Reptiles in the North Rim of Black Canyon of the Gunnison National Park

Charles T. Hanifin, Kristopher B. Pedersen

Please cite this publication as:

Hanifin, C.T., and K.B. Pedersen. 2026. Reptiles in the North Rim of Black Canyon of the Gunnison National Park. Science Report NPS/SR—2026/487. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/2318308

Abstract

Reptile fauna of the intermountain west are historically understudied and yet represent important components of desert ecosystems. The Northern Colorado Plateau Network (NCPN) developed an inventory needs list in 2007 identifying amphibians and reptiles as high priority; this need was reaffirmed as a high priority in 2017 as a component of Inventories 2.0 scoping. Because NCPN parks are managed for multiple use (e.g., grazing as well as human visitation and recreation), lack of good data associated with herpetofauna could place local populations at risk. Key pieces of information such as species presence and distribution data are required for risk management decisions and to enhance management of these taxa. Black Canyon of the Gunnison (BLCA) was not included in earlier herpetofauna inventory efforts by the NCPN but was specifically identified as a priority for further reptile inventory assessment (Platenberg and Graham 2003). Absence of good herpetofauna species presence and distribution data has also been identified as potential data gaps that might hinder progress of future grazing allotment management planning at BLCA. Work reported here was supported by the Inventory & Monitoring group and provides critical data for BLCA. We used a combination of standardized herpetological inventory tools as well as quasi-randomized habitat-based sampling to assess reptile presence (Robertson et al. 2017) at BLCA. As recommended in previous herpetofauna inventories we used the presence of water features and other unique habitat elements to maximize the effectiveness of surveys. We identified the presence of four reptile species at target sites during a single field season in 2025. Species presence data from our study suggest that even common species of reptiles have a somewhat patchy distribution at BLCA.

A photo of a Greater Short-horned Lizard basking on a rock. The lizard is orange-brown in color with a round mid-section and a short tail, while the rock is dark gray with lighter patches.
Greater Short-horned Lizard (Phrynosoma hernandesi) in Black Canyon of the Gunnison National Park.

Utah State University / Kristopher Pedersen

Acknowledgments

This project was made possible through support from the National Park Service Inventory and Monitoring Division.

Introduction

Background

Reptiles and amphibians (herpetofauna) represent ecologically important fauna in terrestrial and aquatic habitats (Christoffel and Lepczyk 2012). However, these taxa are typically under-represented in wildlife studies, and their distribution and population status are poorly understood relative to groups such as large mammals or fish (Christoffel and Lepczyk 2012). Land managers now recognize the importance of assessing herpetofauna species as components of integrated management plans and there is increasing interest in generating species inventories and assessment of herpetofauna to prevent data gaps.

A Northern Colorado Plateau Network (NCPN) inventory needs list developed in 2007 identified amphibians and reptiles as high priorities and this need was reaffirmed in 2017 as a component of Inventories 2.0 scoping (E. Borgman, pers. comm.). Because parks located in the NCPN are managed for multiple use (e.g., grazing as well as human visitation and recreation) lack of good data associated with herpetofauna could place local populations at risk. Key pieces of information such as species presence and distribution data are required for risk management decisions and to ensure that loss of native herpetofauna species do not negatively impact ecosystem functioning. Current resource management at Black Canyon of the Gunnison National Park (BLCA) identified the need for improved data on reptiles in critical areas of the park as part of the development of this project with the NCPN. Absence of good herpetofauna species presence and distribution data in specific areas of the park were identified as potential data gaps that might present risks associated with grazing and use of riparian zones and water features at BLCA. Work completed under this agreement expands efforts to close these data gaps and provide BLCA and other NPS personnel with improved data and tools for herpetofauna assessment as well as meet specific management needs for individual management units.

BLCA is a member of the NCPN. As with much of eastern Utah and western Colorado, reptiles in BLCA, and in particular the North Rim, have been historically understudied. Surveys by the U.S. Geological Survey (1988), as well as National Park Service assessments and citizen science, provide an estimated species inventory for BLCA but these assessments are incomplete and may be inadequate for management of resources such as grazing allotments or assessing other human impacts on herpetofauna communities. As with Dinosaur National Monument (DINO), BLCA was not included in the 2003 Inventory of Amphibians and Reptiles in the NCPN and there has been no formal survey of reptiles present in the North Rim of the park that we are aware of.

