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Canadian Toad

Limiting Factors

Illustration of a Canadian Toad Conditions that lower habitat quality or decrease survival and reproduction are considered to be limiting factors. Various factors may affect Canadian Toad populations including the Disturbance of Hibernacula, the impacts of the Forestry Industry, Wetland Loss and Alteration as well as Global Changes

Disturbance of Hibernacula:  The probable inability of individuals of this species to tolerate freezing, along with their poor burrowing ability, restricts areas where Canadian Toads can over winter. Toads may therefore be vulnerable to disturbance of hibernation sites by human activity. Because relatively high concentrations of toads occur at some hibernation sites, there is potential for large numbers of individuals (perhaps entire populations) to be adversely affected by such disturbance.

The large hibernaculum on the Alberta/Northwest Territories border is subject to disturbance by vehicles during the winter. Nevertheless, this hibernaculum remains active, indicating that the level of disturbance is not high enough to prevent toad use. However, areas of the site which may be otherwise suitable for toads are not used, possibly due to disturbance by snow plows and mortality of toads on the adjacent road has been reported during the period of spring emergence. Specific threats to hibernacula in Alberta have not been documented, probably because few of these sites have been discovered.

Recent threats to hibernacula may not be restricted to human activity. Western Canada has experienced declining snow cover in recent years which may allow deeper frost penetration. Such conditions may increase mortality of hibernating Canadian Toads.

Forestry Industry:  Canadian Toad populations in the boreal forest may be vulnerable to disturbance resulting from logging, which has only recently begun on a large scale in this area. The impact of widespread logging in the boreal forest on the abundance and distribution on Canadian Toads remains unknown, but several ongoing studies in this region are attempting to investigate the potential impact of such activities.

The aquatic nature of adult Canadian Toads suggests that they may be relatively unaffected by forest harvesting so long as buffer strips are retained around water bodies. The extent of movement away from water bodies, both during the summer, and during migration from breeding sites to hibernacula is largely unknown in this area. However, hibernacula may be several hundred meters from water, and recent information suggests that the species may make extensive use of upslope habitat for post-breeding activities. Canadian Toads may therefore be most vulnerable to forest harvest during the nonbreeding season.

Wetland Loss and Alteration:  Canadian Toads may be relatively restricted to more permanent water bodies during adult and juvenile life history stages. Any loss or alteration of these water bodies may therefore adversely affect Canadian Toad populations. One such impact is drought, such as occurred in Alberta during the 1980s. However, the effects of drought are relatively short term, and should not have long-lasting effects on total populations. Wetland loss to drainage, and modification of wetland basins from agricultural activity are much more likely to have severe and permanent impacts on Canadian Toad populations.

In the last 50 years, the impacts on wetlands in the southern half of Alberta have been dramatic. For example, it is estimated that about 60% of wetlands in the aspen parkland have been drained during this period, and over 90% of wetlands in the prairie and parkland have now been modified by agricultural activities. Although the specific impact of wetland alteration and loss on the Canadian Toad population in Alberta is unknown, it is notable that most changes to wetlands have occurred in areas of the province (grassland and aspen parkland) where declines in Canadian Toad populations have been reported.

Global Changes:  On a global scale, the widespread simultaneous decline of numerous amphibian populations has been linked to a number of factors, including increases in ultraviolet radiation reaching the earth's surface, the spread of pathogens, habitat destruction, acidification of water bodies, and climate change. It is possible that one or more of these factors are important influence on populations of Canadian Toads in Alberta. For example, global increases in ultraviolet light resulting from thinning of the ozone layer have been reported, particularly at high latitudes. Eggs of the Western Toad are known to be vulnerable to damage by UV-B radiation and the Canadian Toad may be similarly susceptible. Pathogens have been linked to declines in Western Toads in Colorado, and "red leg" disease has been reported in declining populations of both the Western and Canadian Toads in Wyoming, and in Northern Leopard Frogs in Alberta. However, it is not clear whether these, or other, factors have been responsible for apparent population declines of the Canadian Toad in Alberta.

Reprinted from Alberta Wildlife Status Report No. 12 (1998), with permission from Alberta Sustainable Resource Development.

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