Western toad (Bufo boreas) COSEWIC assessment and status report: chapter 7


Aquatic habitat

Western toads breed in a variety of natural and artificial aquatic habitats, with or without tree or shrub canopy cover, coarse woody debris, or emergent vegetation. They breed in ponds, stream edges, or the shallow margins of lakes (Olson 1992, Reimchen 1992, Corkran and Thoms 1996), as well as in ditches and road ruts (Gyug 1996, E. Wind and L. Dupuis, pers. obs.). Recorded oviposition depths range from 5 cm to 2 m, but depths greater than one metre are rare (Corn 1998). Adult toads may oviposit in the same locations within breeding sites each year, irrespective of water depth (Olson 1992). Livo (1999) found that toad breeding sites in Colorado had significantly fewer predators such as beetles and salamanders, than sites without and that site selection was a balance between water temperature and the presence of predators. In support of this, toad larvae aggregate in the warm, shallow margins of lakes during the day to accelerate their developmental rate (Poll et al. 1984) and seek cover among emergent vegetation (Olson 1992. They disperse to deeper waters at night (Livo 1999).

The practice of stocking fishless lakes may be one of the biggest threats to Western Toads, where population declines can occur unnoticed in relatively pristine, uninhabited areas. Fish stocking has occurred extensively throughout North America. Although unpalatable amphibian species that contain skin toxins, like toads, may not be directly threatened by fish predation (Kats et al. 1988), they are threatened by diseases associated with fish, including Aeromonas (Carey 1993) and Saprolegnia (Blaustein et al. 1994a, Kiesecker and Blaustein 1997). Davis (2000) hypothesized that the extirpation of Western Toads at Jordan Meadows on Vancouver Island may have been due to the introduction of diseases associated with stocked fish.

The potential declines of toads in the Lower Mainland and on southern Vancouver Island are also the result of habitat degradation and loss. Approximately 75% of wetlands in the Greater Vancouver and Victoria areas have been converted to agriculture and development (Nowlan and Jeffries 1996). Assuming that the remaining 25% of wetlands are suitable for toads, populations are likely isolated and extremely vulnerable to stochastic events. Gibbs (2000) determined that increased urbanization is associated with fewer and more isolated wetlands, beyond the scale at which most amphibian dispersal operates. He calculated that wetlands at least 0.4 ha in size must be protected to maintain the metapopulation structure of most wetland organisms (Gibbs 2000). In B.C., current legislation affords no protection to wetlands less than 0.25 to 1 ha, depending on their location within the province.

Western toads are also exposed to various contaminants in both urban and rural areas. In Quebec, Ouellet (unpub. data) found higher proportions of deformed amphibians in ponds in agricultural areas contaminated by pesticides than in ponds in untreated areas. Pesticides can increase mortality rates and deformities, and reduce growth rates of larval amphibians (Bridges 2000). This is likely true for Western Toads as well. Western toads have been found in the tailings ponds of mines, attracted by the relatively warm waters (Brinkman 1998, R. Brodie, pers. com.). Larvae experience reduced growth when exposed to even low levels of cadmium (Brinkman 1998). At high elevations, decreased growth could affect recruitment and overwinter survival (Jones 2000).

The degradation of riparian habitat can also negatively impact Western Toads. Like many amphibian species, newly emerged Western Toads congregate in riparian areas before they disperse upland (Jameson 1956, Richter 1997, E. Wind, pers. obs.). The sandy shorelines of lakes are often utilized as recreation and cattle grazing areas. Cattle can have a major impact on the structural integrity of the riparian habitat and the survival of young amphibians through trampling and grazing (Friend and Cellier 1990). Davis (2000) found a greater incidence of deformities among Western Toadlets in riparian areas than upland. He concluded that predators, people, and freshwater clams might have injured toadlets, which in turn affected their dispersal abilities (Davis 2000).

