Western toad (Bufo boreas) COSEWIC assessment and status report: chapter 9

Limiting factors

The fact that B. boreas populations have declined so severely in the southern half of the range, in so short a period, and in relatively pristine areas, is cause for concern. No satisfactory hypotheses have emerged to explain the declines. Mass mortality episodes of all life stages of toads have been recorded under a variety of conditions including predation, desiccation, UV radiation, and fungal infection. Climatic conditions each year can also have a major impact on recruitment (Jones 1999a). Spring storms, summer drought and early fall freezing have lead to mass mortality of young toads, larvae and eggs. Furthermore, Western Toads tend to lay their eggs in the same location within a breeding site, temporally and spatially (Blaustein et al. 1995). These factors, along with the persistence of PMAs, makes them vulnerable to mass predation (Sullivan 1994).

The southern region of B.C. is the most populated portion of the Western Toad’s range in Canada. High levels of agricultural and urban development have cause the species to be exposed to intense encroachment and/or its associated factors such as increased road traffic, habitat deterioration, isolation, pesticides, and disease, predation or competition with introduced exotics such as bullfrogs and stocked fish. Consequently, populations in the Georgia Depression Ecoprovince are fragmented and have likely suffered declines (e.g., Haycock and Knopp 1998; Dupuis 1998; Davis 2000). Small, isolated populations like this one are most vulnerable to extirpation from environmental stresses (Corn 1994; Blaustein et al. 1995).

Western toads may be particularly vulnerable to pathogens and disease. Davis (2000) suspects that the only known local extirpation of B. boreas within Canada may be linked to pathogens associated with fish, including the water mold, Saprolegnia. This same pathogen was linked to a 50 to 95% mortality rate of Western Toad eggs at three breeding sites in Oregon (Blaustein et al. 1994a). The authors suggest that populations stressed due to habitat degradation and/or increased UV-B may be more susceptible to infection. The communal egg laying habits of B. boreas increases infection rates (Kiesecker and Blaustein 1997).

A chytrid fungus has been identified from amphibians in Australia, Central America and North America, as the cause of amphibian declines of many montane amphibian species, including Western Toads (Berger et al. 1998, Daszak et al. 1999). In Colorado, B. boreas experienced population declines in the late 1970’s and the early 1980’s, and again from the late 1990’s to the present (C. Carey, pers. com.). Carey (pers. com.) found that four decreasing populations of B. boreas in Colorado in the 1990’s had chytrid infections from Batrachochytrium dendrobatidis at levels sufficient to kill amphibians exposed in laboratory studies. Based on similarities in the patterns of population declines and the presence of chytrids in museum specimens, Carey believes that the B. boreas declines witnessed in the late 1970’s and early 1980’s in Colorado were also due to this disease. Virtually nothing is known of how this pathogen kills its host, how it is spread among populations, or where it originated. This virulent pathogen has the ability to drive its’ host population to extinction because it can survive outside of the host on keratinized material after the population has died off, and it can reproduce saprophytically (Daszak et al. 1999). Batrachochytrium develops most rapidly at low temperature (Daszak et al. 1999), which may explain why montane populations are susceptible. A histologically similar pathogen, Basidiobolus ranarum, has been described in endangered wild Wyoming toads, Bufo baxteri (Taylor et al. 1999) and captive dwarf African clawed frogs, Hymenochirus curtipes (Groff et al. 1991), a widely introduced species in the United States in the late 1980s in ornamental garden ponds. The introduction of this latter species may have been involved in the dissemination of Batrachochytrium in North America.

Worrest and Kimeldorf (1975) exposed B. boreas tadpoles to high levels of artificial UV-B, resulting in spinal, corneal and epidermal deformities. In Oregon, studies have demonstrated greater mortality of Western Toad larvae exposed to natural levels of UV-B radiation compared to shielded larvae, potentially due to relatively low levels of photolyase within this species (Blaustein et al. 1994b, Hays et al. 1996). However, similar studies conducted in Colorado failed to find any effect of UV radiation on toad larvae (Corn 1998). Licht and Grant (1997) suggest that current levels of UVB are not high enough to support the hypothesis of UV alone as a causative factor of amphibian declines, but acknowledge that radiation levels will likely increase in the next decade and that some species may be more vulnerable to these changes. UV in combination with some stressor(s) may encourage infection by pathogens.

Amphibian deformities have received a lot of attention in the last decade. On Vancouver Island, Davis (2000) found increased deformities of toadlets along the shoreline relative to upland, and concluded that deformities may affect dispersal abilities of toads. The North American Reporting Center for Amphibian Malformations has two reports of deformed Western Toads from within Canada; two individuals from the Alberni-Clayoquot area (July 1998), and one in the East Kootenays (Sept. 1998). Over the past five years, T. Dickinson (pers. com.) has encountered numerous deformed Bufo boreas toadlets in the region of Isobel Lake, B.C. (51º 23'W 50º 51'N), approximately 5 km from Kamloops. He claims that the amphibians have a “higher than average incidence of limb deformity; 50% for Rana pretiosa and Hyla regilla, and 30% for Bufo boreas. In all cases there is a prevalence of multiple appendages at anywhere from the pelvic girdle to the metatarsals”. People have reported seeing deformed Western Toads in the Little Shuswap Lake area as well (T. Dickinson, pers. com.). The reasons for the deformities are unknown. Johnson et al. (1999) identified a parasite that caused high rates of deformities in Hyla regilla (15 to 45%) and B. boreas tadpoles in California. An aquatic snail (Planorbella tenuis) appeared to be the first host of the trematode parasite (Ribeiroia sp.), which infected the tissue around the pelvic girdle and hind limbs of the amphibians, resulting in abnormal or extra limbs. Under experimental conditions, even low parasite densities resulted in deformities and low survival rates of tadpoles. They suggest that the increased incidence of deformities we have been seeing in many amphibian species may be due to increased densities of one of the parasite’s host species and/or changes in the environment such as accelerated eutrophication due to organic pollution that has caused the snail population to increase.

Toads are often abundant in clearcut habitats both terrestrially and at breeding sites in many areas of B.C. However, the condition of individuals inhabiting these areas has not been studied. Ward and Chapman (1995) suspect that in northern regions, direct toad mortality from timber harvesting may be reduced because harvesting occurs in winter. Increased warmth in cut-over areas may be beneficial where the growing season is limited.On the other hand, cut-over areas may harbour more snakesand other predators (Raphael 1991), which could increase the predation rate on newly metamorphosed young. Studies are needed to address whether the increased abundance of toads in cut-over areas is a true reflection of habitat suitability or whether these areas act as reproductive sinks. Because toads are attracted to open areas, they may spend a great deal of time on roads, which increases mortality. The small metamorphs are particularly vulnerable, and easily decimated. For example, Wind (unpublished data) observed large numbers of toadlets trapped in road ruts 20-30 cm deep that were heavily used by all terrain vehicles. The toads either entered the ruts when they were attempting to cross the road, using the road for basking, or they were attracted to the water trapped inside the ruts. The mass die off of toadlets in northern B.C., mentioned earlier, may have been influenced by the presence of the road. Davis (2000) found road surveys an effective way of sampling Western Toads.

The relatively high rate of development and fragmentation associated with south-coastal areas of B.C. has likely pushed Western Toad populations in that area to a stress level that has made them particularly vulnerable to all of these contributing factors.

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