Description
Anaxyrus williamsi are small, robust toads found in wetlands east of Dixie Valley, Nevada in the United States. They have a snout-vent length ranging from 44.01 to 69.97 mm and a relatively robust body. The head is slightly shorter than wide. The dorsal outline of the snout is truncated and only interrupted by the toad’s protruding eyes. In the lateral view the snout is long. The dorsolaterally directed nostrils are protuberant and located closer to the eye than the snout. The canthus rostralis is distinct and ends abruptly at the nares. Both the canthus rostralis and loreal region are concave. The internarial distance is nearly two-thirds of the interorbital distance. The closely-spaced, prominent eyes are large relative to body size and protrude. The tympanum is also large, distinct, and present in an ovoid shape with a supratympanic fold. The paratoid glands begin near the posterior margin of each eye, are raised towards the dorsal side, sub-elliptical (longer than wide), and smaller than the eye diameter. The dorsal surface is bumpy with interspersed glands, which slightly increase in size towards the posterior end. There are two weak, broken rows of longitudinal glands that start at the interorbital space and continue to the sacral hump. The skin between the tubercles is almost smooth. There are concentrations of small tubercles where the lips are closest to the tympanum that extends to the axillary region. The forelimbs are mostly smooth, while the hind limbs are scattered with tubercles and glands, with a particular concentration of tubercles along the mid axillary line of the upper hind limb and with the glands being about the width as the parotoids. The ventrum is granular. The forearms of male A. williamsi are robust. The hands have a large palmar tubercle that is distinct, subovid, and separated from a smaller, but prominent thenar tubercle, which is round and raised. There are also small, round accessory palmar tubercles. The hands of males have distinct nuptial pads that are raised on the dorsal side of finger I. The unwebbed fingers have a relative length of III > IV > I > II, and have moderate and round subarticular tubercles. The hind limbs are relatively short and robust, with the femur being slightly longer than the tibia. There is a tarsal fold. The foot has a prominent, elevated inner metatarsal tubercle that is elliptical, and a smaller, conspicuous outer metatarsal tubercle that is oval. There are also many small planter tubercles. The toes have proximal webbing and a relative length of IV > III > V > II > I. The toe tips are rounded and the moderate subarticular tubercles are small and round (Gordon et al. 2017).

Anaxyrus williamsi is the smallest species within the Anaxyrus boreas species complex, which also includes A. exsul, A. nelsoni, and A. canorus. It has a characteristically short and narrow head, which also sets it apart from other members of this complex. They have large eyes and tympanum openings, which is uncommon in related A. boreas and A. nelsoni. The dorsal coloration of A. williamsi is distinct from other members of the species complex, but it shares a similar black-spotted, white ventral coloration with B. exsul (Gordon et al. 2017).

The dorsal coloration in life of A. williamsi is characterized by a base of olive hues scattered with irregularly shaped black speckles. The eyes are a glossy black that contains dark pupils and a golden iris. Parotoid gland coloration is tan and circumscribed with black. Similarly, the tubercles across the dorsum are a rust color and bordered by black. This same patterning is true for tubercles found on the dorsal portion of the hind limbs. The ventral coloration stands in contrast as a creamy white background with black specks similar to those found on the dorsum. The white coloration becomes gray near the posterior. The throat is white and free of black spots. The subarticular and palmar tubercles on both hind and forelimbs are a striking bright orange. In preservative, specimens of A. williamsi display more muted coloration. Dorsal background coloration becomes a greenish gray and tubercles become more of a dull tanned color. The formerly bright orange tubercles on the hands and feet become white with brown tips (Gordon et al. 2017).

Male A. williamsi develop nuptial pads along their thumbs, which is found in most members of Bufonidae. These pads assist the male in gripping the female during reproductive amplexus. There is no variation in snout-vent length that can be attributed to sexual dimorphism (Gordon et al. 2017).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: United States

U.S. state distribution from AmphibiaWeb's database: Nevada

View distribution map in BerkeleyMapper.

The extant population of A. williamsi is found in wetlands of the western edge of the Dixie Valley Playa, east of Dixie Valley, Churchill County, Nevada, United States of America. The geographic range of this population is approximately 6 km² . These wetlands are fed by groundwater spring sources and are situated within an otherwise arid desert environment, keeping the species endemic to this specific location (Gordon et al. 2017).

