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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
stevemorey@fs.fed.us
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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Citation: AmphibiaWeb. 2023. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 26 Mar 2023.
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