Dryophytes wrightorum

Duellman et al. (2016) restricted Hyla to Eurasia and North Africa and referred the North American and east Asian sister taxon of this group to Dryophytes. This taxonomy was accepted by Amphibian Species of the World and followed here. It is not followed by the Society for the Study of Amphibians And Reptiles (SSAR) with Crother (2017) stating, "acceptance of this taxonomy within the community is not clear at this point."

Hyla wrightorum was previously regarded as a synonym of Hyla eximia, but Duellman (2001) recognized H. wrightorum as a distinct species. This treatment was adopted by Crother et al. (2003) and Crother (2008). Molecular data (allozymes and mtDNA), as well as advertisement calls, support continued recognition of Hyla eximia (central-southern Mexico) and Hyla wrightorum (disjunct populations in the Sierra Madre Occidental of northern Mexico, the Huachuca Mountains and adjacent Canelo Hills of southeastern Arizona, and the mountains of central Arizona and western New Mexico) as distinct species (Gergus et al. 2004). MtDNA data of Gergus et al. (2004) suggest that populations on the Mogollon Rim, Huachuca Mountains/Canelo Hills, and Sonora have been evolving independently of one another. However, the low level of genetic differentiation among these populations indicates this isolation likely occurred relatively recently (i.e., late Pleistocene) (Gergus et al. 2004).

Faivovich et al. (2005) redelimited this monophyletic taxon to include only North American and Eurasian species. Hua et al. (2009) discussed relationships within the group. Fouquette and Dubois (2014) recognized a suite of subgenera based on genetic and morphological evidence, but pending a more thorough evidentiary review, Crother (2017) hesitates to employ this taxonomy. Zhang et al. (2019) reported on phylogenetic placement.

From the mountains of Arizona, New Mexico, and northern Mexico, into the Sierra Madre Occidental of Sonora and Chihuahua, Mexico. The Mexican distribution is clouded by lack of surveys at higher elevations in intervening mountain ranges, and by taxonomic uncertainties. It appears to be relatively stable overall, but better information on trends are needed. Threats, number of occurrences and population size is uncertain in Mexico.

Three geographically separated populations make up Hyla wrightorum, including from north to south Mogollon Rim, Huachuca-Canelo and Mexico populations. The Mogollon Rim population is found from the mountains of central Arizona, from the Williams area west of Flagstaff, east along the Mogollon Rim, southeastward to the western part of central New Mexico where it is fairly common in Catron and Sierra counties. The Huachuca-Canelo population is an isolated population in the Huachuca Mountains and Canelo Hills in southeast Arizona, that extends across the border to Ranchero Los Fresnos in Sonora, Mexico. The Mexico population ranges further south in the Sierra Madre Occidental of Sonora and Chihuahua, Mexico. It may extend to the area just north of Mexico City, though this is uncertain due to difficulties differentiating between this species and the similar species H. eximia within this contact zone. Populations of the Mogollon population were recently found away from the Gila River basin in New Mexico, at El Malpais National Monument, Cibola County (Monatesti et al., 2005, Herpetol. Rev. 36:74-75), and at Marquez Wildlife Area, McKinley County (Giermakowski et al., 2010 Herptol. Rev. 41(3):375). Elevational range is from 910-2,900 meters (3,000-9,500 feet) (Stebbins 2003); most populations occur above 1,830 meters (6,000 feet) in Arizona (Arizona Game and Fish Department 2005).

In a recent personal communication with Dr. Tom Jones (Arizona Game and Fish Department 2017), "little is known about the distribution in Sonora; there are big geographic gaps that are largely "no information" rather than "they are not there." There is a population, albeit of unknown size or status, at Rancho Los Fresnos, which is just south of the border (below the Huachucas). And as he recalls there are no data from there to the Sierra Madre. Jim Rorabaugh (pers. comm. 2017), indicated the species certainly could occur in some of the Sonora sky islands between the Huachucas/Los Fresnos and the next nearest locality near Nacori Chico. However, Tom Van Devender has led biological expeditions to a lot of those mountain ranges since 2009 and so far no Hyla wrightorum have been found. It is a frog that could be pretty localized and its fairly seasonal. Still, the more work that is done, the more it appears they are scarce at best, and may be absent in those intervening mountain ranges.