This reptile survey addresses several management needs at BLCA regarding disturbance from livestock, restoration and invasive species, and human visitation. Climate change is an overarching concern that affects many of these management applications. Information on reptiles will contribute to upcoming decisions related to climate change such as identifying climate refugia, managing resilience and prioritizing management actions. Many reptile species are insectivorous and may be impacted by changes to insect availability or phenology associated with drought, habitat conversion to invasive plants, or climate change. Seeps, springs, and wetlands in desert environments may be hotspots of reptile diversity and may represent critical microhabitat for reptiles. Habitat changes associated with drought, habitat conversion to invasive plants, or climate change have the potential to negatively impact reptile populations and data reported here can be used to identify potential restoration areas for reptiles with an eye toward protection of climate refugia for wildlife like reptiles.

Specific Project Goals

The goal of this project was to determine the presence and distribution of reptiles in select grazing allotments in BLCA through a mixture of formal transect surveys coupled with ad hoc surveys of targeted sites (e.g., seeps and springs) as well as evening road surveys. Rigorous sampling strategies generated data that can help NPS personnel develop predictive models for presence/absence of some reptile taxa in the future. Initial data from this project provides updated species inventories for BLCA and includes taxonomic revisions as well as assessments of the presence of state or federal reptile species of concern.

Methods

Efforts associated with this project focused on quantifying species presence and distribution in specific, selected areas at BLCA rather than a complete inventory of reptiles of the park. Work under this agreement incorporated three phases: (1) literature reviews of herpetofauna records from eastern/central Utah and western Colorado and coordination with NPS personnel to finalize the sampling design and establish feasibility of access and timing to those sites, (2) field work and data collection, and (3) data organization and analysis, generation of final species, lists and distribution maps, as well as completion of this report. Initial literature reviews were used to identify changes to species taxonomy and/or nomenclature for NPS records and to generate a list of species predicted to be present in study sites.

Literature and Data Review

Investigation of National Park Service literature documented that BLCA was not formally surveyed as part of the 2001–2002 NCPN Herpetofauna Inventory (Platenberg and Graham 2003). We generated a list of predicted herpetofauna species inventory using the park master list, data from iNaturalist, GBIF, and IUCN (GBIF 2023; IUCN 2024; NPS 2023) and preexisting published resources (e.g., McGinnis and Stebbins 2018). We followed accepted best practices and consulted authoritative taxonomic references to ensure current taxonomy and nomenclature when generating predicted species lists and final inventories (Table 1). Key sources included the Society for the Study of Amphibians and Reptiles and the Center for North American Herpetology.

Table 1. Predicted or possible reptile species present in surveyed areas based on multiple sources.
Family Scientific Name Common Name
Colubridae Coluber taeniatus A
(Masticophis taeniatus)		 Striped Whipsnake
Lampropeltis gentilis A
(Lampropeltis triangulum)		 Utah Milk Snake
Opheodrys vernalis A
(Liochlorophis vernalis)		 Smooth Green Snake
Pituophis catenifer Gopher Snake
Thamnophis elegans Terrestrial Garter Snake
Crotaphytidae Crotaphytus collaris Eastern Collared Lizard
Phrynosomatidae Sceloporus graciosus Sagebrush Lizard
Sceloporus tristichus A
(Sceloporus undulatus)		 Plateau Fence Lizard
Urosaurus ornatus Ornate Tree Lizard
Phrynosoma hernandesi A
(Phrynosoma douglasii)		 Greater Short-horned Lizard
Teiidae Aspidoscelis velox A
(Cnemidophorus velox)		 Plateau Striped Whiptail

A Accepted current nomenclature with older name listed in parentheses below. We use standard binomial taxonomic assignments and related common names rather than subspecies throughout this report.

From this list we developed appropriate surveys and tools to assess those species and worked with NPS personnel to identify appropriate sites for assessments. We prioritized inventory assessments and habitat sampling over developing estimates of population size or density. Similarly, we focused on surveying multiple habitat types rather than exhaustive surveys of a single habitat. The exception to this strategy focused on habitats and sampling strategies that maximized the likelihood of identifying rare or cryptic species.

