AmphibiaWeb - Spea intermontana


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Spea intermontana (Cope, 1883)
Great Basin Spadefoot
family: Scaphiopodidae
genus: Spea

Joseph S. Dixon
© Museum of Vertebrate Zoology, University of California, Berkeley (1 of 31)

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Conservation Status (definitions)
IUCN Red List Status Account Least Concern (LC)
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National Status None
Regional Status None
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bookcover The following account is modified from Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo (©2005 by the Regents of the University of California), used with permission of University of California Press. The book is available from UC Press.

Spea intermontana  (Cope, 1883)
            Great Basin Spadefoot

Steven R. Morey1

            There are two recognizable groups of North American spadefoot toads, Scaphiopus (Holbrook, 1836) and Spea (Cope, 1863).  With respect to those species that are referable to Spea, the literature is divided, with some authors following Bragg (1944, 1945b), Stebbins (1951, 1985), Blair (W.F., 1955c), Zweifel (1956b), and Hall (1998), who treat the two groups as subgenera.  We follow B.C. Brown (1950), Smith (1950), Tanner (1989b), Wiens and Titus (1991), Maglia (1998, 1999), and Crother et al. (2000), who recognize the generic distinctness of Spea.

1. Historical versus Current Distribution.  Great Basin spadefoot toads (Spea intermontana) occur from south-central British Columbia, Canada, south into the United States where they range from eastern Washington, Oregon, and California through Nevada and Utah, into southern Idaho, northwestern Colorado, and southwestern Wyoming (Stebbins, 1985; Leonard et al., 1993; Hall, 1998).  Hall (1998) gives a detailed review of the distribution, which can be confusing because of the complicated taxonomic history of this species and the North American pelobatids in general.  The historical and current ranges are similar.  The distribution within some parts of the range differs from the historical pattern due to human activities.  Great Basin spadefoot toads no longer live in areas where urbanization and other land uses have destroyed habitat (Orchard, 1992; Leonard et al., 1993).  On the other hand, they have colonized some new areas where human land uses, such as the construction of reservoirs, have inadvertently created artificial breeding sites where none previously existed.  Hovingh et al. (1985), for example, found that 57% of the Great Basin spadefoot toad breeding sites in the Bonneville Basin, Utah, were manmade water sources. 

2. Historical versus Current Abundance.  In earlier reports, Great Basin spadefoot toads were usually considered to be common in suitable habitats (Grinnell and Storer, 1924; Tanner, 1939; Wright and Wright, 1949), and they continue to be today.  There can be little doubt, however, that patterns of abundance have been influenced by human activities.  In grazing country, which includes most of their range, springs and streams have been dammed, diverted into ditches and impoundments, or otherwise altered, and reservoirs have created artificial water sources where natural water sources do not exist.  In some cases, Great Basin spadefoot toads have been able to capitalize on these changes, becoming more abundant than under pristine conditions; but where urbanization, agriculture other than grazing, and other land conversions have destroyed or harmed habitats, Great Basin spadefoot toads are absent or less abundant than under pristine conditions. 

3. Life History Features.

            A. Breeding.  Reproduction is aquatic.

                        i. Breeding migrations.  Adults are terrestrial and must move from winter refuges to reach breeding sites.  Adults become active on the surface during the first warm evenings of spring.  Activity is greatest during or following evening rainfall, but daytime activity is not extraordinary.  The first such evenings sufficiently warm to promote activity usually occur in April, but newly emerged adults do not necessarily move immediately to breeding sites.  The factors that stimulate breeding are not well known.  Linsdale (1938), Hovingh et al. (1985), and others have noted that breeding need not be stimulated by rainfall, as it often is in other North American spadefoot toads.  In some areas, breeding occurs in playas or pools that form following spring or summer showers, and the observation by Leonard et al. (1993) that water diversions for irrigation can stimulate breeding is a common one, particularly in pastures and on the margins of agricultural fields.  Breeding has been observed in April–July (Wright and Wright, 1949; Nussbaum et al., 1983).  Not only is there a great deal of year-to-year variation in the timing of breeding, it is also asynchronous at a site in the same year.  For example, in 1989 at Mono Lake, California, the first clutches of eggs were laid on 2–5 June, with other clutches appearing in the same pool on 23 and 25 June.  At the same site in 1990, the first clutch appeared on 20 April, and new clutches were laid on 28 April.  Males can be expected to chorus intermittently at breeding sites any time from April–June, occasionally as late as July.  Linsdale’s (1938) estimate of adult migrations of ≤ 0.8 km (0.5 mi) seems reasonable, but most adults are encountered much closer to breeding sites.  Hovingh et al. (1985) seem to suggest that migrations to breeding sites of up to 5 km are not out of the question.

