Western spiderwort (Tradescantia occidentalis) COSEWIC assessment and status report: chapter 4

Distribution

Global range

Western spiderwort is distributed in the central United States, from Texas, New Mexico and Arizona in the south to Montana and the Dakotas in the north (Figure 2). Western spiderwort probably occurs in northern Mexico, but no records are available. The species occurs in Canada in four disjunct areas, with the Manitoba sites being more contiguous with the main range of the species to the south (Smith 2001).

Figure 2. North American distribution of western spiderwort.

Figure 2. North American distribution of western spiderwort.

Canadian range

Western spiderwort occurs in the provinces of Alberta, Saskatchewan and Manitoba (Figure 3). It is considered rare in all three provinces. The species occurs at only four sites in these three prairie provinces, with the sand dune habitats subjected to disturbance from grazing and invasive weeds. Substantial populations of the species occur at two of the three Manitoba locations and at one Alberta site.

Figure 3. Canadian distribution of western spiderwort.

Figure 3. Canadian distribution of western spiderwort.

Legend:

  1. Pakowki Lake Sand Hills (Manyberries site), Alberta.
  2. Elbow Sand Hills (Douglas Provincial Park site), Saskatchewan.
  3. Lauder Sand Hills (includes Western and Eastern populations), Manitoba.
  4. Routledge Sand Hills (includes the ESA and surrounding lands), Manitoba.

Alberta

Western spiderwort occurs only at the Pakowki Lake Sand Hills near Manyberries in southeastern Alberta.

Saskatchewan

Western spiderwort occurs at only one site in south-central Saskatchewan in the Elbow Sand Hills, east of Diefenbaker Lake in Douglas Provincial Park.

Manitoba

Western spiderwort occurs at the Routledge Sand Hills and at two locations in the Lauder Sand Hills (western population, including Manitoba Habitat Heritage Corporation (MHHC) site and an eastern population) in southwestern Manitoba. The eastern population, first discovered by Ken DeSmet in 1995, is approximately 1.5 kilometres northeast of the MHHC site. The three Manitoba sites have a total coverage of 12 quarter-sections of land. The Manitoba populations are isolated from those in the adjacent United States, and represent a northern extension of the species' distribution. Several additional sand hill regions in southwestern Manitoba have been searched without success (Hohn 1994, DeSmet 1995, Goulet and Kenkel 1997).

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