Soapweed (Yucca glauca) COSEWIC assessment and status report 2013: chapter 10
The two Alberta populations have been monitored repeatedly over the last several decades using a variety of approaches. Methodologies for estimating abundance of Soapweed have varied greatly from 1976 to 2011 (see Abundance).
Onefour
Abundance of Soapweed at Onefour has been determined on three occasions using different methods and producing highly variable estimates. In 1976, B. Milner estimated the population to contain about 55,000 clones by trying to count each individual along a 2 km stretch of coulee (Milner 1977; B. Milner pers. comm. 2011). In 1998, a second survey attempted to count all clones in the population using a spotting scope from high vantage points, and a hand-held tally clicker, and estimated rosettes per clone using randomly placed belt transects (2 X 50 m) along the coulee slopes (Csotonyi and Hurlburt 2000). Using these methods, roughly 28,000 rosettes among 8,500 clones were estimated, although the authors acknowledge that small clones and those hidden by rolling terrain would not be visible using this technique. In 2006, a third survey was conducted using restricted random sampling and 103 – 2 X 20 m quadrats. This approach yielded an estimate of 45,000 to 72,000 clones and 154,000 to 247,000 rosettes (Bradley et al. 2006). Estimates from 1977 and 2006 are roughly consistent with one another despite differing methods, while the low numbers estimated in 1998 are likely an artifact of the methodology used in that survey rather than an indication of actual fluctuation in population size.
There are some indications that the population has spread onto the prairie uplands since Milner's work. From 1999 to 2003, a large proportion of the clones growing on the uplands flowered for the first time placing them at about 20-25 years of age (Hurlburt 2004). Although Milner (1977) made no mention of Soapweed growing on the prairie, only mentioning plants on the coulee slopes, recent examination of old photos suggests there were a few sparsely distributed clones north of the prairie rim trail (B. Milner pers. comm.). Onefour staff reported that a large prairie fire in the late 1970s burned vegetation up to the coulee edge, and may have created more favourable conditions for germination due to a reduction in competition from grasses (Hurlburt 2004). Bradley et al. (2006) reported 4,004 rosettes among 1,107 clones on the prairie side of the coulee rim trail. It is believed that most of these clones resulted from germination after the fire (Hurlburt, pers. obs.).
Pinhorn
Population abundance of Soapweed at Pinhorn Grazing Reserve was estimated in 1976, 1998 and 2004 using differing methods. In 1976, Milner (1977) counted about 450 clones. In 1998, 1,383 rosettes among 404 clones were counted (Csotonyi and Hurlburt 2000), and in 2004, 3,510 rosettes among 1,366 clones (Foreman et al. 2006). The 2004 survey included a patch of 19 mature clones that were not located in 1998. Methodological differences concerning the identification of clones contributed to differences in abundance between years, and in particular, the high numbers reported by Foreman et al. (2006) may be an overestimate. A population assessment in 2011 did not locate any seedlings at the site (Hurlburt 2011), which suggests that the increase in abundance in 2004 was not due to recruitment. The estimate for the number of clones at this site is based on 1998 data (404 clones, Csotonyi and Hurlburt 2000), as these are comparable to estimates for the Onefour site, with the addition of the 19 clones from the previously uncounted patch (Foreman et al. 2006), resulting in an estimate of 423 clones.
Rockglen
The Rockglen site has contained the same six clones since its discovery in 2000, and was revisited in 2005 and last observed in 2011 (D. Henderson, pers. comm. 2012). The number of rosettes has not been assessed at this site (D. Henderson, pers. comm. 2012).
Populations of Soapweed are not expected to exhibit much fluctuation among years in mature life history stages because of their 50+ year longevity and low mortality. The survival rate of mature clones was estimated to be 0.997 at Onefour over the period from 1999 through 2007 (Hurlburt 2004). Germination fluctuates greatly among years with increased seedling establishment following years of high fruiting and disturbance events (e.g. fire, or increased erosion following heavy rain events). Stage-based projection models and elasticity analyses suggested that Soapweed at Onefour had a slightly positive population growth rate (λ = 1.004) and that stasis in adult reproductive stages was more important to the persistence of the population than the recruitment of new seedlings (Hurlburt 2004).
A critical component of Soapweed population health in the long term is the persistence of the mutualism and the degree of benefit achieved through the interaction with the Yucca Moth. Measures of mutualism health, including viable seeds per fruit and fruits produced per clone, are highly variable among years and sites (Table 1). Data collected between 1998 and 2003 for Onefour and the closest populations in Montana are used as benchmarks for population maintenance and recovery (ASYMRT 2006). The mean number of fruits per clone at Onefour was lower in 2007 (Hurlburt 2007; Table 1) and 2011(Hurlburt 2011) than previously reported; however, fruit production in other years since 2003, such as in 2006, was high but not estimated quantitatively (Bradley et al. 2006). Fruiting rates at Pinhorn have increased since 2004 (there was no fruit production between 1998 and 2003), but remain significantly below those in the closest populations (Table 1).
Soapweed populations in Canada are approximately 100 km from the nearest known native populations located along the Milk River in Montana. There may be closer populations in Montana, but tributary valleys branching north from the Milk River have not been searched. Canadian populations are isolated from United States populations by distance and unsuitable habitat, including heavily converted cropland dominated by strip farming. Immigration from populations to the south is unlikely to occur naturally.
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