                        ii. Breeding habitat.  Great Basin spadefoot toads breed in springs, sluggish streams, and other permanent or ephemeral water sources (Wright and Wright, 1949; Nussbaum et al., 1983; Stebbins, 1985; Hall, 1998; and references therein).  In the Bonneville Basin, Utah, over half of the breeding sites are manmade reservoirs, the remainder being permanent or temporary springs (Hovingh et al., 1985).  In the extreme western edge of the range, just east of the central Sierra Nevada, Morey (1994) found that breeding was restricted to permanent streams and springs.  This is because little rainfall occurs in the California portion of the Great Basin, and snowmelt is insufficient to fill pools.  Moving eastward, spring rainfall is more common in the central Great Basin, until, on the Colorado Plateau, most rainfall occurs in the form of summer showers (Kay, 1982) that can be torrential, easily filling playas and other temporary pools.  Thus, the reliance on temporary rain-filled pools for breeding increases from west to east across the range.  In order to support metamorphosis, the breeding site must remain filled long enough to accommodate the period between egg deposition and hatching (2–4 d) and the minimum larval period, which in the wild is about 36 d (Morey, 1994).  In the western portion of the range, larval mortality due to drying is uncommon except when humans divert flows before larval development is complete. 

            B. Eggs.

                        i. Egg deposition sites.  In the water of temporary rain-filled pools.

                        ii. Clutch size.  Stebbins (1985) reports that females lay 300–500 eggs in packets of 20–40.  Leonard et al. (1993) report that females may lay as many as 800 eggs.  I have estimated or counted 300; 400; 855; 980; and 1,000 fertilized eggs in clutches produced by captive pairs. 

            C. Larvae/Metamorphosis.

                        i. Length of larval stage.  The eggs and larvae are described by Stebbins (1985).  In the wild, eggs usually hatch in 2–4 d.  In the laboratory at 25 ˚C, embryos hatch in 2 d (Hall, 1998).  In the wild, larval development (hatching to the emergence of the first forelimb) is completed in about 47 d, with a range of 36–60 d (Morey, 1994).  Hall (1993) and Hall et al. (1997) describe larval development and report a larval period of about 31 d at 25 ˚C in the laboratory.  Brown (H.A., 1989b) reports a larval period of 36 d at 23 ˚C in the laboratory.  As with other Spea, the larval period is flexible.  Morey (1994, Chapter 2) reared tadpoles at 27 ˚C and was able to increase the larval period from 16–26 d by manipulating the food supply.  Morey and Reznick (2000) demonstrated that slow-growing larvae transform near the minimum size possible, while fast-growing larvae delay metamorphosis beyond the minimum, presumably to capitalize on growth in the larval environment.  In the wild, metamorphosis usually occurs from late May to September (Wright and Wright, 1949).

                        ii. Larval requirements.

                                    a. Food.  Specifics of the larval diet have not been reported.  The larvae of other spadefoot toads eat animal and plant foods and organic detritus (Pomeroy, 1981; Pfennig, 1990).  Great Basin spadefoot toad larvae are routinely reported to feed on conspecific carcasses (Linsdale, 1938) and carrion (Nussbaum et al., 1983).

                                    b. Cover.  Amount of cover varies.  Unlike other desert spadefoot toads that usually breed in turbid pools, Great Basin spadefoot toads often breed in clear springs and streams.  The amount of emergent vegetation varies from rain-filled pools that have been scoured free of vegetation, to playas and alkaline streams that are ringed with emergent vegetation but otherwise bare, to perennial springs that are choked with aquatic vegetation.  Hovingh et al. (1985) reported that successful breeding sites were characterized by being partially free of aquatic vegetation.

                        iii. Larval polymorphisms.  Cannibalism has been reported (Bragg, 1946, 1950f; Durham, 1956).  The carnivorous larval morph characteristic of some other spadefoot toad species (Pomeroy, 1981) does not seem prevalent in the wild.  Hall and Larsen (1998) and Hall et al. (2002) mention a carnivorous morph, but it has not been described in detail.

