Anaxyrus microscaphus

The genus Anaxyrus was split from Bufo by Frost et al. (2006). However, taxonomy within the genus Bufo remains controversial and many references still use the long-established Bufo.

This species formerly was regarded as a subspecies of Bufo (now Anaxyrus) woodhousii by some authors. Gergus (1998) examined allozyme variation in the microscaphus complex and found discrete differences among the three subspecies (microscaphus ,californicus, and mexicanus), though the morphological differences are minor. Formerly included A. mexicanus and A. californicus as subspecies, but Gergus concluded that under the phylogenetic species concept they should be recognized as species.

See Sullivan and Lamb (1988), Malmos et al. (2001) and Schwaner and Sullivan (2009) for information on hybridization with Anaxyrus woodhousii in central Arizona.

This species is endemic to the southwestern U.S. in southwestern Utah, southern Nevada, Arizona, and western New Mexico. The population has declined in area of occupancy and abundance, although overall distribution has changed little. Damming riverine, alterations to isolated wetland habitats, and increasing frequency and severity of droughts continues to threaten this species.

This species occurs in the southwestern United States in scattered locations in southwestern Utah, southern Nevada, Arizona, and western New Mexico (Price and Sullivan 1988, Sullivan 1993, Gergus 1998, Stebbins 2003, Brennan and Holycross 2006, Ryan et al. 2014). It is found from the Colorado and Virgin river basins in southern Nevada and southwestern Utah southeast through the lower mountains of Arizona mostly below the Mogollon Rim into the Mogollon Plateau of west-central New Mexico (Nigro and Rorabaugh 2023). Using Global Biodiversity Information Facility (GBIF) (2025) records from 2005-2025, range extent is estimated to be 208,600 km² (RARECAT 2025).

It is presumed extirpated from the small historical distribution in California (Clark, Jr. 2011).

Applying a 10 km separation distance to GBIF (2025) records from 2005-2025, 175 occurrences are estimated (RARECAT 2025).

Ryan et al. (2017) mapped 28 occupied sites in New Mexico. Price and Sullivan (1988) mapped about 45 collection sites in Arizona, Utah, and Nevada. In Arizona, at least 55 populations were documented from 1980-1995 (Nigro and Rorabaugh 2023).

Drought and lack of persistent wetlands is the primary threat to this species (Ryan et al. 2014, Oyler-McCance et al. 2024). As smaller isolated wetlands dry up and local extirpations occur, this species has shifted to primarily occupying riverine habitat (Driver et al. 2023). This species is suffering localized declines where riverine habitats have been dammed or otherwise converted to slow-moving or still water habitats (e.g. reservoirs, cattle ponds, golf course ponds) and symaptric A. woodhousii has out-competed A. microscaphus (Sullivan and Lamb 1988, Sullivan 1993, Schwaner and Sullivan 2005, Schwaner and Sullivan 2009, Wooten et al. 2019, Nigro and Rorabaugh 2023). In New Mexico, where A. woodhousii is not living sympatrically with this species, this species does occupy these human-modified wetlands (Ryan et al. 2014, Ryan et al. 2017a).

Hybridization between A. woodhousii and A. microcephalus has occurred in Arizona (Sullivan and Lamb 1988, Sullivan 1993), Nevada (Schwaner and Sullivan 2005), and Utah (Schwaner and Sullivan 2009), but apparently not in New Mexico (Ryan et al. 2014, Ryan et al. 2017a). Hybridization between these species occurs after riverine habitats are converted to reservoirs by damming in areas where both species occur, and A. woodhousii subsequently moves into these slower-moving or still water environments (Sullivan and Lamb 1988, Sullivan 1993, Ryan et al. 2014, Wooten et al. 2019). Despite earlier suggestions of hybridization eliminating A. microscaphus (Sullivan 1986), there is no evidence that this is occurring (Schwaner and Sullivan 2009, Ryan et al. 2017a, Wooten et al. 2019).

Chytrid has been detected in this species in New Mexico, but it has not been associated with mortality (Ryan et al. 2014). However, an unidentified skin fungal pathogen was associated with mortality events at two sites in New Mexico (Ryan et al. 2014).