Soapweed (Yucca glauca) COSEWIC assessment and status report 2013: chapter 11
There are a number of natural and anthropogenic factors that may limit the distribution and abundance of Soapweed and therefore also affect its associated Yucca Moth, Non-pollinating Yucca Moth and Five-Spotted Bogus Yucca Moth in Canada. Soapweed relies primarily on asexual reproduction in its Canadian range, but lack of sexual reproduction may impact the long-term persistence of Soapweed, in addition to having immediate and severe effects on the persistence of the three moth species listed above. As a result, factors that affect sexual reproduction of Soapweed are important to consider even if they do not appear to be critical to short-term persistence.
Wild Ungulate Herbivory
Herbivory by Pronghorn and Mule Deer on Soapweed inflorescences, flowers and fruit has a negative impact on Soapweed sexual reproduction and the recruitment of yucca moths in some years and sites (Hurlburt 2004). Pronghorn eat individual Soapweed flowers, whereas Mule Deer most often eat large portions of or the entire flowering stalk. Fruits are also susceptible to consumption by both Pronghorn and Mule Deer. Herbivory can directly destroy adult moths within flowers, moth eggs in flowers or early stage fruit, and larvae within older fruits.
When the number of available inflorescences is low, the impact of herbivory can be severe (affecting 80-100% of flowers). Under such situations, herbivory can be considered a threat rather than a limiting factor because of the severity of the impact, especially at Pinhorn. A small number of Mule Deer (2 or 3) can consume hundreds of Soapweed stalks in a single evening. In years of high flowering at Onefour, the impact of herbivory has been low (less than 1% of flowers) (COSEWIC 2002). At Pinhorn, the number of clones is smaller and Mule Deer almost always eat most or all available inflorescences (Csotonyi and Hurlburt 2000, Hurlburt 2001, COSEWIC 2002, Hurlburt 2004). Prior to the construction of exclosures, sexual reproduction and yucca moth populations were severely impacted by wild ungulate herbivory.
Over the short term, the impact of herbivory on population projections for Soapweed in Canada is expected to be low because individuals are long-lived and populations rely mostly on asexual reproduction, as shown in the population projection models presented in Hurlburt (2004). The population projection models incorporated patterns of flowering and intensity of herbivory into population projections for Soapweed based on data for the Onefour population.
At Pinhorn failure of Soapweed to set fruit between 1997 and 2002 was attributed to repeated, intense herbivory of inflorescences, which resulted in the apparent loss of Yucca Moth from this site (COSEWIC 2002). Prior to 1997, the last known occurrence of fruiting was 1975 (Csotonyi and Hurlburt 2000). Since the Yucca Moth assessment in 2002, there have been some improvements for Soapweed and Yucca Moth at Pinhorn. In 2004 and in 2007, several fruits with Yucca Moth emergence holes and/or larval damage to seeds were located at the site (Foreman et al. 2006; Environment Canada 2011). In 2004, 3 clones produced a total of 5 fruits with a total of 6 Yucca Moth emergence holes (Foreman et al. 2006). In August 2007, a single old Soapweed fruit was observed at Pinhorn. The fruit, thought to have originated from 2006 (D. Johnson pers. comm. 2011), contained seeds with Yucca Moth larval damage from feeding.
In 2008, Alberta Fish and Wildlife constructed deer exclosures to protect a portion of the Soapweed plants at the Pinhorn site. Soapweed fruit set and observations of yucca moths have increased steadily within the exclosures since 2008. By 2010, 160 fruits were produced by 38 clones, with a total emergence of 36 moths (AB Fish and Wildlife, unpublished data). In 2011, 71 fruits were produced at the site; all but 1 were within the exclosures. At the time of sampling in 2011, moth larvae had not yet emerged from the fruits. These results indicate the potential to recover fruit set and restore a healthy population of Yucca Moth to Pinhorn with ongoing protection from herbivores. However, it is not clear whether Yucca Moth abundance at the site is adequate for long-term survival of the moth and the mutualism, as fruiting and moth emergence remain low compared to elsewhere (Hurlburt 2004, Hurlburt 2011).
Extreme Weather Events
The Dry Mixedgrass prairie, where the Alberta populations occur, is characterized by extreme weather events, including high temperatures, high or low levels of rainfall and high winds. Days of heavy rain or high winds are not uncommon and can have considerable impacts on the reproductive success of Soapweed within a given year.