Life History, Abundance, Activity, and Special Behaviors
Anaxyrus williamsi is abundant within its limited wetlands habitat. The Dixie Valley Playa is located in a desert environment, which is known for wide variations in day and night temperatures. Anaxyrus williamsi is closely associated with the presence of water. Both sexes have been commonly observed swimming in the waters of the Dixie Valley wetlands, but males are more likely to exhibit aquatic activity than females (Halstead et al. 2021). On average, individuals observed on soil were only 4.2 m from the nearest body of water.

Anaxyrus williamsi is nocturnal and thus emerges onto moist soils or shallow water with vegetation only during the cooler nights. During the daytime, these toads reside in underground burrows. Burrows are also used for brumation, a period of inactivity or dormancy, which begins in late autumn and ends in the spring (Gordon et al. 2017).

Breeding season closely follows emergence from brumation, from March to June. During this time, sexually mature males gather in shallow waters. Courtship is relatively simple among A. williamsi and does not involve any advertisement vocalization (Gordon et al. 2017). The only vocalizations are release calls similar to those of other members of the A. boreas species complex. These are often short, high-pitched “chirps” used to inform other males that they are performing amplexus with a male partner (Keinath and McGee 2005). Males have been observed performing amplexus with multiple female partners (Gordon et al. 2017).

Females lay a single clutch of numerous small eggs. Egg masses, often containing numerous indiscriminate clutches, and their resulting tadpoles develop in the same shallow waters where breeding takes place. Like other members of Bufonidae, A. williamsi does not exhibit parental care. This species undergoes a biphasic complex lifecycle. Most tadpoles become fully metamorphosed into toadlets within 10 weeks after hatching (Gordan et al. 2017).

The distribution of sites for breeding and the resulting development of tadpoles are distinct from those commonly associated with adult toad activity during the non-breeding months (Rose et al. 2022). Breeding sites are located further from the groundwater spring sources in shallow water that ranges in surface temperature from 20 °C to 28 °C. Breeding sites were also more closely associated with depths less than 10 cm, whereas adults could be found in deeper areas. Adults could also tolerate more dense vegetation coverage. Therefore, the distribution of adult and larval individuals of A. williamsi vary across the wetlands of the western Dixie Valley Playa.

The marsh vegetation of the A. williamsi wetland habitat is characterized by rushes (Juncaceae), salt grass (Distichlis spicata), common reed (Phragmites australis), cattail (Typhaceae) and some composite forbs (Asteraceae; Rose et al. 2022). The toads themselves can be found among this vegetation in shallow waters of the wetland and in moist soils located nearby the water (Gordon et al 2017).

Larva
Most tadpoles become fully metamorphosed into toadlets within 10 weeks after hatching (Gordan et al. 2017).

Trends and Threats
Region-wide reviews of conservation studies in the American Southwest accurately correspond to the threats that A. williamsi is currently facing. Mims et al. (2020) reported that hydrologic alteration, change to precipitation, and introduced species were some of the largest threats facing amphibian populations in the Southwest. Geothermal energy projects and introduced bullfrogs fall under these categories.

Groundwater spring fed-ecosystems, like the wetland habitat of A. williamsi, are known to be local biological hotspots with high rates of species endemism, that are vulnerable to environmental changes (Shepard 1993). Anaxyrus williamsi has not yet been evaluated by the IUCN, but as of 2023 it is proposed and awaiting approval for federally listed status as an endangered species (U.S. Fish and Wildlife Service 2022). The habitat of A. williamsi is threatened by geothermal energy production expansion, which may interfere with the groundwater spring sources that feed into the wetlands. The fragility of these wetlands and the lack of any nearby suitable habitats or metacommunities means that damage to the Dixie Valley wetlands could seriously imperil A. williamsi (Gordon et al. 2017).

Anaxyrus williamsi individuals are also threatened by introduced species and pathogens. North American bullfrogs (Rana catesbeiana) have been introduced south of the Dixie Valley wetland habitats. These are larger anurans that could outcompete the native Dixie Valley toads (Gordon et al. 2017). Additionally, bullfrogs are known to carry the fungal pathogen responsible for chytridiomycosis, a serious terminal disease responsible for global declines of amphibian populations (Berger et al. 2016).

Possible reasons for amphibian decline

General habitat alteration and loss
Prolonged drought
Drainage of habitat
Predators (natural or introduced)
Disease

Comments
Prior to the species’ official description, the Dixie Valley Toad population was suggested as a “distinct management unit” with regards to conservation or to be an entirely different species (Wildlife Action Plan Team 2012; Rose et al. 2015; Forrest et al. 2017).

