Anaxyrus microscaphus - (Cope, 1867)
Arizona Toad
Other English Common Names: Arizona toad
Synonym(s): Bufo microscaphus microscaphus (Cope, 1867 "1866") ;Bufo microscaphus (Cope, 1867 "1866")
Taxonomic Status: Accepted
Related ITIS Name(s): Anaxyrus microscaphus (Cope, 1867) (TSN 773525)
Unique Identifier: ELEMENT_GLOBAL.2.105766
Element Code: AAABB01110
Informal Taxonomy: Animals, Vertebrates - Amphibians - Frogs and Toads
Kingdom Phylum Class Order Family Genus
Animalia Craniata Amphibia Anura Bufonidae Anaxyrus
Genus Size: D - Medium to large genus (21+ species)
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Concept Reference
Concept Reference: Gergus, E.W.A. 1998. Systematics of the Bufo microscaphus complex: allozyme evidence. Herpetologica 54:317-325.
Concept Reference Code: A98GER01NAUS
Name Used in Concept Reference: Bufo microscaphus
Taxonomic Comments: 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. Gergus concluded that under the phylogenetic species concept they should be recognized as species.

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

NatureServe Status

Global Status: G3G4
Global Status Last Reviewed: 02Jul2015
Global Status Last Changed: 02Jul2015
Ranking Methodology Used: Ranked by inspection
Rounded Global Status: G3 - Vulnerable
Reasons: Wide range and many scattered locations in southwestern Utah, southern Nevada, Arizona, and western New Mexico; still widely distributed and locally common, but now absent from many historical localities in Arizona and Nevada where the riparian corridor has been altered dramatically through the construction of impoundments, leading to replacement by and hybridization with Woodhouse's toad. Further evaluation of the species' staus using up-to-date data is needed.
Nation: United States
National Status: N3N4 (02Jul2015)

U.S. & Canada State/Province Status
United States Arizona (S3S4), Nevada (S2), New Mexico (S2?), Utah (S3)

Other Statuses

IUCN Red List Category: LC - Least concern

NatureServe Global Conservation Status Factors

Range Extent: 20,000-200,000 square km (about 8000-80,000 square miles)
Range Extent Comments: Range encompasses 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).

Area of Occupancy: Unknown 4-km2 grid cells
Area of Occupancy Comments:  

Number of Occurrences:  
Number of Occurrences Comments: Number of occurrences has not been determined using standardized criteria, but this species is represented by a large number of collection sites and locations (as defined by IUCN). Degenhardt et al. (1996) mapped approximately 70 collection sites in New Mexico. Price and Sullivan (1988) mapped about 45 collection sites in Arizona, Utah, and Nevada. However, the number of crurrently occupied locations needs further study.

Population Size: 10,000 - 1,000,000 individuals
Population Size Comments: Total adult population size is unknown but presumably exceeds 10,000. Several hundred adults have been found along individual stream lengths of 1.0-1.5 kilometers (see Schwaner and Sullivan 2005).

Overall Threat Impact: Medium
Overall Threat Impact Comments: Sullivan and Lamb (1988) presented evidence that A. woodhousii has replaced A. microscaphus in some areas of altered (dammed) habitat in central Arizona. Schwaner and Sullivan (2005) cited unpublished data of Bradford et al. indicating that similar replacement also has occurred in southern Nevada. In Arizona, A. microscaphus is now absent from historical localities where the riparian corridor has been altered dramatically through the construction of impoundments (Sullivan 1993). Additionally, hybridization between woodhousii and microscaphus has occurred in and near these and other altered habitats (e.g., golf courses) (Schwaner and Sullivan 2005), with genetic introgression extending dozens of kilometers from the location of a hybrid swarm (Schwaner and Sullivan 2009). However, one study showed that even after several decades of hybridization there was no obvious trend toward replacement of microscaphus by woodhousii (Schwaner and Sullivan 2009).

Drought associated with climate change presumably will detrimentally affect this species' habitat to some degree within the foreseeable future.

Short-term Trend: Decline of <30% to relatively stable
Short-term Trend Comments: Trend over the past 10 years or three generations is uncertain, but distribution and abundance probably have been relatively stable or slowly declining. Further study is needed.