Gyug (1996) surveyed amphibian larvae in wetlands of forests and clearcuts from 1993 to 1995, and found no correlation between the proportion of breeding sites used by toads, distance to the nearest forest, or time of harvest. He suggests that the major impact of forest harvesting on aquatic-breeding amphibians may be the creation of small breeding ponds of under 0.02 ha and at least 20 cm deep that potentially act as population sinks. Created as a result of industrial activity ditches and wheel ruts become more attractive to amphibians because they warm early in the spring, yet even under normal air temperature and precipitation levels, they dry up too early for successful metamorphosis. Furthermore, they usually lack thermal and predatory cover for developing larvae, in the form of downed wood or emergent vegetation. In the absence of such sites amphibians would have bred in natural areas, leading to improved reproductive success. This attraction to ponds in clearcuts may have long-term implications for population persistence (Gyug 1996, Waldick et al. 1999). Conversely, road ruts have been actively used as an amphibian conservation tool in some areas, such as Kentucky, and appear to be effective where their depth and surface area are suitable (Adam and Lacki 1993).

Terrestrial habitat

Outside of the breeding season, adult toads move to summer ranges that often include wetland habitats not necessarily used for egg laying (Jones 1999a). Radio-tracked toads use terrestrial habitats up to 90% of the time (Bartelt and Peterson 1994). Adults disperse into forested areas, wet shrublands, avalanche slopes, and subalpine meadows (Poll et al. 1984). Western toads appear to favour dense shrub cover, where they are protected from desiccation and predation (Bartelt and Peterson 1994, Davis 2000). Toads are often found in clearcuts, and may favour these habitats to closed canopy forests in coastal habitats (Raphael 1988, Dupuis 1998, Davis 2000, Matsuda, unpublished data) and interior habitats (Ward and Chapman 1995, Gyug 1996). Radio-tracked toads in Colorado utilized spring seep areas and roadside ditches in proportion to their availability, and selected for lake and rocky, grass habitats (Jones 1999a).

Toads make use of their ‘pelvic patch’, area of specialized epidermis, to take up moisture from their surroundings when in terrestrial environments (Green and Campbell 1984). They take shelter during the day by burying themselves in loose soils or hiding in animal burrows (Erhardt 1996, Davis 2000). They also occupy other microsites, including moist depressions (Green and Campbell 1984), dense ground vegetation, and tree root tangles (Davis 2000).

Western toads use a large variety of upland habitats within their home ranges. They do not appear to be dependent upon mature or old-growth forest and are frequently found within cut-over areas. It is unclear what the long-term effects of forest harvesting might be on the population dynamics of this species. Their use of dense shrubs for thermal and predatory cover may preclude them from dense young forest stands, which are typically characterized by an undeveloped understory (Franklin 1988). Populations on the south coast may in part be declining in response to a combination of urbanization and a decreased proportion of early seral stands (Davis 2000; Huggard, pers. com.).

Western toads in the southwestern B.C. are likely isolated from mainland populations due to their geographical location and urbanization. Eastern Vancouver Island and the Lower Mainland are the most heavily populated areas of B.C. Rapid housing and industrial development, as well as extensive transportation corridors, are increasingly isolating wetlands in these areas. In addition, Vancouver Island populations are physically isolated from mainland populations, and Lower Mainland populations have limited immigration potential due to the occurrence of the Fraser River that bisects the area. Fragmentation of metapopulations resulting from land development may result in a loss of genetic diversity and increase susceptibility to disease in remaining populations.

Habitat protection needs

State listing of Western Toads in Colorado, New Mexico and Wyoming provides the species itself with some level of protection, but not the species’ habitat. Western toads do not receive any specific habitat protection measures in Canada, either. Western toads require wetland and upland habitat, and movement corridors between these areas for their survival. Relatively shallow wetlands that retain water for the 3 months from early spring until mid-late summer (Stebbins 1951) are important for breeding and should be protected. Buffers around breeding sites have been suggested for amphibian management (Semlitsch 1998), but their effectiveness in maintaining Western Toad populations has not been determine. Buffers may be insufficient given that post-metamorphs need upland habitats with some measure of suitable cover to reduce exposure to climatic extremes, and that population viability requires that some individuals disperse between wetlands. Habitat corridors between wetlands would likely be beneficial but the extent of protection they would offer, and the details for optimal design unknown. Overwintering areas must also be protected, however, nothing is known of hibernacula requirements for this species in Canada. Based on montane populations in Colorado, upland areas near seeps, stream banks, and underground burrows are important areas used for hibernation (Jones and Goettl 1998).

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