Sample Design

Study Area

Black Canyon of the Gunnison National Park is a 12,444-ha park in western Colorado that was established as a national monument in 1933 and redesignated as a park in 1999. The scope of this project was limited to the Grizzly Gulch grazing allotment and nearby water features (Figure 1).

alt = Figure 1. Map of the Grizzly Gulch and Dead Horse Trail survey locations and reptile occurrences in Black Canyon of the Gunnison. There are two areas that were surveyed; one is in the north-east part of the park, straddling a road, and is highlighted in orange. In this area, the transects that were surveyed are shown as black lines. The other area was on the eastern side of the park and follows a winding trail. In both areas, instances where reptiles occurred are denoted with red circles.
Figure 1. Black Canyon of the Gunnison survey locations.

Utah State University / Kristopher Pedersen

Areas of interest were provided by NPS staff. Specific sampling approaches were created in consultation with the park based on need and management questions. Two sample design approaches were used in this study: (1) transect surveys that cover Grizzly Gulch grazing allotment, and (2) ad hoc surveys at water features on the North Rim.

Transect surveys allowed coverage of a wide range of microhabitats in survey regions and provided rigorous sub-sampling of large areas. Ad hoc surveys provided surveys at times and/or locations outside of our standard survey protocols. Broadly, efforts consisted of 64% effort associated with transects and 36% effort associated with ad hoc surveys. Although outside the scope of this project, transect sampling provides data for future analyses to compare and contrast different areas of the park and could be used to generate predictive models of reptile distribution within BLCA. In addition to ad hoc and transect surveys, we conducted road running surveys. These surveys were unsuccessful and are not recommended for future reptile surveys on the North Rim.

Transect Selection and Design

We completed nine transect lines in the Grizzly Gulch grazing allotment. Transects were 1,000 meters in length. Starting points were selected using balanced acceptance sampling (BAS) through the MBHDesign library in R (Foster 2021). BAS uses quasi-random numbers to design spatially balanced surveys (Robertson et al. 2017). Due to the rugged nature of the terrain, transect direction was selected manually by looking at maps and ground truthing locations upon arrival. Transects were made up of between two and seven surveyors spread approximately 15–30 meters apart. Surveyors followed transect lines with minor weaving to increase coverage between people, particularly to check locations where rock outcrops or vegetation increased the chance of reptile presence. Data was collected on all identified reptiles.

Ad Hoc Surveys

Ad hoc surveys consisted of water features of interest and target of opportunity surveys. Ad hoc surveys were less structured than transect surveys. Surveys of water features consisted of looking for animals while walking to the water feature location. This included a survey along the Dead Horse Trail. Target of opportunity surveys occurred when a rare species was observed outside of normal survey methods.

In addition to these ad hoc methods, we conducted evening road running surveys. Road running occurred along the North Rim Road (Figure 1). Road running consisted of driving slowly along park roads while watching the road for reptiles. We conducted approximately six road running trips for a total of approximately 25 person hours. No reptiles were observed during these surveys, therefore road running is not included in ad hoc results.

Data Collection

Each unique survey was collected as an event as per National Park Service (NPS) guidelines. Events included start times, end times, and a GPS starting point. Transects also included a start and end location. Animals were recorded as an occurrence, as per NPS guidelines. Each occurrence was linked to the event in which it occurred. Occurrences also included time of occurrence and GPS location.

Animals were captured and photographed when possible. We also collected temperature and biometric data (i.e., mass, snout to vent length, sex) when possible. Where possible, we collected up to two tail clips per species, one male and one female. Tail clips were stored in 90% EtOH and placed into a −80°C freezer as soon as possible (usually within a few days). Tail clips and photographs were collected as a non-lethal alternative to full specimen collection. This allowed us to collect vouchers and DNA for future study without euthanizing and removing animals from the park. Photographs and tail clips are stored at the Monte L. Bean Life Science Museum at Brigham Young University (BYU) along with relevant geospatial data.

Survey area polygons were created in Google Earth using a combination of tracking data from our GPS, landmarks, and GPS data from occurrences and events. Survey area polygons were created as soon as possible after finishing surveys (typically within 24 hours).

