Anaxyrus cognatus (Say in James, 1823)
Great Plains Toad
Basis for Listing
The Great Plains Toad (Anaxyrus cognatus) is widespread in the Great Plains of the United States and occurs in western Minnesota (Prairie Parkland and Tallgrass Aspen Parklands provinces) in open grasslands and cultivated fields. In parts of its range, populations appear more localized and isolated than historical records indicate. Reproductive activity is triggered by heavy spring rain events at which time adults emerge from burrows and move to shallow wetlands and flooded fields to breed. Because reproduction is dependent on rainfall, toads may forego breeding during years with insufficient precipitation. As a result, population densities of Great Plains Toads can undergo substantial fluctuations during periods of prolonged drought.
Amphibian and reptile surveys conducted by the Minnesota Herpetological Society and the Department of Natural Resources' Minnesota Biological Survey between 1985 and 2007 found mixed results for this species. Surveys of counties in far western Minnesota in 1985 and 1988 (Moriarty 1985; 1988) yielded no observations; however, these surveys took place during drought conditions, which may explain the lack of records. Records of Great Plains Toads were obtained in two of six counties in far northwestern Minnesota in the early to mid-1990s. The species appeared to be abundant in a few isolated areas of southwestern Minnesota during surveys in the 1980s and mid-to-late 2000s, though it was absent from several historical localities. The lack of records may be attributed, in part, to the difficulty in sampling for this species, which resides in burrows outside the breeding season.
Several potential threats, including loss and degradation of grassland habitats and associated wetland breeding sites raise great concerns about the future of Great Plains Toads in Minnesota. The draining of ephemeral prairie wetlands for the production of row crops has severely reduced breeding habitats (Minnesota's Remaining Native Prairie). As a result, reproduction of Minnesota’s populations now usually occurs in flooded agricultural fields. In recent years, only a small number of breeding choruses have been documented from natural habitats. In most years, flooded agricultural fields may dry-up before metamorphosis can occur. Other threats include the heavy use of pesticides and herbicides, road mortality, climate change, and disease. Given its unique life history characteristics and reduced habitat availability, the Great Plains Toad was designated a species of special concern in Minnesota in 2013.
The Great Plains Toad is large, measuring 11.4 cm (4.5 in.) for females and 9.5 cm (3.7 in.) for males. General coloration is brown, gray, or olive, with distinct pairs of large oblong blotches on the back. The blotches contain many small warts and are greenish, olive, brown, or gray, with light borders. The underside is white or cream and unmarked, except for a dark breast spot on some. A raised oblong parotoid gland is present behind each eye. Cranial crests on the head near the eyes connect to form a boss, or raised bump, near the snout. The crests wrap behind the eye and are in contact with the parotoid gland. A dark keratinized spade is found on each hind foot. During the breeding season, males have loose dark skin on their throat (vocal sac) and a dark patch on the inner surface of the thumb. When calling, the vocal sac is oblong and extends above the snout. Minnesota’s two other toad species lack the large paired oblong blotches on the back and have dark mottling on the undersurface. Males give an extremely long riveting metallic call that may last for 40 seconds or more. Larvae are dark, sometimes with a golden sheen, and have a clear tailfin.
In the American south and west, this species occupies deserts, shrub land, chaparral, and grassland habitats. In western Minnesota, it formerly occurred in the extensive dry tallgrass prairie and open grasslands but is now found primarily in agricultural areas and in tiny remnant prairies and grasslands. Breeding sites consist of highly ephemeral shallow water-filled prairie depressions with little or no emergent vegetation. Nearly all recent breeding choruses in Minnesota have been documented in flooded agricultural fields. Associated amphibian species in breeding habitats include the American Toad (Anaxyrus americanus), Canadian Toad (A. hemiophrys), Cope’s Gray Treefrog (Hyla chrysoscelis), and Boreal Chorus Frog (Pseudacris maculata); and in terrestrial habitats include the Eastern American Toad (A. americanus americanus), Canadian Toad, and Northern Leopard Frog (Lithobates pipiens). Open habitats, sometimes associated with sandy soils, are preferred for overwintering.