                        iv. Features of metamorphosis.  In nature, body mass at metamorphosis (Gosner stage 42; see Gosner, 1960) averages 3.6 g (range 1.8–6.5 g; Morey, 1994).  As with other Spea, once the front forelimbs emerge (Gosner stage 42), transforming larvae begin to make short, temporary excursions onto land even while still possessing a long tail.  The nature of these excursions, which in Great Basin spadefoot toads can occur by day or night, is not known, but may have something to do with avoidance of aquatic predators.  The time between emergence of the front forelimbs and the complete resorption of the tail is 2–6 d.  During this time, transforming individuals do not eat, losing 30% or more of their body mass and about 16% of their total fat reserves.  Cope’s (1889) much repeated observation of transforming juveniles, some with complete tails, hopping about on land and gorging on grasshoppers, stretches the imagination, because juveniles have great difficulty feeding on even small, slow-moving prey until tail resorption is complete or nearly so. 

                        v. Post-metamorphic migrations.  Juveniles emigrate from their natal site a few days to several weeks after metamorphosis.  Little is known about how far they travel or how they survive the harsh, dry conditions that are typical in the Great Basin when these movements usually take place.  Migrations by juveniles away from the natal site sometimes coincide with rainfall, but summer rains are unpredictable over much of the range.

            D. Juvenile Habitat.  Once they leave the margin of the natal site, the habitat characteristics of juveniles are probably similar to adults.  Juveniles and adults can be found together on roads on warm or rainy nights.

            E. Adult Habitat.  Found primarily in sagebrush country.  Also found in bunchgrass prairie, alkali flats, semi-desert shrublands, pinyon-juniper woodland to open ponderosa pine communities, and high elevation (2,800 m) spruce-fir forests (Nussbaum et al., 1983; Stebbins, 1985; Leonard et al., 1993).  Hall (1998) cites several other authorities on the habitat associations of Great Basin spadefoot toads.

            F. Home Range Size.  Unknown.

            G. Territories.  There is little evidence of agonistic or territorial behavior in Great Basin spadefoot toads.  Males seem to maintain individual space while chorusing.  Other Spea are solitary during periods of inactivity in burrows (Ruibal et al., 1969).

            H. Aestivation/Avoiding Dessication.  Great Basin spadefoot toads spend long periods of cold weather, generally October–March, in self-constructed, earth-filled burrows.  Nussbaum et al. (1983) indicate that mammal burrows may be used instead of self-made burrows, but no details are provided.  Great Basin spadefoot toads are similar to other spadefoot toads, which burrow as deep as ≤ 1 m (Ruibal et al., 1969) and survive osmotic stress during long periods of dormancy by accumulating urea in their body fluids.  This allows them to absorb water from the surrounding soil as long as the soil has a higher water potential than that of the body fluids (Shoemaker et al., 1969; Jones, 1980). 

            I. Seasonal Migrations.  Not known for juveniles and subadults.  Adults make seasonal movements to and from breeding sites.  These movements are usually nocturnal and do not necessarily coincide with rainfall.  Little is known about what proportion of the adult population moves to breeding sites each year or how far individuals move between the winter burrow and the breeding site. 

            J. Torpor (Hibernation).  From April–September, periods of inactivity are spent in shallow burrows; if it is not too cold, individuals can be encountered just after sunset with only their eyes protruding above the surface.  Svihla (1953) describes adults retreating during the day beneath rocks near a breeding site in Washington.

            K. Interspecific Associations/Exclusions.  One notable feature of the breeding sites used by Great Basin spadefoot toads is the absence of other amphibians.  In the western part of the range in California, the author has observed no other amphibians breeding at spadefoot sites, and sites occupied by breeding populations of either western toads (Bufo boreas) or introduced tiger salamanders (Ambystoma tigrinum) seem to be avoided by Great Basin spadefoot toads.  Of 151 sites inventoried by Hovingh et al. (1985), only one site contained another amphibian species.  An exception to this generality occurs in Deep Springs Valley, California, where Great Basin spadefoot toads and black toads (Bufo exsul) use the same breeding sites.  A fascinating experience in the Great Basin is the predictable appearance of intermountain wandering garter snakes (Thamnophis elegans vagrans) at breeding sites just as Great Basin spadefoot toad larvae reach their maximum size and approach metamorphosis.  Over about 1 wk, these predators eat large numbers of larvae (between Gosner stages 38 and 42).  Garter snakes usually ignore or are unable to capture smaller, less developed larvae, if any are present.

