Amistad National Recreation Area (NRA), located on the U.S. - Mexico border in southwest Texas, is in a transition zone between three major plant communities: the Chihuahuan Desert, Edwards Plateau, and Tamaulipan Shrubland. Three major rivers (Pecos River, Devils River, and the Rio Grande) and the Amistad Reservoir occur in Amistad NRA.
These diverse terrestrial and aquatic systems contribute to Amistad NRA’s high level of biodiversity. Searches for reptiles and amphibians were primarily focused along the eastern shore of the reservoir. Surveys were also conducted in other parts of the park including along the rivers. Areas where salt cedar or tamarisk had invaded were difficult to survey. The inventory took place from May to September 2003 and from June to September 2004—134 person-days in total. During the May 2003 survey, Amistad NRA was unusually dry. From June to September 2003, rainfall was about 30% above average. In 2004, spring rainfall was 85% above average and about 20% above average from June to September.
James Borgmeyer
Methods
Several search approaches were implemented to find reptiles and amphibians: foot surveys, incidental observations, pitfall traps, road cruising, and turtle traps. Different approaches were used each year (2003 and 2004) to maximize the likelihood of finding different species. For each reptile or amphibian observed, location, time, weather, and species were recorded.
Foot surveys were conducted at times when reptiles and amphibians were most likely to be active (morning around sunrise and evening around sunset). Searches were attempted in all habitat types in Amistad NRA, but also focused on areas such as canyons and riparian areas that were likely to support a high diversity of species or rare species. Most of the foot surveys were conducted off-trail. While on foot, searches included looking under rocks and logs, and mirrors and flashlights were used to illuminate crevices.
Incidental observations were recorded while the researchers were not conducting formal searches (e.g., on days off or while driving to a search area). Pitfall traps were used to capture small reptiles and amphibians running or crawling across the ground. In 2003, three sets of pitfall traps were checked by Amistad NRA staff and volunteers daily. The traps were checked 33 times over the course of the survey and were left in place for staff to use in future monitoring efforts. Road cruising surveys were conducted night to find amphibians and nocturnal snakes and lizards.
Turtle traps were used to document species of turtles living in the reservoir. Reservoir levels fluctuate quickly and the traps had to be moved often. Although the traps were checked daily, the water level rose rapidly enough in 2003 that four turtles drowned before the traps were checked. In total, seven turtles (three species) were caught in the traps.
Voucher specimens were collected for nearly each species in Amistad NRA to provide proof that the species actually occurred there during the inventory and to help confirm it was correctly identified in the field. These specimens provide important data about reproduction, diet, health, and morphology. DNA collected from voucher specimens can be used to examine population questions and genetic relationships within and between species. The voucher specimens from Amistad NRA and four other parks are on permanent loan to and stored at the Museum of Southwestern Biology, University of New Mexico, Albuquerque, and are freely available to researchers and managers.
Results
In total, 3,874 reptiles and amphibians were found at Amistad NRA during this inventory, representing 45 species: nine frog and toad, 15 lizard, 17 snake, and four turtle species. One lizard, the southwestern fence lizard (Sceloporus cowlesi), described in 2002, is nearly identical to the eastern fence lizard (Sceloporus undulatus). Distinguishing these species is currently based on current range distribution due to lack of published genetic work. Although 14-person days of foot survey data were lost due to a computer crash, the researchers were able to reconstruct the snake data from memory, though many lizard and amphibian observations were lost. No data documenting new species to Amistad NRA were lost.
The majority of observations were made during foot surveys: 3,168 animals of 42 species. Incidental observations accounted for 532 animals of 26 species and 92 animals of nine species were caught in pitfall traps. Four road cruising surveys produced 76 animals of five species. A total of 42 voucher specimens comprising 40 species were collected.
Four species found in Amistad are listed as state threatened: Texas horned lizard (Phrynosoma cornutum), Texas indigo snake (Drymarchon melanurus erebennus), Trans-Pecos black-headed snake (Tantilla cucullata), and Berlandier’s tortoise (Gopherus berlandieri). The observation of Berlandier’s tortoise is based on finding two small plates from a shell.
One non-native species was found during the inventory, the Mediterranean house gecko (Hemidactylus turcicus). The species occurs widely in urban areas in the southern United States. It expanded its range into Amistad NRA after 1977. Although this gecko is typically found on buildings near lights where it is attracted by insects, in Amistad NRA it is found on cliff faces away from artificial light. Though it is invading natural areas, it does not pose an obvious threat to other native species because the native Texas banded gecko (Coleonyx brevis) typically feeds on the ground, not walls. However, the species may be competing in subtler ways.
Discussion
In order to determine what percentage of the actual total herpetofauna community was documented in this inventory, the results were compared to lists of species likely to occur in Amistad NRA based on previous sightings, museum records, species range maps, and habitat preferences. It is estimated that there are nine snake species and one lizard species that are likely to occur in the park that were not observed, meaning this inventory may have documented 82% of the reptile and amphibian species. However, the majority of Amistad NRA consists of a narrow strip of land around a reservoir and rivers, and it is likely that some of these “likely to occur” species only visit the park and do not inhabit it permanently.
Most of Amistad NRA is altered by the reservoir and consists of heavily disturbed areas which may not be attractive habitat to some reptiles and amphibians. This and other factors combined with the lack of previous intensive surveys in the park gives little confidence in the total estimate of species that occur in Amistad NRA.
This inventory can be used as baseline data for future monitoring. Long-term monitoring and/or repeated inventories may help detect changes within reptile and amphibian communities caused by climate change, drought, invasion by non-native species, wildland fire, or other factors. Reptile and amphibian populations can change dramatically between surveys in a given year due to the timing and amounts of precipitation. Future monitoring or inventories should survey each area multiple times in a season.
See Chapter 7 (Literature Cited and Project Contacts) for a link to the full report including a list of species found during this inventory.
Part of a series of articles titled Chihuahuan Desert Network Reptile and Amphibian Inventories.
Last updated: August 8, 2023