Soapweed (Yucca glauca) COSEWIC assessment and status report 2013: chapter 9

Soapweed populations are maintained through asexual and sexual reproduction (Csotonyi and Hurlburt 2000); those at the northern edge of range are maintained predominantly through the asexual production of new rosettes or ramets (Hurlburt 2004). Lack of reproduction by seed prevents the species from expanding into new habitats (Csotonyi and Hurlburt 2000). Seeds germinate in spring or early summer, about 10 months after dispersal from the parent plant (Hurlburt 2004). A single rosette derived from seed is thought to become mature at about 20-25+ years of age (Figure 4; Hurlburt unpubl. data); whereas a rosette derived asexually can flower within several years. Each rosette is capable of producing a single inflorescence (Kingsolver 1984; Hurlburt 2004). After flowering, individual rosettes die (Kingsolver 1984); clones, however, likely persist in excess of 50 years (Figure 8). A closely related yucca, Yucca angustissima, is known to live over 100 years in the Grand Canyon area of Arizona (Bowers et al. 1995). Fewer than 1 in 5000 seeds germinated under natural conditions, almost always in areas of disturbed or eroded soils (Hurlburt 2004). Seeds only remain viable for one year and there is no seedbank (Webber 1953; McCleary and Wagner 1973).

Figure 8. Growth habit of Soapweed showing multiple rosettes, which each flower once and subsequently die.

Photo of the Soapweed (see long description below).

Photo: D. Hurlburt.

Description of Figure 8

Photo of the Soapweed showing multiple rosettes with flower stalks. Flowering appears to have finished, and the leaves on some of the rosettes have yellowed.

The Yucca Moth, as the sole pollinator, is essential to the survival (over evolutionary time) and sexual reproduction of Soapweed. The plant and moth have an obligate mutualistic relationship where the Yucca Moth pollinates the Soapweed, and the Soapweed provides food (i.e., seeds) and incubation sites for developing Yucca Moth larvae (Riley 1873, 1892). Obligate mutualistic systems are those relationships in which each partner requires the other to survive or reproduce, and as a result, each derives a net benefit from the interaction (Addicott 1995).

Based on observation from Alberta, most Soapweed plants in Canada flower between early June and late July, with peak flowering occurring in late June and relatively few plants flowering into early September (Hurlburt 2004). Canadian populations exhibit longer flowering seasons than those reported anywhere else in the species’ range (Hurlburt 2004) and exhibit large interannual variation in flowering rates (Figure 10). During the flowering period, adult Yucca Moths emerge from the soil and reside in open flowers during the day, where mating takes place. At dusk, female moths gather pollen into balls using specialized appendages called maxillary tentacles. Females then fly about the population actively stuffing the pollen into the stylar canals of freshly opened Soapweed flowers. After pollinating the flowers, the moths oviposit through the walls of the ovary, depositing club-shaped eggs among the ovules (Riley 1873).

Studies of pollination and fruit set at the Onefour population have shown that Soapweed plants only retain about 10% (range = 7.6 to 15%) of pollinated flowers and mature them into fruit (Hurlburt 2004; Hurlburt 2007). Plants selectively abscised flowers that were pollen-limited or had fewer moth ovipositions (eggs) (Hurlburt 2004). This is in contrast to most yucca species, in which plants are thought to selectively abscise flowers/fruit with more moth larvae. The latter strategy prevents overexploitation of the plant by the moth, thus preserving the benefit of the relationship to both species (Shapiro and Addicott 2003, 2004). Clones produced an average of 3.8 + 0.6 fruits per clone per year from 1998 to 2003 at Onefour (Table 1, Figure 9; Hurlburt 2004). An additional feature of Alberta populations is their ability to tolerate selfing (which still requires Yucca Moth as a vector for pollen transfer), with no evidence of early inbreeding depression in offspring produced by selfing (Hurlburt 2004). Fruits dehisce from mid- to late fall through May of the following spring.

Table 1. Measures of mutualistic benefit between Soapweed and Yucca Moths in Alberta and in the nearest populations in Montana (Hurlburt 2004a; Hurlburt 2007b; Hurlburt 2011c; Alberta Fish & Wildlife – unpublished datad). Calculations for Pinhorn include the portion of the site protected by exclosures.
Population Viable seeds/ fruit (+ st. dev. (n fruit)) Fruits/ flowering clone (+ st. dev. (n fruit))
Onefour, AB (1999-2003)a 146.3 + 93.9 (221) 3.76+ 0.63 (589)
Fort Belknap, MT (2000-03)a 147.4 + 89.0 (100) 3.55+ 2.675 (162)
Loma, MT(1999-2003)a 142.4 + 87.9 (112) 3.19 + 1.93 (353)
Fort Benton, MT (1999-2003)a 139.6 + 75.3 (101) 3.56 + 0.67 (241)
Onefour (2007)b 194.6 + 59.3 (20) 2.65 + 0.43 (302)
Onefour (2011)c N/A 1.25 (71)
Pinhorn (2004)d N/A 1.67 (5)
Pinhorn (2008)d N/A N/A[*]
Pinhorn (2009)d N/A 2.56 (23)
Pinhorn (2010)d N/A 4.21 (160)
Pinhorn (2011)c N/A N/A[*] (71)

*The number of flowering clones was not recorded at Pinhorn in 2008 and 2011.