Long-term Trend: Decline of <30% to relatively stable
Long-term Trend Comments: Long-term trend is uncertain, but distribution and abundance probably have decreased to a relatively small degree (Schwaner and Sullivan 2005). A decades-old survey in Arizona indicated local declines but no obvious major trend (Sullivan 1993).

Intrinsic Vulnerability: Moderately vulnerable

Environmental Specificity: Moderate to broad.

Other NatureServe Conservation Status Information

Global Range: (20,000-200,000 square km (about 8000-80,000 square miles)) Range encompasses 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).

U.S. States and Canadian Provinces
Color legend for Distribution Map
Endemism: occurs (regularly, as a native taxon) in multiple nations

U.S. & Canada State/Province Distribution
United States AZ, NM, NV, UT

Range Map
Note: Range depicted for New World only. The scale of the maps may cause narrow coastal ranges or ranges on small islands not to appear. Not all vagrant or small disjunct occurrences are depicted. For migratory birds, some individuals occur outside of the passage migrant range depicted. For information on how to obtain shapefiles of species ranges see our Species Mapping pages at

Range Map Compilers: IUCN, Conservation International, NatureServe, and collaborators, 2004

U.S. Distribution by County Help
State County Name (FIPS Code)
AZ Apache (04001), Coconino (04005), Gila (04007), Graham (04009), Greenlee (04011), La Paz (04012), Maricopa (04013), Mohave (04015), Navajo (04017), Yavapai (04025)
NM Socorro (35053)
NV Clark (32003), Lincoln (32017)
UT Garfield (49017), Iron (49021), Kane (49025), San Juan (49037)*, Washington (49053)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
13 Elephant Butte Reservoir (13020211)+
14 Upper Lake Powell (14070001)+, Escalante (14070005)+*, Paria (14070007)+, Chinle (14080204)+*, Lower San Juan (14080205)+*
15 Kanab (15010003)+*, Lake Mead (15010005)+, Grand Wash (15010006)+, Hualapai Wash (15010007)+, Upper Virgin (15010008)+, Fort Pierce Wash (15010009)+, Lower Virgin (15010010)+, Muddy (15010012)+*, Meadow Valley Wash (15010013)+, Las Vegas Wash (15010015)+, Middle Little Colorado (15020008)+, Chevelon Canyon (15020010)+, Canyon Diablo (15020015)+*, Havasu-Mohave Lakes (15030101)+*, Big Sandy (15030201)+, Burro (15030202)+, Santa Maria (15030203)+, Bill Williams (15030204)+, San Francisco (15040004)+, Upper Gila-San Carlos Reservoir (15040005)+, San Carlos (15040007)+, Black (15060101)+, Upper Salt (15060103)+, Carrizo (15060104)+, Tonto (15060105)+, Lower Salt (15060106)+, Big Chino-Williamson Valley (15060201)+, Upper Verde (15060202)+, Lower Verde (15060203)+, Agua Fria (15070102)+, Hassayampa (15070103)+
16 Escalante Desert (16030006)+
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History
General Description: This is a uniformly warty toad with the eyelids and the front of the oval parotoid glands usually pale; usually there is a light area on each sacral hump and in the middle of the back; cranial crests weak or absent; dorsal color varies with substrate color; both males and females have a pale throat; snout-vent length 5-8 cm; young have red-tipped tubercles on dorsum and the underside of the feet is yellow (Stebbins 1985).
Diagnostic Characteristics: This species differs from A. californicus in having red rather than yellow spots on the dorsum of juveniles (Gergus 1998). It differs from A. woodhousii in lacking a dorsal stripe, having weak or no cranial crests, more rounded parotoid glands, and by the males lacking a dark throat. It differs from A. speciosus in having the light colored areas on the eyelids, front of the parotoid glands, on the sacral humps, and in the middle of the back; also by lacking a sharp-edged tubercle on the hind feet (Stebbins 1985).
Reproduction Comments: In west-central Arizona, breeding occurred February-April, independent of rainfall, and usually occurred for a total of a few weeks each year (Sullivan 1992). In southwestern Utah, breeding peaks in June. At higher elevations, breeding may extend to July or perhaps August (Stebbins 1985).
Non-Migrant: Y
Locally Migrant: Y
Long Distance Migrant: N
Mobility and Migration Comments: Individuals may migrate short distances between nonbreeding terrestrial habitats and breeding pools.
Riverine Habitat(s): CREEK, Low gradient, Moderate gradient, Pool
Palustrine Habitat(s): Riparian
Terrestrial Habitat(s): Cropland/hedgerow, Desert, Shrubland/chaparral, Woodland - Conifer, Woodland - Hardwood, Woodland - Mixed
Special Habitat Factors: Benthic, Burrowing in or using soil, Fallen log/debris
Habitat Comments: Habitat includes rocky stream courses in the pine-oak zone (e.g., Arizona, New Mexico), stream courses bordered by willows and cottonwoods, irrigation ditches, flooded/irrigated fields, and reservoirs (Stebbins 1954, Price and Sullivan 1988, Schwaner and Sullivan 2005).

