Rubber boa (Charina bobttae) COSEWIC assessment and status report: chapter 2

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COSEWIC Executive Summary

Species information

The Rubber Boa, Charina bottae, is a small (75 cm) member of the family Boidae and the only boid in Canada. There are two subspecies of Rubber Boa, the Northern Rubber Boa (Charina bottae bottae) and the Southern Rubber Boa (Charina bottae umbratica)Only the Northern Rubber Boa is found in Canada. The Rubber Boa is easily distinguished from other Canadian snake species by its brownish colouration, rubbery appearance (due to its small, smooth scales), and short, blunt tail that resembles a second head.

Distribution

The Rubber Boa is found throughout western North America from British Columbia south to California, and eastward as far as Montana, Wyoming and northwestern Colorado. Within Canada, populations of the Rubber Boa are patchily distributed within major river basins in the southern third of British Columbia. The only population that has been studied in Canada is in the Creston Valley Wildlife Management Area.

Habitat

Rubber Boas occupy a wide variety of habitats including riparian, grassland, montane forest and, occasionally, vacant city lots. The major habitat requirement of the Rubber Boa are rocky outcrops and an abundance of coarse woody debris which the snakes use for protective cover and to aid in thermoregulation. Rubber boas spend a considerable amount of time underground in abandoned rodent burrows and rock crevices.

Biology

The Rubber Boa is a nocturnal feeder and is active at temperatures much lower than the majority of reptile species (6-28°C); however, in Creston Valley the daily preferred temperature was 30°C. Low temperature activity may be the result of a trade-off between the benefit of feeding at night when fewer snake predators are active and the cost of being active at less than optimal temperatures. Female Rubber Boas give birth to 2 to 8 offspring in late August and early September. Clutch frequency is less than annual and perhaps as infrequently as every 4 years. Rubber Boas are long-lived, surviving from 20 to 30 years in the wild and longer in captivity.

Population sizes and trends

The number and size of Rubber Boa populations and subpopulations in Canada are unknown. Rubber Boas do not seem to be particularly abundant in the Creston Valley Wildlife Management Area, despite being in an area that is considered prime habitat for the species.

Limiting factors and threats

Rubber Boas may be limited in their distribution by the short summers experienced in much of Canada that do not provide pregnant females with enough warm days to complete embryonic development. Where populations do exist in Canada, the population growth rate may be low due to the combination of late maturation and small litter size and, therefore, slow to respond to disturbance. Forestry, agriculture and urban development all reduce the quality and amount of habitat available to the Rubber Boa, particularly if rock outcrops are damaged or destroyed and coarse woody debris is removed from the affected areas.

Special significance of the species

The Rubber Boa is the only member of the ancient family Boidae in Canada, and one of only two boids found outside the tropics and subtropics. It is of interest because of its unusual appearance, its apparent rarity in Canada, and its ability to forage nocturnally at very low body temperatures.

Existing protection or other status designations

Globally, the Rubber Boa is ranked G5 (Secure – Common, typically widespread and abundant) and in the United States it is ranked S5 or S4 in all states except for Wyoming (S2S3). In British Columbia, the species is ranked S3S4 (Vulnerable, but apparently secure -- uncommon, but not rare, and usually widespread. Possibly cause for long-term concern) and is on the provincial Blue List -- vulnerable and at risk. The Rubber Boa is also ‘Identified Wildlife’ under the Forest Practices Code, therefore forest licensees are legally obligated to manage harvest areas and protect critical habitat for the species.

Summary of status report

The Rubber Boa is widely distributed across the northern edge of its range (British Columbia); however, sightings of the species are very rare even when substantial effort is expended in search of the snake. Behavioural traits, such as noctural feeding and thermoregulating under cover, make the Rubber Boa appear rare because it is difficult to capture, or the species may be truly rare in Canada. The distribution and reproductive potential of the Rubber Boa is likely limited by length of the summer (reproductive) season, as short and cool growing seasons do not allow females to complete development of their offspring before hibernation. Assuming that the Rubber Boa is in fact rare in Canada, the species’ patchy distribution and low reproductive potential make it vulnerable to local extinctions where habitat quality is reduced by forest management practices, agriculture or urban settlement, or where increased urban road development leads to higher mortality rates.

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.