increased interest and concern for the species.

Consequently, the Texas Parks and Wildlife Department initially listed the Texas tortoise as a Texas state species-of-concern. Then, in 1982, it raised the tortoise’s listing to threatened species status (Rose and Judd).

Vital Signs

Doctors use thermometers, X-rays, and MRIs to check the health of their patients. At Palo Alto, researchers rely on tortoises to monitor the natural health of the park. Every six months since 2008, the park's natural resource specialist, Gulf Coast Network staff, other partners, and volunteers have combed the brush and prairies of Palo Alto. Their goal is to locate, mark, and track Texas tortoises living in the park.

The Texas tortoise is very sensitive to environmental changes. This makes it a good indicator of the ecological well-being of the park. For example, evidence of increased tortoise mortality can indicate habitat decline long before other hardier species are affected.

On the other hand, a thriving or growing tortoise population, especially the presence of young tortoises, suggests the habitat remains relatively stable. By tracking the health and numbers of tortoises at the park, Palo Alto hopes to keep tabs on the "vital signs" of the battlefield and surrounding lands.

A researcher uses a pair of calipers to measure a Texas tortoise.

NPS photo

Monitoring Texas Tortoises at Palo Alto

The Gulf Coast Network began development of the tortoise monitoring project in 2008 in collaboration with Drs. K. Buhlmann and T. Tuberville of the University of Georgia–Savannah Ecological Laboratory (SREL). Sampling to date has revealed tortoises are widely distributed within the best available habitat areas across the park.

Over 200 unique individuals have been identified and marked. The findings of a fundamentally equal sex ratio in adults, evidence of ongoing reproduction, and frequent observation of hatched egg-shells, suggests a viable population at the park. Trial data also suggest that, in good habitats, the tortoise population at Palo Alto may exceed 8–10 individuals per hectare. That is a density comparable to that reported for other Texas tortoise populations (Kazmaier et al. 2001, Hellgren et al. 2000).

The project is part of a service-wide program to keep a close eye on parks with significant natural resources. The goal of this inventory and monitoring program is to help identify significant environmental issues in National Parks and address them before they reach a catastrophic stage.

To date, the tortoise program has provided positive feedback about the park's condition. Their searches have found a viable and healthy population on park grounds.

Tortoise Mortality

Animals, including feral pigs that have taken up residence on the battlefield, are a real threat to Texas tortoises. The busy roads bordering the park also present a deadly obstacle for tortoises leaving and entering the park. The park has put up tortoise-proof fencing along portions of the boundary to reduce this hazard.

It doesn't help that the tortoises seem to prefer the dense chaparral, a habitat declining quickly in the developing lands surrounding the battlefield.