Egg deposition occurs among gravel, leaves, or sticks, or on mud or clean sand, in flowing or shallow quiet waters of perennial or semipermanent streams (Dahl et al. 2000) or shallow ponds.

Adult Food Habits: Invertivore
Immature Food Habits: Herbivore
Food Comments: Diet includes: snails, crickets, beetles, ants; sometimes cannibalizes newly metamorphosed individuals. Larvae probably eat algae, organic debris, and plant tissue.
Adult Phenology: Hibernates/aestivates, Nocturnal
Immature Phenology: Hibernates/aestivates, Nocturnal
Phenology Comments: These toads are inactive in cold temperatures. Adults are primarily nocturnal except during the breeding season (Stebbins 1985). Adults are active at ambient temperatures of about 22-35 C. Newly metamorphosed individuals often are active during daylight hours.
Colonial Breeder: Y
Length: 8 centimeters
Economic Attributes Not yet assessed
Management Summary
Biological Research Needs: Better information is needed on current area of occupancy, abundance, and trend.
Population/Occurrence Delineation
Group Name: Bufonid Toads

Use Class: Not applicable
Subtype(s): Breeding Site
Minimum Criteria for an Occurrence: Occurrences are based on evidence of historical presence, or current and likely recurring presence, at a given location. Such evidence minimally includes collection or reliable observation and documentation of one or more individuals (including larvae or eggs) in or near appropriate habitat where the species is presumed to be established and breeding.
Separation Barriers: Busy major highway such that toads rarely if ever cross successfully; roads with nonpermeable barriers to toad movement; urbanized areas dominated by buildings and pavement.
Separation Distance for Unsuitable Habitat: 1 km
Separation Distance for Suitable Habitat: 5 km
Separation Justification: Opportunistic observations of various toad species in lowland habitats indicate regular movements of up to at least several hundred meters from the closest known breeding site (G. Hammerson, pers. obs.). Sweet (1993) recorded movements of up to 1 km in Bufo californicus. In defining critical habitat for B. californicus, USFWS (2000) included breeding streams and upland areas within a 25-m elevational range of each essential stream reach and no more than 1.5 km away from the stream. In northwestern Utah, Thompson (2004) recorded movements of Bufo boreas of up to 5 km across upland habitat between two springs during the summer-fall season. Another toad moved 1.3 km between May of one year and May of the next year; the following June it was back at the original breeding location (Thompson 2004). Most studies of toad movements have not employed radiotelemetry and were not designed to detect long-range movements or dispersal.

The separation distance for unsuitable habitat reflects the nominal minimum value of 1 km. The separation distance for suitable habitat reflects the good vagility of toads, their ability to utilize ephemeral or newly created breeding sites, and the consequent likely low probability that two occupied locations separated by less than several kilometers of suitable habitat would represent truly independent populations over the long term.

Inferred Minimum Extent of Habitat Use (when actual extent is unknown): .5 km
Date: 27Apr2005
Author: Hammerson, G.
Population/Occurrence Viability
U.S. Invasive Species Impact Rank (I-Rank) Not yet assessed
NatureServe Conservation Status Factors Edition Date: 02Jul2015
NatureServe Conservation Status Factors Author: Hammerson, G.
Element Ecology & Life History Edition Date: 09May2013
Element Ecology & Life History Author(s): Hammerson, G.

Zoological data developed by NatureServe and its network of natural heritage programs (see Local Programs) and other contributors and cooperators (see Sources).

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  • Gergus, E.W.A. 1998. Systematics of the Bufo microscaphus complex: allozyme evidence. Herpetologica 54:317-325.

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