Series: Chihuahuan Desert Network Reptile and Amphibian Inventories

Big Bend National Park Reptile and Amphibian Inventory

Big Bend National Park (NP), located along the U.S. - Mexico border in Texas, is the largest protected area of Chihuahuan Desert in the United States, covering 1,253 square miles. The park ranges in elevation from 1,800 feet along the Rio Grande to 7,800 feet in the Chisos Mountains.

Camouflaged, curled up trans-Pecos ratsnake
Trans-Pecos ratsnake (Bogertophis subocularis subocularis) at Big Bend National Park.

Dave Prival

This great range in elevation contributes to variations in moisture and temperature, contributing to a high level of diversity in the park’s plant and animal communities. The reptiles and amphibians of many of the most diverse areas of the park were already well known, so this inventory focused primarily on three mountain ranges that were not previously surveyed: the Sierra Quemada south of the Chisos Mountains, the Sierra del Caballo Muerto on the eastern edge of the park, and the Mesa de Anguila in the southwestern corner of the park. The ranges are exposed and rocky with little or no water and were logistically challenging to survey. Backpacking trips to survey the area took place from May to August 2003 and from June to September 2004, resulting in four trips to the Sierra Quemada, two to the Sierra del Caballo Muerto (Deadhorse Mountains), and four to the Mesa de Anguila. Several shorter surveys into each of these ranges and other areas of the park occurred as time permitted. A total of 131 person-days were spent surveying.

Methods

Several search approaches were implemented to find reptiles and amphibians: foot surveys, incidental observations, and road cruising. 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. Since an existing multi-year study of amphibians at Big Bend NP was in progress at the time of this inventory (Dayton 2005), the researchers focused their efforts on finding reptiles and only recorded amphibians when they happened to see them, but also did not record every amphibian they saw, particularly if more than 10 were observed in the same place.

Foot surveys were conducted at times when reptiles and amphibians were most likely to be active (morning around sunrise and evening around sunset). 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). Road cruising surveys were conducted at night to find amphibians and nocturnal snakes and lizards.

Voucher specimens were collected for nearly each species found in the park, and for each species in each of the three mountain ranges to document the distribution of species in different mountain ranges. Voucher specimens provide proof that the species actually occurred in the park during the inventory and help to confirm identification. 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. For this inventory, the reptile voucher specimens 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. The amphibian voucher specimens were loaned to Texas A&M University to supplement the collection efforts from the multi-year amphibian study in progress at the time.

Results

In total, 2,259 reptiles and amphibians were found at Big Bend NP during this inventory, representing 59 species: nine frogs and toads, 21 lizards, 26 snakes, and three turtles. The majority of observations were recorded during foot surveys: 1,008 animals of 48 species. Incidental observations accounted for 817 animals of 49 species. Thirty-six road cruising surveys resulted in 434 animals of 25 species. A total of 96 voucher specimens comprising 51 species were collected.

Four species found in Big Bend NP are listed as state threatened: Texas horned lizard (Phrynosoma cornutum), reticulate banded gecko (Coleonyx reticulatus), Trans-Pecos black-headed snake (Tantilla cucullata), and Texas lyre snake (Trimorphodon biscutatus vilkinsonii).

Two non-native species were found during this inventory: the Mediterranean house gecko (Hemidactylus turcicus) and American bullfrog (Rana catesbeiana). The Mediterranean house gecko occurs widely in urban areas in the southern United States. This gecko is typically found on buildings near lights where it is attracted by insects. In Big Bend NP, it was found in developed areas: on the wall of Barker House near the Rio Grande Village, the Rio Grande Village, and at Panther Junction. The native Texas banded gecko (Coleonyx brevis) typically feeds on the ground, not walls. However, these species may be competing in subtler ways.

American bullfrogs were heard calling from the Mexican side of Santa Elena and were also found in the park at Rio Grande Village. This species is native to the eastern United States and was introduced to the west in the early 1900s. The first reports of bullfrogs in Big Bend NP are from the 1980s (R. Skiles, pers. comm.). American bullfrogs compete with and prey on native species, particularly leopard frogs and gartersnakes.

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 the park based on its 2002 reptile and amphibian checklist. While the emphasis of the study was on reptiles, the researchers attempted to document as many species as they could. It is estimated that there are probably 10 species that are likely to occur in Big Bend NP that were not observed (one toad, one salamander, one lizard, four snakes, and three turtles), meaning this inventory may have documented 86% of the reptile and amphibian species in the park. It is unknown what percentage of species were documented in the three mountain ranges because the ranges were not previously surveyed.

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.