Results

Surveys were conducted between June 7 and June 29, 2025. We conducted a total of nine transect surveys accounting for 68.41 person hours and covering an area of approximately 215.80 hectares (ha) (Table 2 and Table 3). We conducted a total of three ad hoc surveys accounting for 38.95 person hours. Two planned survey trips were cancelled due to the South Rim Fire which caused parkwide closures starting July 10, 2025.

Table 2. Sample area and number of animals by location for Black Canyon of the Gunnison National Park. Total number of species in parentheses is the number of expected species.
Location Hectares
Sampled		 Number of
Animals		 Animals per
Hectare		 Number of
Species
Grizzly Ridge 200.25 21 0.1 3
Dead Horse Trail 15.55 4 0.2 2
Total 215.80 25 0.1 4 (11)

Table 3. Survey effort and number of animals by survey type for Black Canyon of the Gunnison National Park. Ad hoc survey time does not include time spent during road running surveys.
Type Number of
Surveys		 Time
Spent		 Percent of Effort Number of
Animals		 Animals per
Hour Surveyed
Transect 9 68.41 64 19 0.2
Ad Hoc 3 38.95 36 6 0.1
Total 11 107.39 100 25 0.2

Grizzly Ridge Grazing Allotment

Surveys of the Grizzly Ridge Grazing Allotment occurred between June 7 and June 29, 2025 (Figure 2). We conducted a total of nine transects and two ad hoc surveys covering approximately 198.81 ha. We surveyed three stock ponds within, and one stock pond located near, the grazing allotment. We found a total of 22 animals representing three of the eleven species expected in the park. No reptiles were found near the stock ponds. All four stock ponds, however, were dry when surveyed. Animals were found between 2,387.4 m and 2,524.8 m elevation. Species found consisted of Greater Short-horned Lizard (Phrynosoma hernandesi), Sagebrush Lizard (Sceloporus graciosus), and Smooth Green Snake (Opheodrys vernalis). Of the 21 animals observed within the allotment, 19 were Greater Short-horned Lizards.

[Figure 2. Map showing the survey area, transect lines, and reptile occurrence locations in the Grizzly Gulch grazing allotment. The area surveyed is in the north-east part of the park, straddling a road, and is highlighted in orange. The nine transects that were surveyed are shown as black lines, and all instances where reptiles occurred are denoted with red circles. Some transects had multiple reptile occurrences, while others had one occurrence or none.
Figure 2. Map of the Grizzly Gulch Grazing Allotment survey.

Utah State University / Kristopher Pedersen

Dead Horse Trail

Dead Horse Trail was surveyed on June 8, 2025 (Figure 3). The trail was surveyed as part of our effort to visit water features along the North Rim. Our specific target consisted of four stock ponds along an unnamed branch trail which travels north towards Poison Spring Gulch. In addition to the four stock ponds, we surveyed an unmarked water feature along Dead Horse Trail. We did not observe any reptiles along Dead Horse Trail or around the unmarked water feature. Along the branch trail we observed two Terrestrial Garter Snakes (Thamnophis elegans). We also observed a Smooth Green Snake at the stock pond labeled Poison Springs 4 and a Terrestrial Garter Snake at Poison Springs 2. Water was present in the unmarked stock pond. The Poison Springs stock ponds did not have water, but the presence of mud suggested they had dried recently. Thamnophis elegans identified at this site could also be identified to the subspecies level (Thamnophis elegans vagrans; Wandering Garter Snake) but are reported here and in project related data as T. elegans to maintain data consistency.

Figure 3. Map showing the survey area, trail, and reptile occurrence location along Dead Horse Trail. The entire trail was surveyed, and reptile occurrences are denoted with red circles. The three reptile occurrences all occurred fairly far from the trailhead, where the trail meets a road.
Figure 3. Map of the Dead Horse Trail survey location.

Utah State University / Kristopher Pedersen

Voucher Specimens

As a part of this survey, we collected voucher photos and tail clips from a subset of animals (Table 4). Photographs were collected from a total of ten animals and tail clips from a total of four animals. These are stored at the Bean Life Science Museum at BYU.

