Anaxyrus exsul - (Myers, 1942)
Black Toad
Other English Common Names: black toad
Synonym(s): Bufo exsul Myers, 1942
Taxonomic Status: Accepted
Related ITIS Name(s): Anaxyrus exsul (Myers, 1942) (TSN 773519)
Unique Identifier: ELEMENT_GLOBAL.2.101917
Element Code: AAABB01070
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
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Concept Reference: Frost, D. R. 1985. Amphibian species of the world. A taxonomic and geographical reference. Allen Press, Inc., and The Association of Systematics Collections, Lawrence, Kansas. v + 732 pp.
Concept Reference Code: B85FRO01HQUS
Name Used in Concept Reference: Bufo exsul
Taxonomic Comments: Bufo exsul formerly was regarded as a subspecies of B. boreas by some authors.

Molecular data indicate that Bufo exsul is phylogenetically nested within Bufo canorus; further data are needed to determine whether Bufo exsul should be subsumed with Bufo canorus (Shaffer et al. 2000).

Phylogenetic analyses of mtDNA data from throughout the range of the Bufo (Anaxyrus) boreas species group (including boreas, canorus, exsul, and nelsoni) by Goebel et al. (2009) identified three major haplotype clades. The Northwest clade (NW) includes both subspecies of boreas (boreas and halophilus) and divergent minor clades in the middle Rocky Mountains, coastal, and central regions of the west and Pacific Northwest. The Southwest (SW) clade includes exsul, nelsoni, and minor clades in southern California. Bufo (Anaxyrus) canorus, previously identified as paraphyletic, has populations in both the NW and SW major clades. The Eastern major clade (E) includes three divergent lineages from southern Utah, the southern Rocky Mountains, and north of the Great Basin at the border of Utah and Nevada. Goebel et al. (2009) tentatively suggested that some or many of the clades might warrant recognition as distinct species. However, the authors refrained from delineating new species circumscriptions, noting that additional research might suggest different taxonomic outcomes (e.g., recognizing the traditionally defined Bufo canorus as two distinct species or, conversely, combining it with other minor groups and thus broadening its scope).
Conservation Status
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NatureServe Status

Global Status: G1
Global Status Last Reviewed: 17Jul2014
Global Status Last Changed: 17Jul2014
Ranking Methodology Used: Ranked by calculator
Rounded Global Status: G1 - Critically Imperiled
Reasons: Small range in Deep Springs Valley, Inyo County, California; habitat is appropriately managed, and population has been relatively stable in recent decades; molecular data suggest that A. exsul may be conspecific with A. canorus--further study is needed.
Nation: United States
National Status: N1 (05Nov1996)

U.S. & Canada State/Province Status
United States California (S1)

Other Statuses

IUCN Red List Category: VU - Vulnerable

NatureServe Global Conservation Status Factors

Range Extent: <100 square km (less than about 40 square miles)
Range Extent Comments: Range includes several springs feeding Deep Springs Lake (playa), in Deep Springs Valley, Inyo County, California: Buckhorn Spring, Corral Spring and adjacent pond (Schuierer and Anderson 1990), and Bog Mound Springs at an elevation of 1,520 meters (about 5,000 feet). A population at Antelope Springs at about 1,710 meters, about 7 kilometers northwest of Deep Springs Lake, may have been introduced (Schuierer 1962, Murphy et al. 2003, Stebbins 2003). An introduced population occurs in a flowing well near Salt Lake in Saline Valley, Death Valley National Park, Inyo County, California (Murphy et al. 2003). Black toads were introduced at Cottonwood Springs in the Owens Valley in the 1960s (Schuierer 1962), but no there are no subsequent records for that area (Fellers 2005). The species was introduced at Batchelder Spring, Westgard Pass, Inyo County, but apparently it is extirpated there.

Area of Occupancy: 2-5 4-km2 grid cells
Area of Occupancy Comments:  

Number of Occurrences: 1 - 5
Number of Occurrences Comments: Entire range can be regarded as one occurrence; or perhaps subdivided into a half dozen subpopulations.

Population Size: 10,000 - 100,000 individuals
Population Size Comments: Abundant at Corral Springs. Kagarise Sherman (1980) estimated the Corral Springs population at 7,897 toads in 1977 and 9,744 toads in 1978. In the same location, Murphy et al. (2003) estimated the population at 8,538 (95% confidence interval = 5,335-21,366). Population size in the much more extensive habitat at Buckhorn Springs and in other locations is unknown. Total population size in the early 2000s probably was around 24,000 (J. Murphy and E. Simandle, pers. comm., cited by Fellers 2005).

Overall Threat Impact: Very high - medium
Overall Threat Impact Comments: "Kagarise Sherman (1980) noted tadpole mortality associated with water diversion and a low level of adult mortality presumably caused by cattle trampling. In response, Deep Springs College abandoned irrigation ditch maintenance and related agricultural practices (e.g., raking and burning) that were potentially detrimental to the toads. Cattle exclosures were built around the spring sources in the early 1970s, presumably to protect toad hibernacula from disturbance." (Murphy et al. 2003). Since then, Deep Springs College has abandoned all agricultural practices within black toad habitat, except periodic cattle grazing (Murphy et al. 2003). Cattle exclosures and other areas at Corral Springs have become thickly vegetated; the impact of this on the toads deserves further study (Murphy et al. 2003). Trampling by cattle occurred at Antelope Spring in at least the 1970s.

