© 2004 Stanley Trauth (1 of 16)
Spea bombifrons Cope, 1863
1. Historical versus Current Distribution. Plains spadefoot toads (Spea bombifrons) were originally described by Cope (1863) with type localities near Fort Union on the Missouri River (latitude 48 ˚N, on the Platte River 322 km [200 mi] west of Fort Kearney) and Llano Estacado, Texas (Degenhardt et al., 1996). Their range has been described as southern Alberta to northern Mexico, west into southeast Arizona (Shannon, 1953, 1957) or questionably to near Gila Bend (Walters, 1955), east to Nebraska and western Missouri, including central and western Oklahoma and southern Texas (Conant, 1958a). More recently, their range has been extended to include southwestern Manitoba (Preston and Hatch, 1986), new localities in southern Alberta (Lauzon and Balaus, 1998), Saskatchewan (Morlan and Matthews, 1992), the Missouri River Floodplain and Loess Hills in western Iowa (Huggins, 1971; Christiansen and Mabry, 1985; Farrar and Hey, 1997), and extending across Missouri along the Missouri River (Metter et al., 1970; Easterla, 1972; Femmer and Metter, 1979). Disjunct populations occur in extreme southern Texas and northeast Mexico (Conant and Collins, 1991), along the Arkansas River in Arkansas (Plummer and Turnipseed, 1982; Trauth et al., 1989a), and in southern Colorado (Conant and Collins, 1991).
2. Historical versus Current Abundance. Relative abundance has not been well documented. The local distribution and secretive nature of plains spadefoot toads make them difficult to census (Farrar and Hey, 1997). Plains spadefoot toads appear to be stable or perhaps increasing in abundance in certain regions, if conclusions are drawn based solely on range expansion records.
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Migration to breeding sites over distances of at least 1 km/night (Landreth and Christensen, 1971) occur during or following heavy rains.
ii. Breeding habitat. Ephemeral pools such as cattle tanks, flooded farm fields, or playa lakes.
i. Egg deposition sites. Eggs are deposited on the bottom or attached to partly submerged vegetation or other objects (Collins and Collins, 1993).
ii. Clutch size. Females lay up to 2,000 eggs in masses of 10–250 each (Collins and Collins, 1993). Hatching takes place in 20 hr at 30 ˚C (Justus et al., 1977).
i. Length of larval stage. Time to metamorphosis takes from 13–20 d (King, 1960; Voss, 1961), depending on temperature and other environmental factors. Larvae tolerate high (39–40 ˚C) and greatly fluctuating temperatures (Bragg, 1945a).
ii. Larval requirements.
a. Food. Some tadpoles are cannibalistic (see below) or carnivorous, taking small invertebrates; others eat algae and detritus and form aggregations for feeding (Bragg, 1965, 1967; Pomeroy, 1981; Black and Sievert, 1989; Farrar and Hey, 1997). Plains spadefoot toads are stronger competitors for fairy shrimp (Anostraca) than are Mexican spadefoot toads (S. multiplicata) when living in sympatry (Pfennig and Murphy, 2002).
b. Cover. Unknown, but larvae live in pools that are frequently murky with little aquatic vegetation (personal observations). Some larvae form aggregations for protection (Bragg, 1965a, 1967; Pomeroy, 1981; Black and Sievert, 1989; Farrar and Hey, 1997).
iii. Larval polymorphisms. Tadpoles occur as two trophic morphs (Bragg, 1956; Bragg and Bragg, 1958; Farrar and Hey, 1997) that appear to be similar in morphology and behavior to those described for Mexican spadefoot toads (Pomeroy, 1981; Pfennig, 1990). Carnivorous morphs are large, flat-headed tadpoles with enlarged jaw muscles, a long snout, and beak with upper cusp and lower notch. Omnivores have smaller jaw muscles, flat beaks, and round bodies. Carnivores develop more rapidly, metamorphose at a larger size (Pfennig et al., 1991; Farrar and Hey, 1997; Hey and Farrar, 1997), and metamorphose sooner than omnivores (unpublished data). Cannibalism is common, and the frequency of cannibalistic morphs may depend on kinship patterns (Pfennig and Frankino, 1997).
D. Juvenile Habitat. Newly metamorphosed animals burrow in mud along the edge of their natal ponds (Bragg, 1945a), hide in cracks in the hard earth, or seek cover in litter near the breeding site (Cornejo, 1982).
E. Adult Habitat. Nocturnal, feed on the surface under humid conditions. When not feeding they occupy shallow summer burrows or deeper winter burrows (Bragg, 1944). Plains spadefoot toads require loose, well-drained soils such as those found in floodplains, prairies, or loess hills in the northeastern part of their range (Christiansen and Bailey, 1991), and grasslands, sandhills, semi-desert shrub, and desert scrub in the southwest (Bragg, 1965; Hammerson, 1986).
