Soapweed (Yucca glauca) COSEWIC assessment and status report 2013: chapter 3
Soapweed
Yucca glauca
Wildlife Species Description and Significance
Soapweed (Yucca glauca) is a long-lived perennial that grows as rosettes of long, sharp leaves. Each rosette can produce a single inflorescence up to 85 cm tall that typically contains 15 to 75 large, nodding, white flowers. Fruit are kiwi-sized and dehiscent, each usually containing six rows of flat, black seeds.
Soapweed engages in a mutualistic relationship with its pollinator, the Yucca Moth, whose larvae depend on Soapweed fruit as a food source; this is a relatively rare type of interaction. Soapweed is also the obligate host plant to the Non-pollinating Yucca Moth, the Five-spotted Bogus Yucca Moth and the Strecker’s Giant Skipper.
Soapweed and Yucca Moths reach their northern range limits in Canada, and have some unique characteristics that allow them to persist despite highly variable biological and environmental conditions. Alberta Soapweed have the longest documented flowering seasons for the species. Not all fruits of Soapweed reach maturity, but Soapweed plants in Alberta have the ability to selectively abort fruits with fewer moth eggs, thus maximizing the reproductive potential of the moths. Soapweed in Alberta have also been shown to be capable of self-pollination, and offspring show no evidence of early inbreeding depression.
Distribution
The native range of 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 two sites in Alberta, along the Milk River and its tributary, the Lost River. A previously reported small population of Soapweed near Rockglen, Saskatchewan is growing in natural conditions, and is included as a third Canadian population.
The nearest populations of Soapweed to the south are roughly 100-120 km away along the Missouri River in Montana. Rescue from those populations is unlikely because of the limited dispersal distance of Soapweed seeds and because most of the intervening habitat has been converted to strip-farming, and is unsuitable for Soapweed.
Habitat
Soapweed occupies well-drained, sparsely vegetated, south-facing coulee slopes along the Milk River drainage in southeastern Alberta and southwestern Saskatchewan. The area has hot, dry summers and low precipitation with large daily variation and weather extremes including high winds and heavy rains.
Coulee habitat of this nature is rare and naturally limiting to Soapweed. Intervening prairie, which is needed for range expansion, may have declined in quality for Soapweed because of fire suppression and declines in other forms of natural disturbance such as wallowing by bison.
Biology
Soapweed populations are maintained by asexual (clonal) production of rosettes and sexual reproduction through seeds. Soapweed seedlings flower at 20-25+ years of age, but a rosette derived asexually can flower within several years. Each rosette flowers once and then dies, although clones (genetically distinct individuals) are thought to live over 50 years. Sexual reproduction can only be achieved through an obligate pollination-seed predation mutualism with the Yucca Moth. In Canada, Soapweed reproduced almost exclusively asexually; very few seedlings are able to become established, and thus sexual reproduction currently contributes little to population persistence (though it is essential for survival of the mutualism with yucca moths.
Soapweed flowers from early June through September. Yucca Moths mate in Soapweed flowers during the day. Female moths disperse to other flowers at dusk, where they complete pollination and lay eggs in the flowers’ ovules.
Only about 10% of pollinated flowers mature into fruits; the rest of the fruits are selectively abscised (dropped before they mature). Based on surveys conducted on the population of Soapweed at the Onefour Research Station (Onefour), Soapweed produces 3-4 fruits per flowering stem
Population Sizes and Trends
Total number of Soapweed clones at Onefour was estimated as between 45,000 and 72,000 in 2006, which is consistent with the 1977 estimate of 55,000 clones. There has been some expansion of the Onefour population onto the adjacent prairie since the late 1970s (~1000 clones), likely in association with a prairie fire that reduced the density of competing grasses.
Total population size at Pinhorn has apparently been stable, with ~400-450 clones since 1977. However, improvements in clone identification in 2004 resulted in a revised estimate of 1366 clones. No mortality or recruitment of new clones has been documented since 1998.
The Rockglen population first discovered in 2000 contained six clones in 2000, 2005 and 2011.
Survival of mature clones was estimated at 0.997 at Onefour. Germination fluctuates greatly among years with increases in seedlings following large fruiting years and years with activities creating some disturbance of the substrate. The population growth rate at Onefour was estimated to be slightly positive at λ = 1.004 using population projection models.
Threats and Limiting Factors
Soapweed is naturally limited in Canada by its relationship with the Yucca Moth. Other possible limiting factors in Canada include herbivory by wild ungulates and insects, and extreme weather events such as high winds or heavy rains.
The primary sources of anthropogenic threats to Soapweed include habitat alteration through lack of disturbance (including fire suppression) and degradation through agriculture, oil and gas development and off-road vehicle use. Soapweed is collected for the horticultural trade and for medicinal use.
Protection, Status, and Ranks
Soapweed is listed as Threatened under Canada’s Species at Risk Act and as Endangered under Alberta’s Wildlife Act. Soapweed is the subject of an Alberta Recovery Plan and an Environment Canada Recovery Strategy. Soapweed is assessed as globally secure (G5) throughout its range and as critically imperiled in Canada (N1), and in Alberta (S1).
In Canada, the species occurs on public land that is managed by Alberta at the Pinhorn Grazing Reserve, and by Agriculture and Agri-Food Canada at the Onefour Research Substation. Its habitat is protected through critical habitat designation under SARA and regulations associated with Alberta natural areas. The land ownership of the Rockglen population is mostly private, with one plant on provincial land (roadside). Because this population has not previously been part of the species’ assessment, critical habitat has not been designated at this site.
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