            L. Age/Size at Reproductive Maturity.  Unknown.  Morey and Reznick (2001) reared closely related western spadefoot toads under a variety of conditions in the laboratory and in outdoor enclosures and found that under high-food conditions, most males developed secondary sexual characters by the beginning of the first breeding season following metamorphosis.  Females reared under similar conditions made the transition from juvenile to adult dorsal coloration, but had small ovaries that had not reached the vitellogenic stage of the first ovarian cycle.  Thus, it seems reasonable that males mature in the first 1–2 yr after metamorphosis, while females probably are not sexually mature until at least the second breeding season after metamorphosis.  Nussbaum et al. (1983) speculated that individuals could achieve adult size by their third summer.  Nussbaum et al. (1983) report that males mature at a body length of about 40 mm; females mature at about 45 mm.  Stebbins (1985) reports adult body lengths of 37–62 mm.  Wright and Wright (1949) report adult males as 40–59 mm and adult females as 45–63 mm.  In California, males average 57 mm (range 51–65 mm, n = 18) and females average 57 mm (range 51–66 mm, n = 33; unpublished data).  Females under 51 mm were not reliably gravid.

            M. Longevity.  Unknown.  Hall (1998) describes a male that must have been at least 6–7 yr old.  Tinsley and Tocque (1995) analyzed skeletal growth rings to estimate age structure in a population of Couch's spadefoot toads (Scaphiopus couchii).  They estimated that females live ~13 yr and males ~11 yr in the wild.

            N. Feeding Behavior.  Tanner (1931, summarized in Whitaker et al., 1977) found the diet of Great Basin spadefoot toads consisted mostly of ants, with smaller proportions of tenebrionid beetles, adult and larval carabid beetles, larval dytiscid beetles (Coleoptera), Gryllidae, and Ichneumonidae.  Adults are generally nocturnal, but a number of authors (e.g., Linsdale, 1938) have noted that juveniles will feed in the open during the day.

            O. Predators.  Reports of predators on adult Great Basin spadefoot toads include rattlesnakes (Crotalus viridis), coyotes (Canis latrans; Wright and Wright, 1949), and burrowing owls (Athene cunicularia; Gleason and Craig, 1979; Green et al., 1993).  Larvae are preyed upon by American crows (Corvus brachyrhynchos; Harestad, 1985) and intermountain wandering garter snakes (Wood, 1935).  In the eastern Sierra Nevada, California, rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) sometimes gorge on larvae or transforming juveniles when rising stream waters flood quiet overflow pools where adults breed.  In 1989 near Mono Lake, California, I startled four snowy egrets (Egretta thula) as I approached a stream occupied by large larvae (Gosner stage 38–40).  All that remained of the entire cohort of larvae when I arrived were thousands of coiled intestines resting in the stream bottom and a few large surviving larvae that were injured and trailing long lengths of intestine.  Apparently the intestines were distasteful, and the egrets “popped” the larvae and flicked away the intestines, swallowing the empty carcass.  Only smaller, less developed larvae from a younger cohort were uninjured.

            P. Anti-Predator Mechanisms.  As with other Spea, injured or handled adults produce volatile skin secretions that cause an allergic reaction (sneezing and a runny nose) in some humans.  Stebbins (1951) and Waye and Shewchuk (1995) describe the smell as being similar to popcorn or roasted peanuts.  In the eyes or nose, the sticky skin secretions of an injured Great Basin spadefoot toad can cause a burning sensation.  Nussbaum et al. (1983) believe the skin secretions are noxious and probably repulse predators.

            Q. Diseases.  Unknown.

            R. Parasites.  Unknown.  Other spadefoot toads are host to polystomatid monogenean trematode parasites (Tinsley and Earle, 1983).  In nature, infections from these trematodes apparently do not lead to major disease outbreaks (Tinsley, 1995).

4. Conservation.  Great Basin spadefoot toads continue to be common in suitable habitats, however, patterns of abundance have been influenced by human activities.  Throughout most of their range, springs and streams have been dammed or diverted into ditches and impoundments, and reservoirs have created artificial water sources where natural water sources did not exist.  In some cases, Great Basin spadefoot toads have been able to capitalize on these changes, becoming more abundant than under pristine conditions; where urbanization, agriculture other than grazing, and other land conversions have destroyed or harmed habitats, Great Basin spadefoot toad populations have been extirpated or have declined.  In Colorado, Great Basin spadefoot toads are listed as a Species of Special Concern. 

            Acknowledgments.  Thanks to Sean Barry, Robert W. Hansen, and Michael Westphal for constructive comments on an earlier version of this account. 

1Steven R. Morey
U.S. Fish and Wildlife Service
911 Northeast 11th Avenue
Portland, Oregon 97232

Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.

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