In addition to habitat concerns, potential threats include chytridiomycosis and introduction of non-native species such as bullfrogs. The small range of the species makes it highly vulnerable to sudden declines that could occur with the introduction of a non-native predator or disease (Fellers 2005).

These toads appear to tolerate periodic droughts and other climate variations (Schuierer 1972) and so may not be highly vulnerable to climate change unless it results in significant reductions in spring outflows.

Excessive collecting by herpetologists may have been a threat in the early 1970s (Schuierer 1972).

Short-term Trend: Relatively Stable (<=10% change)
Short-term Trend Comments: Population was regarded as more or less stable in the early 1970s (Schuierer 1972, Bury et al. 1980); no significant changes have been recorded since then (Kagarise Sherman 1980, California Department of Fish and Game 1990, Schuierer and Anderson 1990, Murphy et al. 2003, Fellers 2005).

Long-term Trend: Decline of <30% to relatively stable
Long-term Trend Comments: Long-term trend is uncertain, but the overall population likely has been relatively stable in extent of occurrence and has declined probably less than 25% in abundance, area of occupancy, and number/condition of occurrences.

Intrinsic Vulnerability: Moderately vulnerable

Environmental Specificity: Moderate. Generalist or community with some key requirements scarce.

Other NatureServe Conservation Status Information

Inventory Needs: Range is well known.

Distribution
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Global Range: (<100 square km (less than about 40 square miles)) Range includes several springs feeding Deep Springs Lake (playa), in Deep Springs Valley, Inyo County, California: Buckhorn Spring, Corral Spring and adjacent pond (Schuierer and Anderson 1990), and Bog Mound Springs at an elevation of 1,520 meters (about 5,000 feet). A population at Antelope Springs at about 1,710 meters, about 7 kilometers northwest of Deep Springs Lake, may have been introduced (Schuierer 1962, Murphy et al. 2003, Stebbins 2003). An introduced population occurs in a flowing well near Salt Lake in Saline Valley, Death Valley National Park, Inyo County, California (Murphy et al. 2003). Black toads were introduced at Cottonwood Springs in the Owens Valley in the 1960s (Schuierer 1962), but no there are no subsequent records for that area (Fellers 2005). The species was introduced at Batchelder Spring, Westgard Pass, Inyo County, but apparently it is extirpated there.

U.S. States and Canadian Provinces
Color legend for Distribution Map
Endemism: endemic to a single state or province

U.S. & Canada State/Province Distribution
United States CA

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 www.natureserve.org/conservation-tools/data-maps-tools.

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


U.S. Distribution by County Help
State County Name (FIPS Code)
CA Inyo (06027)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
18 Eureka-Saline Valleys (18090201)+
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History
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Basic Description: A small, mostly black toad.
Reproduction Comments: Breeding occurs between mid-March and late April or early May and possibly into June (mainly mid-March to late April according to Schuierer and Anderson 1990). About 10-100 breeding pairs congregate in marsh waters. Eggs hatch usually in about 4-5 days; tadpoles metamorphose about 3-5 weeks later. Individuals become sexually mature probably by the end of their second year.
Non-Migrant: Y
Locally Migrant: N
Long Distance Migrant: N
Riverine Habitat(s): Low gradient, Pool, SPRING/SPRING BROOK
Lacustrine Habitat(s): Shallow water
Palustrine Habitat(s): HERBACEOUS WETLAND, Riparian
Special Habitat Factors: Benthic, Burrowing in or using soil, Fallen log/debris
Habitat Comments: Habitat includes watercourses and marshes (grass, sedge, dwarf bulrush and watercress) formed by water flow from springs, in areas surrounded by desert with low bushes. These toads rarely move away from riparian areas. Adults are more aquatic than other toad species in California and prefer habitats with short plant cover and unobstructed access to still or slowly flowing water (Schuierer and Anderson 1990). Breeding occurs in shallow marsh and pond waters (Schuierer and Anderson 1990). In winter, the toads retreat to rodent burrows or other refuges.


Adult Food Habits: Invertivore
Immature Food Habits: Herbivore
Food Comments: Diet includes flies, beetles, ants, and lepidopterous larvae. Larvae probably eat algae, organic debris, and plant tissue.
Adult Phenology: Diurnal, Hibernates/aestivates
Immature Phenology: Diurnal, Hibernates/aestivates
Phenology Comments: These toads are inactive in cold temperatures. They are primarily diurnal but may be active on warm nights. Most activity occurs between late March and mid-September.
Colonial Breeder: Y
Length: 6 centimeters
Economic Attributes Not yet assessed
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Management Summary
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Stewardship Overview: Bury et al. (1980) made the following recommendations: prevent introduction of predators such as bullfrog; exclude livestock by fencing; discontinue marsh burning and channel modification; formalize agreement with owner to protect toad and habitat. Deep Springs College has stopped diverting spring outflows for irrigation and has implemented the livestock management and marsh modifications as recommended by Bury et al. (1980). Better information is needed on the effects of various management regimes (e.g., livestock grazing). To the extent possible, actions should be taken to ensure the stability of spring flows and availability of standing water.