Table 4. List of species including voucher collection and type.
Species Common Name Number of Animals Tail Clips Photographs
Opheodrys vernalis Smooth Green Snake 2 1 1
Phrynosoma hernandesi Greater Short-horned Lizard 19 2 8
Sceloporus graciosus Sagebrush Lizard 1 0 0
Thamnophis elegans Terrestrial Garter Snake 2 1 1

Summary and Conclusions

This reptile inventory and assessment provides data to address management related to grazing and other management concerns at BLCA. This report also provides baseline data for future work on North Rim reptiles. We documented four of eleven possible reptile species on our BLCA master list (Tables 1 and 4). Data reported here indicate that 36% of the species expected in the park are present in the North Rim study area. The majority of animals observed, 19 of 25, were Greater Short-horned Lizards (Phrynosoma hernandesi). We also observed a single Sagebrush Lizard (Sceloporus graciosus), two Smooth Green Snakes (Opheodrys vernalis), and three Terrestrial Garter Snakes (Thamnophis elegans).

Species Richness and Density

While our study methods do not provide a true estimate of actual population densities, observed reptiles per hectare at BLCA were low compared to other parks in our study (0.2 reptiles/ha compared to between 0.8 and 1.09 reptiles/ha for Dinosaur National Monument (DINO; Hanifin and Pedersen 2026b), Capitol Reef National Park (CARE; Hanifin and Pedersen 2026a), and Colorado National Monument (COLM; Hanifin and Pedersen, in press). We also observed limited species richness within our study area (four species). As a result, our study suggests that reptile population densities of even common species may be limited in BLCA. This could, in turn, suggest that reptile populations may be at more risk from human activities.

However, the low number of reptiles per hectare and low species richness are likely the result of multiple factors. Specifically, it is likely that abiotic conditions in the North Rim are significant factors in reducing reptile abundance, since BLCA is a mostly high-elevation park and this is especially true of the North Rim. All work done within the park occurred above 2,400 meters. As with other high-elevation sites in the intermountain west, the growing season for reptiles is highly limited because of temperatures, snow fall, and snow coverage. Because reptiles are ectotherms, low environmental temperatures resulting from elevation can constrain foraging, reproduction, and predator avoidance (Zamora-Camacho et al. 2013). At sites such as the BLCA North Rim, lizards are likely less able to maintain preferred body temperatures which will likely reduce local fitness and impact population densities (Diaz 1997). We have seen similar patterns at two high elevation locations within Dinosaur National Monument: Wild Mountain and Round Top, which also had low animals per hectare (0.23 and 0.07, respectively; Hanifin and Pedersen 2026b).

Other elements of habitat quality may contribute to the apparent low observed species richness in our North Rim study sites. Reptile fitness is closely associated with high quality thermal habitat (Huey and Kingsolver 1989). Quality of thermal habitat is closely associated with the presence of perch sites with temperatures that match lizard thermal preferences, and the presence of thermal heterogeneity among perch sites (Hertz et al. 1993; Sears et al. 2016). In desert habitats these types of microhabitats are typically associated with rocky outcrops. North Rim locations surveyed in this study were mostly shrubland, with only limited and widely dispersed rock outcrops. This apparent lack of thermally preferable perches in shrubland habitats may limit lizard success within these locations. Although our study did not allow us to directly assess this hypothesis, comparisons of our study area with nearby locations in the North Rim of BLCA provide some insight. We generated estimates of species richness from North Rim sites near the canyon that were similar in elevation to both the Grizzly Gulch and Dead Horse Trail sites using iNaturalist and Global Biodiversity Information Facility (GBIF). This analysis identified three lizard species which are not present at our study locations (GBIF 2023), including the Plateau Striped Whiptail (Aspidoscelis velox), the Plateau Fence Lizards (Sceloporus tristichus), and the Ornate Tree Lizard (Urosaurus ornatus), as well as the lizard species we found at survey sites. These results suggest that, at least on the North Rim, variation in thermal microhabitat quality and availability might impact species richness and/or density. It is important to note, however, that the differences in data collection between these two locations make a direct comparison tenuous, and that future studies which include more habitat types may provide a better understanding of reptile communities within BLCA.

