This species' range is continuous along the Virgin River and its tributaries in southwestern Utah, Clover Creek, Meadow Valley Wash and possibly the lower Muddy River in southern Nevada, USA (Blair 1955; Price and Sullivan 1988; Schwaner and Sullivan, in press), and scattered in locations across Arizona, and western New Mexico (Sullivan 1993). Known populations of the Arizona toad range from 365-1,770m asl (Blair 1955).
Habitat and Ecology
Rocky stream courses in pine-oak zone in Arizona and New Mexico, although irrigated cropland and reservoirs increasingly used (Price and Sullivan 1988). In Utah, occurs primarily along streams and washes bordered by willows and cottonwoods (Stebbins 1954; Blair 1955), but has been recorded in impounded areas such as Enterprise Reservoir and Gunlock Reservoir (Blair 1955), and more recently, in Quail Creek Reservoir (Sullivan 1995). Lays eggs among gravel, leaves, or sticks, or on mud or clean sand, at bottom of flowing or shallow quiet waters of perennial or semi permanent streams (Dahl et al. 2000) or shallow ponds. Breeding season is not correlated with rainfall (Blair 1955). Calling males heard as early as February at lower elevations, March-April at higher elevations (Schwaner and Sullivan, in press). Individual males, not all calling at the same time, strung out along the shores of streams and lakes (Blair 1955), sometimes in dense aggregations that include many terrestrial satellite males (Schwaner and Sullivan, in press). Most breeding is accomplished within two weeks (Schwaner et al. 1997), although males may continue to call until June or July (Schwaner, unpublished). Amplexus is axillary and males use the basket method (Krupa 1988) to maximize fertilization efficiency (Brown et al. 2000). Clutch size ranges from 3000-4000 eggs (Blair 1955), most of which hatch under ideal conditions. Unless periodic flash floods, natural drying or human alterations to the stream habitat occur, most tadpoles survive to metamorphosis (Schwaner, unpublished). Predation by mammals, birds and snakes, and over-winter mortality, seem to reduce annual production of young and adults (Schwaner, unpublished).
In Utah and Nevada, locally very common in suitable habitat within its small range (Schwaner and Sullivan, in press). Schwaner et al. (1997) estimated a population of 1,000 toads along a 1,500m transect of the Beaver Dam Wash at Lytle Preserve. A recent survey in Arizona indicated local declines but no obvious major trend (Sullivan 1993).
The construction of impoundments (Sullivan 1993). Malmos et al. (2001) reported directional hybridisation by common male (B. microscaphus) and rare female (B. woodhousii) matings. Schwaner (2003) reported a hybrid swarm at a golf course near the junction of the Beaver Dam Wash with the Virgin River and genetic introgression of B. woodhousii into putative pure B. microscaphus populations 60 miles upstream. Temporal separation of the two species by different breeding times (Sullivan 1995) was not evident.
A major threat is hybridisation with B. woodhousii, possibly facilitated by dam construction.
B. microscaphus is a state protected species in Utah, Nevada and Arizona. Protected populations occur in the Virgin River and its tributaries in Zion National Park.
Red List Status
Least Concern (LC)
Listed as Least Concern in view of its wide distribution, tolerance of a degree of habitat modification, presumed large population, and because it is unlikely to be declining fast enough to qualify for listing in a more threatened category.
Anaxyrus californicus and A. mexicanus were formerly included in this species, which formerly was known as the Southwestern Toad.
Geoffrey Hammerson, Terry Schwaner 2004. Anaxyrus microscaphus. The IUCN Red List of Threatened Species 2004: e.T54709A11189148. http://dx.doi.org/10.2305/IUCN.UK.2004.RLTS.T54709A11189148.en .Downloaded on 20 February 2019