F. Home Range Size. Unknown.
G. Territories. Unknown.
H. Aestivation/Avoiding Dessication. Burrow to depths necessary to remain moist (Hammerson, 1986). Summer burrows are shallow; winter burrows may be soil-filled and as deep as 4.6 m under dry conditions (Bragg, 1965). Rodent burrows may be used for overwintering (Hammerson, 1986). Emergence occurs during the monsoon season in the southwest and may be cued by low frequency sound, as is the case for Couch's spadefoot toads (Scaphiopus couchii; Dimmitt and Ruibal, 1980a). In more temperate regions such as Iowa, emergence can occur with spring rains as early as mid April when temperatures rise above 10 ˚C (unpublished data). Animals continue feeding throughout the summer on warm humid nights or after rainfall until mid September (Mabry and Christiansen, 1991).
I. Seasonal Migrations. Unknown.
J. Torpor (Hibernation). See also "Aestivation/Avoiding Dessication" above. Juveniles are thought to be intolerant of freezing and avoid freezing through burrowing and by having low supercooling temperatures (Swanson and Graves, 1995). These authors estimate that to avoid freezing during mild South Dakota winters, juveniles must burrow to depths of 20–50 cm.
K. Interspecific Associations/Exclusions. Three spadefoot toad species often use the same breeding pond in New Mexico (Cornejo, 1982; Woodward, 1984). Zones of interbreeding between Mexican and plains spadefoot toads have been described in Texas, New Mexico, and Arizona (Sattler, 1985; Simovich et al., 1991; Simovich, 1994). Hybridization has been induced in the laboratory between Mexican spadefoot toads and plains spadefoot toads (Forester, 1975) and may occur in nature (Hughes, 1965). Mating calls serve as reproductive isolating mechanisms (Forester, 1973). Two mating call types have been described for plains spadefoots toads (Pierce, 1976).
Throughout the Great Plains, plains spadefoot toads and Great Plains toads (B. cognatus) commonly use the same ephemeral breeding sites. Here, plains spadefoot toad tadpoles feed on the smaller Great Plains toad tadpoles (Bragg, 1940a). Plains spadefoot toads also have been reported to share breeding ponds with Woodhouse's toads (B. woodhousii; Collins and Collins, 1993).
L. Age/Size at Reproductive Maturity. Reproductive maturity is reached during the second year (Trowbridge and Trowbridge, 1937; Mabry and Christiansen, 1991). Iowa males range from 31–38 mm SVL, females 32–40 mm (Mabry and Christiansen, 1991). Females are also heavier than males (Collins and Collins, 1993). Mature males have keratinized nuptial pads on their thumbs.
M. Longevity. As long as 13 yr based on skeletochronology (Tinsley, 1997).
N. Feeding Behavior. Nocturnal, feeding on beetles, crickets, grasshoppers, ants, spiders, termites, moths, caterpillars, and other small arthropods (Bragg, 1944; Whitaker et al., 1977; Collins and Collins, 1993; Anderson et al., 1999b).
O. Predators. Hyrus triagularis (hydrophilid beetle larva), Apus sp. (a crustacean), western terrestrial garter snakes (Thamnophis elegans), and cannibalistic congeners feed on plains spadefoot toad tadpoles (Black, 1968). Predators of adults include prairie rattlesnakes (Crotalus v. viridis), Swainson’s hawks (Buteo swainsoni), and burrowing rodents (Hammerson, 1986).
P. Anti-Predator Mechanisms. Skin gland secretions may cause sneezing, mucous discharge, and watering eyes in people (Conant, 1975) and presumably could affect predators. Tadpole feeding aggregations only occur in the presence of predators and may deter them (Black, 1968).
Q. Diseases. Unknown.
R. Parasites. Tadpoles have been observed with Saprolegnia sp. fungus (Bragg and Bragg, 1957) and adults with intradermal mites (Duszynski and Jones, 1973). They may also serve as hosts for the monogean parasite, Pseudodiplorchis americanus, which is known to infect Couch's spadefoot toads.
4. Conservation. Plains spadefoot toads tend to be locally abundant and currently are not listed by any state or by the U.S. Federal Government. They are listed as Protected in Manitoba (Weller and Green, 1997); their status in Alberta is uncertain (Klassen, 1998). Plains spadefoot toads require loose, well-drained soils for burrowing and aestivating; grasslands (Anderson et al., 1999a) or grassy buffer zones for foraging; and shallow, sparsely vegetated, fish-free, ephemeral wetlands for breeding. In parts of their range, natural breeding habitat has been severely reduced by agriculture, industrial, and other types of floodplain development.
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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