Great Plains Toad
The abundance and status of the Great Plains Toad
(Bufo cognatus), a moderately large toad
species, which ranges into the southeastern comer of Alberta, has been of concern to observers for several years. The most recent provincial status report on the species, prepared for
World Wildlife Fund
Canada, recommended these toads be classified as threatened. The species is included on the
Red list of species which are at risk of declining to nonviable population levels in Alberta.
Recent biological surveys, such as those on the Canadian Forces Base Suffield and through the Alberta Amphibian Monitoring Program, have led to the discovery of a number of new populations of this
species in Alberta.
Great Plains Toads are primarily a grassland species, and occur throughout the grasslands of central North America, from short- to tall-grass prairie.
In Alberta, Great Plains Toads inhabit the dry grasslands in the southeastern corner of the province. This area endures wide climatic extremes, experiencing hot, dry summer conditions and long, cold winters. The area is part of the northern extension of the North American Great Plains, and in southern Alberta is generally classified as mixed-grass prairie. These toads have been described as inhabiting the sand plain and sandhill
habitat types within the mixed grasslands of southeastern Alberta.
The quality and availability of breeding sites is presumed to be a critical component of the persistence of the species in Alberta. Great Plains Toads prefer temporary, clear, shallow pools such as those found in flooded fields, ditches, and buffalo wallows; usually with considerable vegetation protruding. The clarity of the water in breeding sites is important - these toads will not breed in muddy or turbid waters.
In Alberta, the majority of breeding ponds used by Great Plains Toads are seasonal, although some permanent and semi-permanent water bodies may also be used. These temporary water bodies form in the spring, when winter runoff and spring rains have filled prairie depressions, creating shallow sloughs that generally dehydrate later in the summer. Permanent and semipermanent water used for breeding sites include one or more of the following sources: springs, irrigation projects, waterfowl management projects, and dikes in shallow drainages. These more permanent breeding sites remained productive even through long periods of drought, and may be important habitat areas.
Tall emergent vegetation, such as cattails and bulrushes, was found to be uncommon in these breeding ponds (although some shorter, more sparse species were found in more permanent habitats) but submerged vegetation,
such as pond weeds, was abundant in some ponds in non-irrigated areas. Most breeding ponds influenced by irrigation waters were found
to have algal growth. These clumps of algae were used as calling perches by males, and are thought to be used as egg attachment sites and for shelter of tadpoles.
The breeding sites located in irrigated areas appear to be more predictably productive for Great Plains Toads than those located in non-irrigated regions; this is likely due to either seepage from canals, or water tables which have risen following irrigation. These irrigated regions provide shallow breeding sites even under drought conditions.
On the Canadian Forces Base (CFB) Suffield, a relatively large area of native grassland in southeastern Alberta reserved for military purposes, this species was found to occur in a wide variety of water body types, with the exclusion of alkaline sloughs. Great Plains Toads were found in seasonal water bodies, including the flooded margins of dugouts. They were not found in permanent springs.
Great Plains Toads are a relatively large toad species, with head-body (snout to vent) length between 45 - 110
millimetres. In Alberta, the species is described as having a background colour of pale brown-grey or olive, with an irregular pattern of darker spots.
The vocal sac of the males is sausage-shaped when inflated, extending from the throat area for up to 1/3 the length of the body. When deflated, the dark, loose skin of the vocal sac is perhaps the easiest way to identify the males from females. The breeding call of the males is a rapidly repeated, long (up to 50 seconds), harsh, metallic-sounding trill that can be heard from great distances and can be extremely loud at close range. Calling begins
approximately 30 minutes or more after sunset and breeding activity appears to be most intense within the first three hours thereafter.
Generally, breeding activity in the Great Plains Toad has been strongly associated with rainfall. The species is recorded to have an extended breeding season because of this, ranging from March to September, in the central part of its distribution.
These toads may form large breeding assemblages and lay their eggs in a communal mass, often with remarkable site allegiance from year to year. Females have been documented to produce between approximately 1,300 to 45,000 eggs with an average-sized female producing a clutch of just under 9,400 eggs. These eggs, laid in long loose strings over vegetation, hatch in one to five days, depending upon temperature. Tadpoles take anywhere from 18 to 49 days to metamorphose, which is also temperature-dependent. Extremely high tadpole mortality, due to pond desiccation and competition for food, occurs frequently. In conditions where high tadpole densities and cool conditions prevail, even a 45-day larval period may not be enough for metamorphosis to be reached. Low densities and higher temperatures have the opposite effect, causing rapid development but increasing the chance of desiccation through evaporation of the breeding pool. Tadpole mortality has been found to be exceedingly high, with overall tadpole survival reported to be very low in this species, over the entire breeding season.
Newly metamorphosed toadlets will remain in the area of their natal pond for about a month or until it desiccates, ranging out to feed. After the pond dries up, the young toads leave, and do not remain in the area. Juveniles are distinguished from other toad species by their numerous small, brick-red tubercles. It takes young toads from three to five years to reach reproductive age.
Great Plains Toads are very capable burrowers and may spend a great deal of time underground, especially in response to high temperatures. When threatened, these toads inflate themselves, lower their head and extend all four legs as a defensive posture. This posture is probably only assumed when threatened by natural predators, as it has not been noted by those handling these toads in the field in Alberta.
Several of the adaptations of the Great Plains Toad for survival in semiarid environments have been widely studied.
Bufo cognatus was found to be tolerant of a wide range of temperatures when compared with other toad species. Although undocumented in Alberta, Great Plains Toads probably burrow deeply to avoid freezing over the winter. A recent study of freezing tolerance, which included this species, found that they were intolerant of any internal freezing.
The survival of this species of amphibian in such a dry environment has been suggested as being related to longevity; these toads are estimated to survive around 10 years or more routinely, and perhaps live as long as two decades. Perhaps this longevity is the key to their survival, being an adaptation to wait out long dry periods between successful breeding episodes.
Though mostly nocturnal these toads may be active during the day, especially after rain, and they have been described as often almost social in their feeding activities.
Bufo cognatus consume large quantities during favorable periods, then burrow and remain dormant in hot, dry weather. In Alberta, solitary adults have been found foraging at night during periods when it was hot and dry in the daytime.
Great Plains Toads are opportunistic feeders, but apparently avoid eating earthworms, even when plentiful. Ground-dwelling, nocturnal insects are the most commonly consumed prey of Great Plains Toads. Examples of the prey taken by these toads included beetles, ants and spiders, in descending order of relative occurrence. Tadpoles feed on algae and decomposing insect remains or vegetation. The newly metamorphosed toadlets will feed on almost any small insect they can swallow.
Reprinted from Alberta Wildlife Status Report No. 14
with permission from Alberta
Sustainable Resource Development.