Western toad (Bufo boreas) COSEWIC assessment and status report: chapter 6
Population Size and Trend
There are no long-term data sets, or abundance estimates available for this species to provide an estimate of the current numbers of Western Toads within Canada. Based on the species’ extensive range, and the frequency of occurrence reported for this species in British Columbia and Alberta, we estimate that the toad population within Canada is greater than the critical limits set by COSEWIC for listing a species as threatened or endangered, i.e., more than 10,000 individuals occupying an area > 5,000 km².
Western toads can appear to be very common and abundant. Female toads lay thousands of eggs each per breeding season, which results in an abundance of tadpoles at breeding sites. But young toads experience high mortality rates and populations fluctuate extensively from year to year in response to climatic factors. In montane areas of Colorado, individual breeding sites recruit in one out of three years at best (Jones 1999a). Populations can also be cyclical. For example, at a pond in the Blackwater Creek drainage in Pemberton, B.C., there appears to be a major population boom every 5 to 10 years (Dupuis, pers. obs.). Without long-term monitoring it is impossible to estimate current population numbers.
In Alberta, the range of B. boreas appears to be expanding, potentially replacing the Canadian toad, Bufo hemiophrys, which has declined throughout its range within that province (C. Paszkowski, pers. com.).
In the interior of British Columbia, Western Toads are the most widely distributed of all amphibians in Kootenay National Park (Poll et al. 1984), the Prince George Forest District (Kinsey and Law 1998), and the Williston and Dinosaur Reservoir Watersheds west of Fort St. John (Hengeveld 2000), especially at high elevations. Although commonly encountered along the mid-coast of B.C., there are reports of apparent declines or extirpations of Western Toad populations in the more heavily populated south coast of the province (Dupuis et al. 1995, Haycock and Knopp 1998, Dupuis 1998, Beasley et al. 2000, Wind 2000, 2001, B. Matsuda, unpublished data, K. Ovaska, pers. com.), within the Georgia Depression Ecoprovince. Haycock and Knopp (1998) surveyed 94 ponds over an extensive area of the Fraser River Lowlands for amphibians in 1997 and noticed a marked decline in the abundance and frequency of encounter of Western Toad populations, compared to historical figures (R. Haycock, pers. com.). Western toads were once common and abundant on southern Vancouver Island (Carl 1944, Hardy 1949, Oliver 1973). Many long-term residents in the Parksville area claim that toads are less abundant now than 30 years ago (Davis 2000). Hardy (1949) described them as “…by far the commonest amphibian encountered” at Jordan Meadows (an extensive wetland area; 0.5 x 4.5 km), just north of Victoria. Western toads were abundant here during the mid to late 1970’s (approximately 0.8 toads per person hour of searching) but few were encountered during the mid 1980’s, and none were found during intensive searches conducted between 1997 to 1999 (Davis 2000). Records from Jordan Meadows represent the only documented local extirpation of B. boreas within Canada. In the Queen Charlotte Islands, some toad populations may be threatened by predators. Populations along lakeshores on Moresby Island were very abundant during the breeding season up until the early 1990’s, when they were decimated by raccoon predation (Reimchen 1992). Adult treefrogs have been observed feeding heavily on Western Toad eggs (Erhardt 1996). Both raccoons and Pacific treefrogs, Hyla regilla have invaded Moresby Island from Graham Island where they were introduced (Reimchen 1992).
Toad populations in the United States have declined rapidly in many areas (Table 1). Corn et al. (1989) failed to find toads at 49 of 59 known localities within Colorado and Wyoming. The range of the toad in Wyoming may be experiencing elevational contractions (Livo and Yeakley 1997). The species is considered endangered in Colorado and New Mexico and is a federal candidate species because the population in the Southern Rocky Mountains has dropped precipitously over the last 20 years (Jones 2000). There may only be one breeding population left in New Mexico, where only one adult and tadpole were seen in 1996 (Jones 2000). In Colorado, there are four metapopulations of toads composed of two or more breeding sites, with several dozen to several hundred toads each, for a total of 37 known breeding localities (Jones 2000). Researchers are attempting to reintroduce toads to historic but currently unoccupied sites within this state (Jones 2000). There have been severe declines in some parts of Washington. Toads are uncommon in the Puget Lowland ecoregion and the mountain meadows of the North Cascades (Leonard et al. 1993, Hallock and Leonard 1997). The status of toad populations in Utah is unclear, largely because extensive surveys have not been conducted, especially in unpopulated areas (Ross and Esque 1995). In 1992, Drost and Fellers (1996) revisited sites surveyed for amphibians in 1915 in the Sierra Nevada’s of California. Toads had been very abundant at most survey locations previously, but only small numbers of toads were found at one of the six previously occupied sites. The reason for the declines remains unclear but the authors suggested that insecticide use in agricultural valleys below the mountains, drought, and fish stocking may have been factors. In Oregon, there was a severe decline of B. boreas at two sites in the Cascade Range in the early 1990’s, with entire cohorts of eggs and larvae lost. This prompted several studies investigating potential causes such as UV radiation, disease, drought, etc. (Olson, 2001). There has been a steady recovery of these two populations since that time and the collapse some had predicted has not taken place (Olson 2001). The recovery of these two populations appears to be related to the longevity of the species and the alternating breeding schedule of the adults (Olson 2001). Although it is the montane populations of B. boreas that have declined in some areas, not all high elevation populations are threatened, potentially because of the variable ecology montane populations demonstrate across their range (Olson 2001).
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