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Great Plains Toad

Limiting Factors

Illustration of a Great Plains ToadThere are five main limiting factors which may affect Great Plains Toad populations in Alberta; drought, predation, habitat alteration and destruction, hydrological changes, and road kills

Drought:  Prolonged drought conditions potentially limit Great Plains Toad numbers. Without adequate rainfall, many breeding sites, especially in non-irrigated areas, may not even contain water. In Alberta, drought conditions are thought to have resulted in a large decline in Great Plains Toad populations between the mid- 1970s and late 1980s.  Desiccation of breeding ponds, prior to tadpole metamorphosis, was a factor, which likely limits the population size. However, drought conditions are a normal part of the prairie environment, and it is probable that the life history of Great Plain Toads is well adapted to such circumstances.

Predation:  Western Hog-nosed Snakes have been observed to prey upon adult Great Plains Toads on at least two occasions. Western Hognose Snake Crows and other birds are potential predators of adult Bufo cognatus as well. Tadpoles may fall prey to birds, insect larvae, and spadefoot tadpoles. Tadpoles of species that breed in temporary ponds, including Great Plains Toads in New Mexico, were subjected to higher predation when they occurred in more permanent water bodies. This occurred because there was a higher number of aquatic predators in the more permanent pools and because of the nearly constant movement patterns of the tadpoles of the temporary-pond species. Perhaps similar predation factors may play a role in some Alberta populations, especially in areas with irrigation or water management projects, where more permanent ponds are kept up for agricultural or municipal reasons, but this remains unknown. 

Habitat Alteration and destruction:  Wetlands in Alberta have been affected heavily by agricultural practices. In the periodAgricultural Activity between 1981 and 1985, evidence of agricultural impacts were apparent in an average of 66% of wetland basins and 93% of wetland margins in Alberta. These impacts have undoubtedly reduced the available breeding habitat for Great Plains Toads, as well as other amphibian species. In southern Alberta, the widespread construction of dugouts, to contain runoff water for agricultural or other purposes in the depressions within which temporary wetlands form, has likely contributed to the degradation of some breeding sites. Dugouts are designed to accumulate and store the water from the immediate area into a deeper pond having less surface area for evaporation. However, if the dugouts have a low shoreline gradient with shallow marshy edges they may remain suitable as breeding habitat for Bufo cognatus.

Dugouts are not generally suitable as breeding pools for Great Plains Toads because of their depth and steep sides, as well as frequent visitations by livestock. The disturbance caused by livestock walking into the water may cause the pond bottom to become stirred up, clouding the water, therefore making it even less appealing as a breeding site for Bufo cogliatus. Tadpoles, however, have been documented to have fully developed in waters that were disturbed by livestock after egg laying and hatching had occurred. The hummocking that can occur in such areas may also pose a threat to these and other amphibians by entrapping them in the deep hoof prints or simply by making it difficult for them to access or exit the pond. Long-term intensive use of wetlands by cattle could cause lasting damage, especially to shallow pools and permanent spring areas. Such lasting damage, in the form of introduced plant species, hummocking of shorelines, and disturbance of sandy substrates, may further reduce the breeding habitat available to these toads.

Water is a precious commodity in the semiarid southern part of Alberta. The clear, temporary, springtime pools preferred by The Great Plains Toad for breeding may be compromised by the development of some water management projects. Any water control that results in deeper, more permanent water in a breeding habitat constitutes a potential threat. The majority of water management projects in southeastern Alberta, such as those constructed for waterfowl habitat, or for agricultural, industrial or domestic purposes, generally strive to collect and maintain water in permanent catchments. The overall effect of these practices on Great Plains Toads remains unknown.

Conversely, some water management projects, such as shallow pools which form behind dikes, or pools formed through seepage or spillage from irrigation canals, are used as breeding sites during periods of drought. These pools, though generally still ephemeral, are much more consistently available on an annual basis, than those in dry land areas and as such are of benefit to the species at that time.

The disruption or removal of native vegetation by cultivation and the accompanying introduction of pesticides and herbicides, and alteration of the native hydrology of the site, may degrade or destroy breeding sites of Great Plains Toads. Tadpoles of five species of anurans, subjected to varying levels of exposure to pesticides or herbicides commonly used in Canadian crop and forest lands, either were paralyzed or died. 

Large numbers of young toads move into cultivated fields, where the soil was softer, to burrow if native prairie soils have become dried and hardened. However, it is suggested that one of the two leading causes of mortality of these toads in the United States is being run over in cultivated fields by tractors pulling discs.

Hydrological Changes:  The construction of dams and canals, Crop Irrigation and the practice of irrigation itself, combined with rising domestic and industrial water demands over much of southern Alberta, has presumably affected the hydrology in many parts of the region. Naturally occurring permanent-type wetlands that are fed by groundwater discharge may be important breeding sites during periods of drought. Many of these sites may have already been compromised by human-related demands for water in the region.

Road kills:  The phenomenon of large numbers of toads all moving in one direction along roadways has been noted, presumably capturing insects as they went. If similar habits occur in Alberta populations, the possibility exists that large numbers of these toads could be killed within a short time span by passing vehicles. In fact, earlier records that documented the presence of this species in Canada, were acquired by the identification of toads which had been run over. Road kill, especially of migrating juvenile toadlets, has been suggested as one of the most significant causes of mortality for post-metamorphic Great Plains Toads in the central part of their range although no reports of such losses have been recorded in Alberta.

Oil and Gas Exploration and Development:  Oil and gas development is a major industry in Alberta, and many activities associated with hydrocarbon extraction may impact Great Plains Toads. Disruption of groundwater resources, ground and surface water contamination, and the consumptive use of these water sources by drilling, are all potential threats to Great Plains Toads in Alberta. In addition, on CFB Suffield in particular, toads and other small animals may fall into, and become entrapped, in the sunken gas well caissons.

Reprinted from Alberta Wildlife Status Report No. 14 (1998), with permission from Alberta Sustainable Resource Development.

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