Gordon et al. (2017) utilized both Maximum Likelihood and Bayesian Inference methods on haplotype networking, and phylogenetic tree construction using the control region, including D-loop, mtDNA. Those analyses indicate that A. williamsi is mostly closely related to Nevada A. boreas, with Humboldt-Lahontan region A. canorus populations being the next most closest related lineage. However, Discriminant Function Analysis found that morphologically, A. williamsi is most similar to A. exsul (Gordon et al. 2017).

The species epithet, “williamsi,” pays tribute to Robert Williams, the former Field Supervisor of the U.S. Fish and Wildlife Service (Gordon et al. 2017). He is noted for his considerable contribution towards conservation of fauna in Nevada and California.

The extremely limited range of this species has allowed it to remain cryptically undescribed until recently, despite broad surveys of the A. boreas species complex members (Franklin et al. 2018).

References
Berger, L., Roberts, A. A., Voyles, J., Longcore, J. E., Murray, K. A., and Skerratt, L. F. (2016). History and recent progress on chytridiomycosis in amphibians. Fungal Ecology, 19, 89-99. [link]

Forrest, M. J., Stiller, J., King, T. L., and Rouse, G. W. (2017). Between hot rocks and dry places: the status of the Dixie Valley Toad. Western North American Naturalist, 77(2), 162-175. [link]

Franklin, T. W., Dysthe, J. C., Golden, M., McKelvey, K. S., Hossack, B. R., Carim, K. J., Tait, C., Young, M.K., and Schwartz, M. K. (2018). Inferring presence of the western toad (Anaxyrus boreas) species complex using environmental DNA. Global Ecology and Conservation, 15, e00438. [link]

Frost, D. R., Grant, T., Faivovich, J., Bain, R. H., Haas, A., Haddad, C. F. B., de Sá, R. O., Channing, A., Wilkinson, M., Donnellan, S. C., Raxworthy, C. J., Campbell, J. A., Blotto, B. L., Moler, P., Drewes, R. C., Nussbaum, R. A., Lynch, J. D., Green D. M., and Wheeler, W. C. (2006). The amphibian tree of life. Bulletin of the American Museum of Natural History, 297, 1-370. [link]

Gordon, M. R., Simandle, E. T., and Tracy, C. R. (2017). A diamond in the rough desert shrublands of the Great Basin in the Western United States: A new cryptic toad species (Amphibia: Bufonidae: Bufo (Anaxyrus)) discovered in Northern Nevada. Zootaxa, 4290(1), 123-139. [link]

Halstead, B. J., Kleeman, P. M., Rose, J. P., & Fouts, K. J. (2021). Water temperature and availability shape the spatial ecology of a hot springs endemic toad (Anaxyrus williamsi). Herpetologica, 77(1), 24-36. [link]

Keinath, D. and M. McGee. (2005, May 25). Boreal Toad (Bufo boreas boreas): a technical conservation assessment. USDA Forest Service, Rocky Mountain Region. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5182081.pdf. Accessed May 2023

Mims, M. C., Moore, C. E., and Shadle, E. J. (2020). Threats to aquatic taxa in an arid landscape: Knowledge gaps and areas of understanding for amphibians of the American Southwest. Wiley Interdisciplinary Reviews: Water, 7(4), e1449. [link]

Rose, J.P., O.J. Miano, G.R. Cottle , R.E. Lovich , R.L.Palmer, and Todd, B.D. (2015). A herpetological inventory of Naval Air Station Fallon, Churchill County, Nevada. Western North American Naturalist 75, 526–534 [link]

Rose, J. P., Kleeman, P. M., and Halstead, B. J. (2022). Hot, wet and rare: modelling the occupancy dynamics of the narrowly distributed Dixie Valley toad. Wildlife Research, 50(7), 552–567 [link]

Shepard, W. D. (1993). Desert springs-both rare and endangered. Aquatic Conservation, 3(4), 351–359. [link]

U.S. Fish and Wildlife Service. (2022). Endangered and Threatened Wildlife and Plants; Endangered Species Status for the Dixie Valley Toad. https://www.fws.gov/species-publication-action/endangered-and-threatened-wildlife-and-plants-endangered-species-32. Accessed May 2023

Wildlife Action Plan Team. 2012. Nevada Wildlife Action Plan. Nevada Department of Wildlife, Reno, NV. https://www.ndow.org/blog/2012-nevada-wildlife-action-plan/. Accessed May 2023

Species Account Citation: AmphibiaWeb 2023 Anaxyrus williamsi: Dixie Valley Toad <https://amphibiaweb.org/species/8649> University of California, Berkeley, CA, USA. Accessed Apr 18, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 18 Apr 2024.

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