Biology / Life History
The Great Plains Toad is normally active from May to September. Individuals may emerge sporadically over a five-week period, often with the onset of heavy rain (Ewert 1969). When precipitation is great enough to fill prairie depressions and flood agricultural fields, adults move toward breeding sites. The breeding season may last from May to July, with a peak in June. Vocalization and mating occur only within brief periods in this range and are typically initiated by significant rainfall. Breeding choruses may last from a few days to two weeks. Males call nocturnally and are usually perched near vegetation. Females lay strings of up to 20,000 eggs that are fertilized externally. Incubation lasts 5-7 days, and the tadpoles metamorphose in roughly six weeks depending upon water temperature. Following breeding, adults disperse into the upland prairie, 300-1,300 m (984-4,265 ft.) from the breeding site, where they may feed or burrow into the soil (Ewert 1969). Surface activity of adults may vary depending upon local weather conditions. During wet periods, considerable movements can occur; but during dry years this species may not surface at all, skipping the breeding season and remaining underground through the summer and winter. Metamorphs and juveniles may remain on the surface to feed under slightly drier conditions than adults. The Great Plains Toad constructs its own burrows using the spade-like projection on each hind foot, reaching depths of up to 100 cm (39 in.) to escape freezing winter temperatures (Ewert 1969). Little is known regarding age at maturation or life expectancy. Predators include carnivorous mammals, birds, and snakes. When confronted, this species may inflate its body with air to make it appear larger and more difficult for predators to swallow. This species may give a chirping release call and urinate when seized. The parotoid glands produce a noxious substance that may be harmful or distasteful to predators. The primary diet consists of a variety of invertebrates, many of which are agricultural pests.
Conservation / Management
The preferred habitat of the Great Plains Toad is upland prairie and lowland prairie, both of which are scarce in Minnesota (Minnesota’s Remaining Native Prairie). Even more uncommon are the shallow ephemeral swales that, when filled with water, provide the necessary habitat for reproduction. In Minnesota, the Great Plains Toad typically avoids breeding in permanent water, so the construction of permanent wetlands is usually not a viable management option. In the absence of natural breeding sites, this species has adapted to utilizing flooded agricultural fields. These sites are unstable because they are frequently tiled to reduce flooding and they may dry before metamorphosis is complete. Therefore the protection of swales on managed prairie and grassland sites is important for Great Plains Toad reproduction. In the absence of existing swales, the restoration or creation of shallow depressions that temporarily hold water through metamorphosis may be beneficial. Chemical run-off of herbicides and pesticides ismay also be an issue for this species. Conservation or establishment of viable breeding sites in native habitat is necessary. During wet or humid weather, road mortality is a considerable concern. Culvert underpasses, with fencing placed in strategic locations, may reduce road mortality. Periodic burns on prairie and grassland tracts will benefit this species, if they are done ideally either before the toads emerge (April) or after they have burrowed down in the fall (September). Conducting surveys for the Great Plains Toad can be problematic, since it is detectable only under certain conditions. This species may not be observed for years during droughts, then suddenly appear in large numbers during wet years, as was the case in both 1937 and 1941 (Breckenridge 1944). Anuran (frog and toad) call surveys during or following thunderstorms in May, June, or early July are recommended to detect or monitor breeding sites. Nocturnal road surveys adjacent to appropriate habitat during heavy precipitation may also be effective in documenting this species.
Conservation Efforts in Minnesota
Although no work has been done to specifically benefit the Great Plains Toad in Minnesota, prescribed burns are regularly performed on prairie and grassland areas within its range. More research on the distribution and habitat preferences of the Great Plains Toad in Minnesota is needed to help guide conservation efforts. Research is also needed to determine the possible impacts of certain activities, such as the effects of cattle on wet prairie depressions that may serve as potential breeding habitat for the Great Plains Toad. Acquisition of upland prairie tracts, including sites with sandy soils would be beneficial. Tracts that include shallow depressions suitable as breeding sites should be given priority. Conservation efforts directed toward Blanding’s Turtle (Emydoidea blandingii) and shorebird habitats may benefit this species. Distribution surveys should be planned during periods of heavy spring rainfall.
Jeffrey B. LeClere (MNDNR), 2018
(Note: all content ©MNDNR)