Western spiderwort (Tradescantia occidentalis) COSEWIC assessment and status report: chapter 2

Executive Summary

Western Spiderwort
Tradescantia occidentalis

Species information

Western spiderwort (Tradescantia occidentalis (Britt.) Smyth), a perennial species, is a member of the Commelinaceae (spiderwort family). Only one other geographically separate species of this genus occurs in Canada, namely, Tradescantia ohiensis Raf. This species is restricted to southwestern Ontario.

Distribution

Western spiderwort is confined in the wild to western North America. In Canada, the species is restricted to disjunct sites in the southern prairie provinces. Western spiderwort is known from one site in southeastern Alberta, one site in southcentral Saskatchewan, and three sites in southwestern Manitoba.

Habitat

Western spiderwort sites are located on partly stabilized sand dune ridges, usually on south-facing steeper slopes but also on blow-out areas. There is usually some degree of active drifting sand associated with the sites. In Manitoba, the sand dune ridge systems can be fairly extensive. In Alberta and Saskatchewan, western spiderwort is restricted to small areas within larger partly stabilized dune complexes. Common species associated with western spiderwort sites are as follows: sand grass (Calamovilfa longifolia), chokecherry (Prunus virginiana), poison ivy (Rhus radicans), rose (Rosa woodsii), pasture sagewort (Artemisia frigida), prairie sagebrush (Artemisia cana), ground juniper (Juniperus horizontalis), bluebell (Campanula rotundifolia), buckbrush (Symphoricarpos occidentalis), silverberry (Elaeagnus commutata), June grass (Koeleria macrantha), Indian rice grass (Oryzopsis hymenoides), needle-and-thread (Stipa comata), golden bean (Thermopsis rhombifolia), cushion cactus (Coryphantha vivipara), spear grass (Stipa viridula), sand bluestem (Andropogon scoparius), blue grama (Bouteloua gracilis), and brittle prickly-pear (Opuntia fragilis).

Biology

Western spiderwort flowers from May to July and sets seed from early-August to the first snowfall. Each flower lasts only one day. The plant produces above ground shoot buds in the fall, and overwinters in this vegetative state. Flower color ranges from white to pink to the most common form, deep purple. The most common pollinator is the sweat bee. The plant has both fleshy and slender roots, an adaptation for survival in low moisture sand dune habitat.

Population sizes and trends

The total Canadian population is estimated to be currently about 22,000 plants. The majority of plants are found at two of the three Manitoba sites; namely, the Hellman site (9,422 plants) and the Loutit site (4,321).  The third Manitoba site has a population estimate of 619 plants. The Saskatchewan site has approximately 100 plants. The population at the Pakowki Sand Hills in Alberta had a population of 7 plants during 2002, a drought year, but rebounded to 7,450 plants during 2002 when ample precipitation was available.

Limiting factors and threats

Limiting factors have been noted to be cattle grazing, invasion by leafy spurge, shading/dune stabilization and human disturbance. Low to moderate cattle grazing has limited impact and may even serve to offset the effects of dune stabilization. Overgrazing is harmful but is not a concern at the present. Leafy spurge has invaded three of the five Canadian sites but western spiderwort is holding its own at present. Human disturbance of the fragile sand dune habitat through use of off-road vehicles, sand removal and oil exploration is problematic. The Alberta population is in danger of extirpation due to the effects of dune stabilization.

Special significance of the species

Western spiderwort is one of several important species of the fragile sand dune habitat of the southern Canadian prairie provinces. Species of spiderwort are grown horticulturally.

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.

Canadian Wildlife Service

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

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