Soapweed (Yucca glauca) COSEWIC assessment and status report 2013: chapter 7
Soapweed is a perennial species of the semi-arid regions of the Great Plains of North America. The species occurs from Texas north to Alberta, from the Rocky Mountains east to the Mississippi River (Figure 3, Fairbarns 1985). Less than 1% of the global range occurs in Canada. Soapweed’s obligate pollinator/seed predator, Yucca Moth, pollinates several other species of Yucca, and thus has a broader distribution ranging from southern Texas to Canada and from the Rocky Mountains east to the Atlantic seaboard (Pellmyr 1999).
Figure 3. Current global range of native Soapweed
From Hurlburt 2001.
Description of Figure 3
Map showing the current global range of native Soapweed. The Soapweed extends throughout the Great Plains from Texas north to Alberta and from the Rocky Mountains east to the Mississippi River. In Canada, Soapweed occurs in Alberta, along the Milk River and its tributary, the Lost River, and near Rockglen, Saskatchewan.
Soapweed is known to occur in two populations in southeastern Alberta. One population is near Onefour, AB along the Lost River drainage, a tributary of the Milk River. This population is within the boundaries of the Onefour Research Station managed by Agriculture and Agri-Food Canada. The second population is on the Pinhorn Grazing Reserve south of Manyberries (Figure 4).
Figure 4. Known native populations of Soapweed (Yucca glauca) in Canada
Description of Figure 4
Map showing the location of native populations of the Soapweed in Canada. The Soapweed occurs at two sites in Alberta. One population is near Onefour, along the Lost River drainage, a tributary of the Milk River. The second population is on the Pinhorn Grazing Reserve south of Manyberries. A previously reported small population of Soapweed near Rockglen, Saskatchewan is growing in natural conditions and is included as a third Canadian population.
There have been two reports of additional populations of Soapweed in Alberta. Olson (1976) reported Soapweed growing about 11 km SE from the known Pinhorn location along the Milk River, on a rough map. Targeted searches were performed in the area from 1999 through 2008 by Dr. Johan Dormaar, but the site was never relocated. Discussions with Olson did not provide any new information. Likewise, A. Fischer in Csotonyi and Hurlburt (2000) reported a population of Soapweed "about an hour north of the United States border along Highway 4", but this population was never confirmed. These two reports are unconfirmed, and therefore are not considered further in this assessment.
A new occurrence for Soapweed that includes 6 clones was discovered in 2000 near a roadside in Rockglen, Saskatchewan (D. Henderson pers. comm. 2001). In 2003, one flowering stalk observed at Rockglen appeared to have enlarged pedicels, indicating a likely pollination event, and suggesting that Yucca Moth may also have been present (ASYMRT 2006). Although reported as introduced from Montana 15-20 years ago (Environment Canada 2011), the plants at Rockglen occur in undisturbed habitat similar to that of the two Alberta populations (unglaciated upland on native prairie) and include individuals with one to multiple rosettes and evidence of past flowering (Hurlburt, D., pers. comm. 2012, Henderson, D. pers. comm. 2012). COSEWIC Guidelines on Manipulated Populations (COSEWIC 2010) state that even if these individuals were accidentally or intentionally introduced, they can be included as part of the assessment if they are self-sustaining and occur in a natural setting within the natural range of the species. Reproduction in Soapweed includes production of new rosettes (as well as sexual reproduction), and therefore, this set of individuals appears to be self-sustaining. With respect to whether the Rockglen site is part of the natural range, the fact that there are populations of Soapweed in the areas of Montana to the south of both the Alberta and Saskatchewan sites is taken as evidence that the Rockglen site falls within an area that can be considered part of the native range. As a result, the Rockglen population is considered to be part of the wildlife species, and the clones that occur at this site are included as part of the total population in Canada. (There is no evidence that the Rockglen population includes self-sustaining populations of Yucca Moth, Non-pollinating Yucca Moth, or Five-spotted Bogus Yucca Moth, and thus, this site is not considered a location for the moths.)
In contrast, several isolated, small patches of Soapweed are excluded from the assessment. These are either growing in horticultural settings, or do not show evidence of being self-sustaining, and therefore, following COSEWIC Guidelines for manipulated populations, these are not considered in assessing the status of the species. These plants are reported to be transplants originating from Onefour, Pinhorn or the United States, and occur across southern Alberta (Fairbarns 1985; Csotonyi and Hurlburt 2000; Hurlburt 2001, 2007; Saunders and Ernst 1998) and Saskatchewan (Fairbarns 1985; Hurlburt 2001). Soapweed plants, likely derived from wild Alberta stock, can be found growing in gardens with examples known from Etzikom, Medicine Hat, near Manyberries, near Wild Horse, and Lethbridge, AB, as well as in Fox Valley, SK. The plants in Lethbridge occur on coulee slopes, but are not known to have produced rosettes or fruits, and therefore are not considered self-sustaining at this time. At Fox Valley, between 10 and 50 Soapweed plants from potted plants of unknown origin occur in a 5 km square (Maher et al. 1979; Fairbarns 1985; Hurlburt 2001). Two recent occurrences of single Soapweed clones along roadways in southeast Alberta are of unknown origins (Bradley et al. 2006; Hurlburt 2007), but are not considered self-sustaining. Some of these sites may merit inclusion in future assessments.
Plants at the Etzikom Windmill Museum, Etzikom, AB, Police Point Park, Medicine Hat, AB and in several private gardens near Pinhorn and Onefour, have been observed to produce fruit regularly with emergence holes, indicating that the Yucca Moth is also present (ASYMRT 2006, Foreman et al. 2006).
The extent of occurrence has been estimated at 2040 km2, including the Rockglen site, and the index of area of occupancy is 16 km2. The biological area of occupancy of Soapweed in Alberta is 185 ha based on critical habitat designation in Environment Canada (2011).
From 1998-2011, several thousand search hours have been expended looking for native Soapweed populations in suitable habitat in Alberta and Saskatchewan, particularly along the Canada-United States border (D. Hurlburt, pers. obs.). To date, additional search effort has only resulted in the identification of one new population, at Rockglen, SK. Because Soapweed is readily identifiable by the public and highly visible when flowering, and new reports are often provided by non-biologists or naturalists as incidental observations. To date, most reports represent non-natural occurrences.
In 2007 and 2008, a media campaign was launched to encourage the public to report yuccas that had fruited in gardens, as a means of tracking the prevalence of Yucca Moth in Alberta (D. Johnson pers. comm. 2011). Despite the increase in attention through newspapers, newsletters and websites across southern Alberta, no new native plants or populations were reported, although incidents of fruiting in gardens were documented in Lethbridge and Magrath in 2007 (D. Johnson pers. comm. 2011, E.B. Cunningham pers. comm. 2011, P. Harris pers. comm. 2011).
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