Periodic monitoring is needed to check on the possible introduction of non-native predators, competitors, or diseases.


Population/Occurrence Delineation
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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
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U.S. Invasive Species Impact Rank (I-Rank) Not yet assessed
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Authors/Contributors
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NatureServe Conservation Status Factors Edition Date: 21May2013
NatureServe Conservation Status Factors Author: Hammerson, G.
Element Ecology & Life History Edition Date: 21May2013
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).

References
Help
  • Behler, J. L., and F. W. King. 1979. The Audubon Society field guide to North American reptiles and amphibians. Alfred A. Knopf, New York. 719 pp.

  • Blackburn, L., P. Nanjappa, and M. J. Lannoo. 2001. An Atlas of the Distribution of U.S. Amphibians. Copyright, Ball State University, Muncie, Indiana, USA.

  • Bury, R. B., C. K. Dodd, Jr., and G. M. Fellers. 1980. Conservation of the Amphibia of the United States: a review. U.S. Fish and Wildlife Service, Washington, D.C., Resource Publication 134. 34 pp.

  • California Department of Fish and Game (CDF&G). 1990. 1989 annual report on the status of California's state listed threatened and endangered plants and animals. 188 pp.

  • Fellers, G. M. 2005. Bufo exsul Myers, 1942(a). Black toad. Pages 406-408 in M. Lannoo, editor. Amphibian declines: the conservation status of United States species. University of California Press, Berkeley.

  • Frost, D. R. 1985. Amphibian species of the world. A taxonomic and geographical reference. Allen Press, Inc., and The Association of Systematics Collections, Lawrence, Kansas. v + 732 pp.

  • Frost, D. R. 2002. Amphibian Species of the World: an online reference. V2.21 (15 July 2002). Electronic database available at http://research.amnh.org/herpetology/amphibia/index.html.

  • Frost, D. R. 2010. Amphibian Species of the World: an Online Reference. Version 5.4 (8 April 2010). Electronic Database accessible at http://research.amnh.org/herpetology/amphibia/index.php. American Museum of Natural History, New York, USA.

  • Goebel, A. M., T. A. Ranker, P. S. Corn, and R. G. Olmstead. 2009. Mitochondrial DNA evolution in the Anaxyrus boreas species group. Molecular Phylogenetics and Evolution 50:209-225.

  • Kagarise Sherman, C. K. 1980. A comparison of the natural history and mating system of two anurans: Yosemite toads (Bufo canorus) and black toads (Bufo exsul). Ph.D. dissertation, University of Michigan, Ann Arbor.

  • Murphy, J. F., E. T. Simandle, and D. E. Becker. 2003. Population status and conservation of the black toad, Bufo exsul. Southwestern Naturalist 48:54-60.

  • Myers, G. S. 1942. The black toad of Deep Springs Valley, Inyo County, California. Univ. Michigan Museum Zoology, Occas. Pap. No. 469.

  • Schuierer, F. W. 1962. Remarks upon the natural history of Bufo exsul Myers, the endemic toad of Deep Springs Valley, Inyo County, California. Herpetologica 17:260-266.

  • Schuierer, F. W. 1963. Notes on two populations of Bufo exsul Myers and a commentary on speciation within the Bufo boreas group. Herpetologica 18:262-267.

  • Schuierer, F. W. 1972. The current status of the endangered species Bufo exsul Myers, Deep Springs Valley, Inyo County, California. Herpetological Review 4:81-82.

  • Schuierer, F. W., and S. C. Anderson. 1990. Population status of BUFO EXSUL Myers, 1942. Herpetological Review 21:57.

  • Shaffer, H. B., G. M. Fellers, A. Magee, and S. R. Voss. 2000. The genetics of amphibian declines: population substructure and molecular differentiation in the Yosemite toad, BUFO CANORUS (Anura, Bufonidae) based on single-strand conformation polymorphism analysis (SSCP) and mitochondrial DNA sequence data. Molecular Ecology 9:245-257.

  • Sherman, C. K. 1980. A comparison of the natural history and mating system of two anurans: Yosemite toads (BUFO CANORUS) and black toads (BUFO EXSUL). Ph.D. dissertation, Univ. Michigan, Ann Arbor. xiv + 394 pp.

  • Stebbins, R. C. 1985a. A field guide to western reptiles and amphibians. Second edition. Houghton Mifflin Company, Boston, Massachusetts. xiv + 336 pp.

  • Stebbins, R. C. 2003. A field guide to western reptiles and amphibians. Third edition. Houghton Mifflin Company, Boston.

  • Stephens, M. R. 2001. Phylogeography of the Bufo boreas (Anura, Bufonidae) species complex and the biogeography of California. M.S. thesis, Sonoma State University. 62 pp.

  • U.S. Fish and Wildlife Service (USFWS). 2002. 12-month finding for a petition to list the Yosemite toad. Federal Register 67(237):75834-75843.

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