Cattle Grazing

The impact of grazing on reptile populations is unclear. Studies suggest that grazing can increase abundance for some species and have negative impacts for others (Jofré and Reading 2012; Val et al. 2019). One of the key ways in which disturbance affects reptile populations is through changes in microclimate and habitat openness (Rachmansah 2025). Two lines of evidence suggest that grazing at BLCA may not be problematic for the reptiles that are present the Grizzly Gulch site. First, two studies (Germano et al. 2012 and Read and Cunningham 2010) show that the effect of grazing on lizards in arid habitats is either neutral or positive. While the causes are not well understood, a study of Desert Horned Lizard (Phrynosoma platyrhinos) found changes in vegetation structure as a likely mechanism for positive effects, at least in that species (Newbold and MacMahon 2008). Reptile response to minor disturbance is largely affected by physiological traits and biogeographic history (Rachmansah 2025), with a positive association for species which evolved in warmer, more arid habitats (Ferreira et al. 2016; Santos et al. 2019). Because lizards appear to be the dominant herpetofauna at BLCA sites we expect grazing might be less significant. Second, because we expect the general species richness in BLCA sites to be low due to inherent biotic and abiotic conditions, we would expect grazing impacts on herpetofauna to be minimal. There is little to no data on the effect of grazing on snakes because of their general rarity and cryptic behavior. We suspect the impacts are minimal but are unable to draw conclusions or make recommendations based on our data. However, we suspect that managed grazing is probably not a problem for most reptile species at BLCA. One potential concern is that invasive species, such as Cheatgrass (Bromus tectorum), appear to generate negative impacts for some reptiles (Hall et al. 2009; Newbold 2005). In the western US, two main factors affect community resistance to Cheatgrass invasion: competition for nitrogen by native bunchgrasses (Booth et al. 2003), and resistance to Cheatgrass seed germination by biological soil crusts (Reisner et al. 2013). Cattle grazing can increase gaps between plants, as well as damage biological soil crusts, reducing resistance to Cheatgrass invasion (Rayburn et al 2014, Reisner et al. 2013). Although our study did not include control areas to compare distribution and abundance of reptiles, as long as grazing is maintained at levels which limit ecosystem damage, and thus maintain ecosystem resistance to invasion, we would expect minimum impact to these reptile populations.

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Appendix

Figures 4 and 5 show voucher specimens that were collected during the reptile inventory and assessment in Black Canyon of the Gunnison National Park.

Figure 4. Phrynosoma hernandesi photo voucher specimen. In this photo, a person is holding a Greater Short-horned Lizard next to a ruler. We can see that the reptile is about 10 cm in length and has a round midsection and a short tail. It is orangish-gray and brown in color and has bumpy, spiky skin on its back.
Figure 4. Phrynosoma hernandesi.

Utah State University / Hanifin Lab

Figure 5. Opheodrys vernalis photo voucher specimen. In this photo, a person is holding a Smooth Green Snake next to a ruler. We can see that it is much more than 10 cm in length, but it has a thin body less than two centimeters in diameter. It has a small head, a smooth body, and is a brilliant green color. The snake’s tail is wrapped around the person’s hand.
Figure 5. Opheodrys vernalis.

Utah State University / Hanifin Lab

Author Information

Charles T. Hanifin ORCID ID Logo https://orcid.org/0000-0003-4930-7698

Kristopher B. Pedersen ORCID ID Logo https://orcid.org/0009-0001-6128-2123

Utah State University
Department of Biology
5305 Old Main Hill
Logan, UT 84322-0300

About the National Park Service Science Report Series

The National Park Service Science Report Series disseminates information, analysis, and results of scientific studies and related topics concerning resources and lands managed by the National Park Service. The series supports the advancement of science, informed decisions, and the achievement of the National Park Service mission.

All manuscripts in the series receive the appropriate level of peer review to ensure that the information is scientifically credible and technically accurate.

Views, statements, findings, conclusions, recommendations, and data in this report do not necessarily reflect views and policies of the National Park Service, U.S. Department of the Interior. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the U.S. Government.

The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific and other information about those resources; and honors its special responsibilities to American Indians, Alaska Natives, and affiliated Island Communities.

This report is available in digital format from the National Park Service DataStore and the Natural Resource Publications Management website. If you have difficulty accessing information in this publication, particularly if using assistive technology, please email irma@nps.gov.