Western spiderwort (Tradescantia occidentalis) COSEWIC assessment and status report: chapter 10

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Evaluation and Proposed Status

Existing protection or other status

COSEWIC designated western spiderwort as "threatened" based on the status report by Smith and Bradley (1990). The province of Manitoba designated the species as threatened in 1994 (Goulet 1997). Western spiderwort is listed by Alberta and Saskatchewan as a species of concern (ANHIC 2001, Harms, Ryan and Haraldson 2001).

Assessment of status and author’s recommendation

There are five known populations of western spiderwort within Canada. All populations are disjunct from their American counterparts and, for the most part, from each other. Three of the five populations are in Manitoba on two dune fields; namely, the Lauder Sand Hills (two populations, 1.5 km apart) and Routledge Sand Hills (1 population). The Alberta and Saskatchewan sites contain smaller disjunct populations. Two of the Manitoba sites contain a substantial population base, namely, the relatively newly discovered (1995) Eastern Population (Lauder Sand Hills) and the Routledge Sand Hills population. The total Canadian population is estimated to contain in 2001/2002 about 22,000 plants.

Threats to the survival of the species in Canada include: cattle grazing, invasion of habitat by leafy spurge, shading/dune stabilization, and human disturbance. The fragile sand dune habitat is particularly susceptible to damage as a result of human disturbance (off road vehicles, sand removal and oil operations). In most cases the species is holding its own against the invasion of its habitat by leafy spurge in Saskatchewan and in Manitoba at the Western Population (MHHC) in the Lauder Sand Hills and at the Routledge Sand Hills. Cattle grazing has appears to have an impact on the species but in moderation does not apparently pose a severe threat and may be somewhat beneficial in negating the effects of dune stabilization.

A recommendation of 'threatened' is tendered based upon the low number of Canadian sites, the considerable fluctuation in population sizes and the ongoing concerns regarding the sensitivity of the sand dune habitat to varied disturbances.