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Plains Spadefoot

Illustration of the Plains Spadefoot The Plains Spadefoot (Spea bombifrons) is a nocturnal toad that spends most of its life underground. The spadefoot toads are not true toads but belong to the primitive family Pelobatidae. The Plains Spadefoot is considered a Blue List species in Alberta.

Spadefoot toad species are confined to the more arid regions of western North America. The Plains Spadefoot occupies habitats ranging from deserts in the southwest United States to aspen parkland in the Canadian prairies. In Alberta, the Plains Spadefoot primarily occurs in the Grassland Natural Region although there have been observations from the Parkland Natural Region. Within these Natural Regions, the Plains Spadefoot occurs in the Dry Mixedgrass, Mixedgrass, Northern Fescue, Foothills Fescue, and Central Parkland Subregions. Habitats in which it has been found include unvegetated sand dunes, sand dunes with willow and cottonwood, upland prairie, desert, short and mixedgrass prairie, and sagebrush. In Alberta, Plains Spadefoots have been observed in shortgrass prairie, mixed grassland and shrubland, fescue grassland, sand dunes, floodplains, and aspen parkland.

Throughout most of its range, the distribution of the Plains Spadefoot is strongly correlated with the presence of sandy, gravelly, or sandy loam soils. Likewise, the occurrence of the Plains Spadefoot in Alberta is strongly correlated with sandy soils. All known observations of Plains Spadefoots in the province originate from areas of sandy glacial outwash, sand dunes, and sandy stream channels. Similarly, all observations made of Plains Spadefoots during several pipeline projects in Alberta and Saskatchewan were in or adjacent to habitats with sandy soils. This affinity for sandy soils presents the Plains Spadefoot with limited breeding opportunities because sandy aeolian deposits are rapidly drained, thus inhibiting the development of wetlands. Plains Spadefoots therefore rely on pockets of fine-textured, less permeable soils, within sandier habitats where temporary wetlands are formed.

The Plains Spadefoot breeds in a variety of wetland types. Wetlands used for breeding are predominantly temporary with variable amounts of vegetation and may occur in native as well as tame habitats. On the Canadian prairies, Plains Spadefoots have been observed in partially flooded fields (predominantly agricultural fields), roadside ditches, flooded dugouts, shallow temporary wetlands in fallow fields, native prairie and tame pastures, sandy planted fields, temporary ponds in uplands, along streams, semi-permanent ponds, oxbow lakes, and stream meander channels. Nonnative habitats where breeding Plains Spadefoots have been observed include a construction site, flooded soybean fields and cornfields, a flooded wheat field and even driveways and bicycle paths.

Spadefoot Toad Spadefoot toads resemble true toads (Family Bufonidae) in body form but have smoother, thinner skin like that of frogs. Unlike true toads, Spadefoot Toads have a vertical elliptical pupil and no parotoid gland. The spadefoots are named for their large, sharp metatarsal tubercles that are used to dig backwards to depths of almost 1m. The Plains Spadefoot is active in Alberta from late May to fall, but it is seldom seen outside of breeding periods.

Spadefoot toads are well adapted to the dry conditions of deserts and prairies and breed opportunistically depending on suitable environmental conditions. Spadefoot toads emerge quickly and migrate to breeding wetlands during periods of heavy rainfall and warm temperatures. Although large choruses of Plains Spadefoots are associated with heavy rainfall, small choruses have been observed in southern Alberta in temporary wetlands that form as a result of light rain or snow melt. 

Spadefoots breed quickly in order to take advantage of favourable breeding conditions and to allow the eggs and larvae as much time as possible to develop. Males precede females to the breeding wetland and males generally outnumber females at any given time and place. The male Plains Spadefoot's call is loud and harsh and may carry up to 3 kilometres. In Alberta, single Plains Spadefoots could easily be heard from 1 kilometre away. Both sexes are greatly stimulated by the male spadefoot call and larger choruses attract more individuals of both sexes.

The breeding season of the Plains Spadefoot is not well defined. In Alberta and Montana breeding has been observed from early May through June. If suitable environmental conditions do not occur during the active season, Plains Spadefoots may not breed at all. During extended drought periods between 1978 and 1992, breeding did not take place during one or more consecutive years in the Milk River and Empress regions of Alberta. Conversely, breeding may occur more than once in a single year if conditions are particularly favourable.

During periods of drought, Plains Spadefoots do not remain underground for years at a time but, especially during humid weather, emerge to forage during evenings. As summer progresses and the soil becomes drier, Plains Spadefoots burrow deeper into the soil and emerge less frequently. During very dry conditions, Plains Spadefoots may burrow 60 to 90 centimetres below the surface. Plains Spadefoots will most often burrow along the edge of a solid object or near a plant that offers security or shade.

Plains Spadefoots burrow deeply to avoid freezing and desiccation during the winter. In Arizona, Plains Spadefoots have been found at depths of 91 cm during hibernation. There are no published accounts of hibernation depths in Canada. The Plains Spadefoot is intolerant of freezing and must burrow below the frost line during the winter. However, this species does exhibit super cooling to -4.3 degrees Celsius (toads did not freeze at temperatures as low as -4.3 degrees Celsius), which would help the Plains Spadefoot avoid freezing in shallower burrows during the winter.

Spadefoots forage above ground and prey on a variety of insects. Insect prey include flies, moths, beetles and various spiders.

Reprinted from Alberta Wildlife Status Report No. 25 (1999), with permission from Alberta Sustainable Resource Development.

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