Recently the U.S. Fish and Wildlife Service (USFWS) has received information, along with many new location detections in the Huachuca Mountains and Canelo Hills, which indicates that the Arizona treefrog is not only more numerous in this population, but is much more widespread than was known. There are now approximately more than 30 known localities in the Huachuca-Canelo population area. The Mogollon Rim population has the majority of the occurrences for this treefrogs global range, occurring in Arizona and New Mexico. In Arizona, there are approximately 165 occurrences (Arizona Heritage Data Management System 2017), while in New Mexico 25 occurrences represent the population there (Natural Heritage New Mexico 2017). Several areas of the western portion of the Arizona range (near Williams and Flagstaff) have not been revisited in many years, and need to be resurveyed to determine if these subpopulations are still extant. There are several isolated populations in central Sonora and Chihuahua, Mexico, based on ca. 21 occurrences from a total of 45 observations and collections (25 between 1997 and 2013). These general areas are from along MEX Hwy 16 in the vicinity of Yecora, near Madera (west and north of), and near Creel (northwest, and southwest to southeast of). Historically collected as far south as MEX Hwy 24 northeast of El Vergel, Chihuahua, though whether they still are extant here is unknown. In addition, Hyla wrightorum was collected near El Salto west of Durango, however this is far removed from the nearest known populations of Hyla wrightorum (ca. 465 km southeast of Cerocuhui, or ca. 315 km south of the historic MEX Hwy 24 location), and is in the intergrade zone with Hyla eximia. Surveys and genetic work would need to be done to firmly establish the existence of frogs from here and genetically as to what species of treefrog they belong to.

The degree to which this population is threatened is not well known. In the Huachuca Mountains of Arizona, Gergus (1999) found small numbers of adults at each breeding location. He felt that these populations may be vulnerable to extirpation. Small populations such as those in the Huachuca Mountains and Canelo Hills have the potential for low levels of genetic heterozygosity and increased inbreeding depression, and small populations are also more susceptible to local extinction from unpredictable changes in the environment (Gergus et al. 2004). However, metapopulation dynamics of this species are poorly known.

Predation by non-native predators, such as crayfishes, centrarchids, and bullfrogs, or native predators such as gartersnakes, tiger salamanders and giant waterbugs, are a potential threat but are not presently known to be causing declines. However, these likely limit the types of wetlands in which the frog can successfully breed and maintain populations (USFWS 2008). On the Mogollon Rim, Sredl and Collins (1992, in USFWS 2016) found that Arizona tiger salamanders (Ambystoma mavortium nebulosum) were significant predators of Arizona treefrogs. In southeast Arizona, the endangered Sonoran tiger salamander (Ambystoma mavortium stebbinsi) occurred in Scotia Canyon historically, but has not been observed there since 1995. Bullfrogs had occupied perennial pools and ponds in Scotia Canyon, but could not breed in the ephemeral pond used by Arizona treefrogs to breed; bullfrog tadpoles need two years to develop. However, bullfrogs likely preyed upon breeding adults, and in the fall, juvenile treefrogs at this ephemeral pond. Bullfrogs have recently been eliminated from Scotia Canyon, and work is underway to remove them from a five-mile radius of the canyon. This project should continue to benefit the Arizona treefrog by reducing predation. (USFWS 2016).

Past livestock grazing practices may have had a much greater impact on the habitat and the species itself; current effects are not unknown. Restricting grazing during breeding season in streams, wet meadows, ephemeral ponds is important to protect against impacts on adults, larvae and eggs. Excessive livestock grazing can remove shoreline or aquatic vegetation through browsing or trampling. Livestock grazing currently occurs in and near populations of the Arizona treefrog on the Coronado National Forest, but is excluded from Fort Huachuca and Rancho Los Fresnos. However, where grazing occurs, we have no detailed site-specific information to determine the intensity and frequency of this threat on the species (USFWS 2016).

Off Highway Vehicle (OHV) use whether for recreation or hunting is common on many public lands in the United States, however, direct impacts from OHV use to this species is not documented but remains a concern.

Long-term drought and increasing temperatures (can lead to drying of habitat), along with related insect outbreaks and fire (exacerbated by drought) may eliminate small local populations. Although the effects of wildfires on populations of the Arizona treefrogs have not been studied, populations are at risk of post-fire flooding, erosion, scouring, and sedimentation impact that have and are expected to continue to destroy or modify habitat, at least in montane habitats. Reaser and Blaustein (2005) hypothesized that amphibian populations most at risk due to climate change are those that: 1) are already at the upper limit of their physiological tolerance to temperature or dryness or both; 2) depend on small, ephemeral wetlands; or 3) are bound by barriers to dispersal. The Huachuca-Canelo population of the Arizona treefrog breeds in small, ephemeral wetlands located in relatively mesic, relict mountain woodlands and valley cienegas. The only likely barriers to treefrog dispersal are arid environments, but if increasingly arid and warm conditions persist or worsen, relictual mountain top moist forest and cienegas may decline or disappear leaving no place to which the frogs can disperse or establish new populations. (USFWS 2016).

In a pers. comm. with Rorabaugh (2017), he indicated that predation by bullfrogs and fishes at Los Fresnos may have reduced populations or distribution there. It is generally a high elevation species, and in the mountains of Sonora there are threats from climate change and logging. Perhaps heavy grazing in some areas, too.