Figure 9. Three Soapweed fruits on a single inflorescence. Fruits at Onefour exhibit a wide-range of coloration from reddish, as in this photo, to pale green.

Photo of three Soapweed fruits on a single inflorescence  (see long description below).

Photo: D. Hurlburt.

Description of Figure 9

Photo of three Soapweed fruits on a single inflorescence surrounded by a rosette of long, sharp leaves. Fruits can display a wide range of colouration, ranging from reddish (as in this photo) to pale green.

Figure 10. Percent of clones flowering for Onefour, Pinhorn and closest Montana populations (Loma and Ft Benton) from 1998 to 2011 as available (1998-2003: Hurlburt 2004, 2007, 2011).

Chart comparing the percentage of Soapweed clones flowering (see long description below).

Absence of bars indicates years in which flowering was not assessed rather than no flowering. For the Onefour population, data are not available for 2006 and 2010 when flowering was reported to be high (Bradley et al. 2006; Walker, I. pers. comm. 2011).

Description of Figure 10

Chart comparing the percentage of Soapweed clones flowering at the Onefour, Pinhorn and the closest Montana populations at Loma and Fort Benton in the years from 1998 to 2011. Where bars are absent, this indicates years in which flowering was not assessed.

Dispersal and Migration

Soapweed seeds are wingless, smooth and flat, and likely disperse only short-distances by gravity, though dispersal may also be aided by the high winds that are common in these habitats. Most seeds fall around the base of the parent plant (Fairbarns 1985). At Onefour, seeds that colonized the prairie uplands were likely dispersed by wind in the mid- to late-seventies (Csotonyi and Hurlburt 1998), landing on recently burned prairie allowing for germination in a minimally competitive environment. These upland prairie clones are all located within 100 m of the coulee slopes suggesting that even in high winds most Soapweed seeds do not disperse more than 100 m.

Seeds do not survive gut passage and are not dispersed by the few ungulates that feed upon mature fruit. It is plausible, but unlikely, that the occasional seed may be carried on the bodies of animals or that yucca fruit could float by water to new locations. Most Soapweed plants 'migrate' to new locations aided by people for gardens, by vehicles (along railways in Montana ) or possibly in substrate used to build roads.

In addition to Soapweed’s obligate relationship with the Yucca Moth, the plant is the sole host of two other closely related species of moths. The Non-pollinating Yucca Moth is a seed predator and lays its eggs in early-stage Soapweed fruit. Its developing larvae eat a proportion of developing seeds, alongside Yucca Moth larvae (COSEWIC2006b). The second species, the Five-spotted Bogus Yucca Moth, a stem borer, lays its eggs in Soapweed inflorescences (COSEWIC2006a). Neither of these species provide any known benefit to the Soapweed; however, their survival greatly depends upon the mutualism between Soapweed and Yucca Moths.

Aphids, which are tended by several species of ants, often live on Soapweed fruit. This facultative mutualistic relationship between ants and aphids, which is dependent on the obligate mutualism between the Soapweed and Yucca Moth, has some interesting implications for its associates. Soapweed inhabited by ants experienced a 60% increase in the number of viable seeds produced per fruit, because ants reduced ovipositing by the Non-pollinating Yucca Moth. Ants were also less likely to chew on and damage yucca buds when aphids were present (Perry et al. 2004; Snell and Addicott 2008b).

In Alberta and northern Montana, the survival of Five-spotted Bogus Moth larvae is dependent upon the presence of Soapweed fruits and the presence of aphids feeding on the sap associated with the stalk. Five-spotted Bogus Moth larvae only survive in the green portions of the Soapweed stalk, although their eggs are spread throughout. Soapweed stalks only remain green up to the position of the highest fruit along the stalk, unless aphids are present, in which case the stalks remain greener for longer (Snell and Addicott 2008a).

Numerous additional interspecific interactions are less well documented. Kerley et al.(1993) proposed a closely related species of yucca, Parmella (Yucca elata), supported over 70 species of arthropods. Soapweed at Onefour provide a food source for birds (who strip larvae from inflorescences), Mule Deer (Odocoileus hemionus), White-tailed Deer (Odocoileus virginianus), Pronghorn Antelope (Antilocapra americana), Elk (Cervus canadensis) and Nuttall’s Cottontail (Sylvilagus nuttallii). Soapweed also provides shelter to Prairie Rattlesnake (Crotalus viridis), Bull Snake (Pituophis catenifer) and the Greater Short-horned Lizard (Phrynosoma hernandesi) (Hurlburt 2007). Mule Deer and Pronghorn have been observed to feed extensively on Soapweed stalks, flowers and fruits (see Threats and limiting factors).

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