Western toad (Bufo boreas) COSEWIC assessment and status report: chapter 2

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Executive Summary

The western toad (Bufo boreas) has an extensive range throughout western North America, including southern areas of the Yukon Territory, most of British Columbia, and western Alberta. It is one of only a few amphibians inhabiting alpine areas. Two subspecies are recognized, the most widely distributed of which occurs within Canada (B. b. boreas). The western toad lives nine to 11 years. It is an explosive breeder, congregating along the shallow margins of lentic breeding sites for a one- to two-week period each spring. Females lay between 5,000 to 16,500 eggs per breeding season once they reach sexual maturity at four to five years.  Males mature in three years. The black tadpoles metamorphose in approximately three weeks, at which time they typically form large aggregations along the shoreline. Western toads are highly philopatric; most males return to breeding sites annually whereas females return every one to three years. Females travel farther from breeding sites, moving 400 to 600+ m upland to summer ranges. Occasional long distance excursions of up to 7.2 km have been noted for this species. Summer home ranges are distinct, and approximately three to seven hectares in size. Toads are found in a variety of habitats, including forests, wetlands, clearcuts, and grasslands, with their summer ranges usually including a combination of upland and wetland areas. Toads exploit open areas, often basking in the sun to thermoregulate. Toads hibernate for roughly four to six months each winter in animal burrows and under debris where they remain in contact with moisture.

Although B. boreas is listed as secure in British Columbia and Alberta, it is the only IUCNred-listed amphibian species occurring in Canada. This is due largely to its status in southern parts of its range. Many populations in the United States have declined or become extirpated. The Southern Rocky Mountain B. b. boreas is a candidate for endangered species listing by the U.S. Fish and Wildlife Service. Red-leg disease, fungal agents that attack toad eggs (e.g., Saprolegnia), and UVradiation have all been proposed as potentially contributing to the decline of toads. Bufo boreas is vulnerable to mass reproductive failure, especially in areas with small, isolated populations. Newly emerged toadlets are also vulnerable to mass die offs. In addition, spring storms, summer drought and early fall freezing can reduce populations to critical levels, and small populations can be significantly impacted by predators in both terrestrial and aquatic environments.

The south-coast population of B. boreas should be considered to be of special concern in Canada because their numbers on the Lower Mainland of B.C.and eastern Vancouver Island (the Georgia Depression) appear to be declining. Long-term data sets are lacking, but encounter rates were low in all surveys conducted recently in these areas compared to historical records. The majority of Western toad populations in the Georgia Depression are isolated from other mainland populations due to the habitat fragmentation and alteration associated with high urban and agricultural development. Therefore, the potential for rescue through immigration is very limited. Severe and rapid toad population declines in the U.S. in recent decades demonstrate the vulnerability of this widespread and otherwise common species. With the cause of declines largely unknown, and with some of our B.C. populations exposed to similar stresses, including deformities and disease, there is an elevated concern for this species in areas where it has become less common. If trends continue, our interior and northern populations may serve as the stronghold of the species. The remainder of the western toad population is likely not at risk at this time but this should be re-examined on a regular basis.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) determines the national status of wild species, subspecies, varieties, and nationally significant populations that are considered to be at risk in Canada. Designations are made on all native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fish, lepidopterans, molluscs, vascular plants, lichens, and mosses.

COSEWIC Membership

COSEWIC comprises representatives from each provincial and territorial government wildlife agency, four federal agencies (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biosystematic Partnership), three nonjurisdictional members and the co-chairs of the species specialist groups. The committee meets to consider status reports on candidate species.

Definitions

Species

Any indigenous species, subspecies, variety, or geographically defined population of wild fauna and flora.

Extinct (X)

A species that no longer exists.

Extirpated (XT)

A species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A species facing imminent extirpation or extinction.

Threatened (T)

A species likely to become endangered if limiting factors are not reversed.

Special Concern (SC) ^Footnote1

A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events.

Not at Risk (NAR) ^Footnote2

A species that has been evaluated and found to be not at risk.

Data Deficient (DD) ^Footnote3

A species for which there is insufficient scientific information to support status designation.

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list.