Intense wind gusts over 100 km/hr can cause significant loss of Soapweed flowers and buds, reducing availability of Soapweed flowers to moths for pollination or destroying larvae in early stage fruit through premature removal of fruit from the stalk (COSEWIC 2002). In 1999, over half of the flowers and developing fruits at the Onefour site, and 100% of uneaten flowers at the Pinhorn site, were destroyed on a particularly windy day (COSEWIC 2002). Individual plants located at the tops of coulee slopes or on the prairie flats were particularly susceptible. Yucca Moth may be affected further during such adverse conditions because the wind makes it more difficult for moths to fly among inflorescences to collect pollen or to pollinate (Cruden et al. 1976, Aker 1982, Hurlburt 2004); moths have been observed to remain in Soapweed flowers during extreme periods of wind (COSEWIC 2002, Hurlburt 2004). Heavy rains can also lead to erosion of the coulee slopes, and such events could cause mortality of clones, but may also provide open habitat that may favour establishment of seedlings.
Obligate Mutualistic Relationships
Although Soapweed populations can persist through asexual propagation, the absence of sexual reproduction may limit population growth and range expansion and restrict the ability of a species to adapt to changing conditions. Sexual reproduction in Soapweed requires the presence of its highly co-evolved pollinator, the Yucca Moth.
Small or newly established populations of Soapweed may not have adequate numbers of Yucca Moths to sustain successful sexual reproduction. Yucca moths are short-lived, particularly weak flyers, and therefore are unlikely to disperse long distances (Kerley et al. 1993; Marr et al. 2000). Further, there is evidence to suggest that small populations of Soapweed may not contain enough flowering plants to sustain permanent populations of Yucca Moths (Dodd 1989; Dodd and Linhart 1994; COSEWIC 2002).
All three yucca moth species require regular sexual reproduction of Soapweed in order to reproduce and allow yucca moth populations to persist. To some degree, extended prepupal diapause in yucca moths is thought to allow moth populations to persist in times of limited host plant sexual reproduction or flowering; however, the exact duration of diapause is unknown and the extent of host plant reproductive failure moths can withstand is unknown. Repeated reproductive failure of the plant over a 25-30 year period (due to wild ungulate herbivory) is thought to have led to declining Yucca Moth populations on the Pinhorn Grazing Reserve.
Insect Herbivory
The mutualism between Soapweed and the Yucca Moth is likely to be negatively impacted by the presence of the Non-pollinating Yucca Moth (Perry 2001; D. Hurlburt, unpubl. data), whose larvae consume Soapweed seeds (Addicott 1996) and compete with Yucca Moth larvae for food (James 1998). At Onefour, Non-pollinating Yucca Moth larvae are abundant in some years and can consume up to 40% of seeds (COSEWIC 2002; COSEWIC 2006b). The presence of ants on Soapweed reduces the impact of Non-pollinating Yucca Moth (see Interspecific Interactions), resulting in indirect benefits to fruit set of Soapweed and to the Yucca Moth (Snell 2008b).
Ants can, however, significantly reduce the availability of Soapweed flowers in which moths can oviposit, and may kill or harass adult moths in flowers patrolled by the ants (Perry 2001). Ants reduce the availability of Soapweed flowers by chewing on buds and subsequently causing the premature abscission of those buds. In some years, some plants at Onefour lose up to 90% of their buds through ant damage (COSEWIC 2002). Ants are also attracted to Soapweed plants by aphids, but ants tend to be present on Soapweed even in the absence of aphids.
A number of anthropogenic threats have been identified with the potential to threaten persistence and/or sexual reproduction of Soapweed and yucca moths in Canada. At present, these are mostly considered potential threats, as some occurred at the time of the last assessment, but have since been mitigated. Few are known to have ongoing negative impacts on known populations at present, but may impact future trends.
Lack of Disturbance
A lack of natural disturbance limits the availability of open patches that favour the establishment of new Soapweed plants. Historically, heavy grazing and wallowing by bison, as well as periodic fires, may have helped reduce the abundance and density of competitive grasses (Samson and Knopf 1994; Samson et al. 2004). Fire in the mixed-grass prairie was estimated to occur every three to five years prior to European contact (Samson and Knopf 1994; Samson et al. 2004), but is thought to have occurred only once in Soapweed habitat at Onefour during the last 35 years.
Wild Ungulate Herbivory
As noted above (under Natural Limiting Factors), when levels of flowering are low (as is common at Pinhorn, Table 1), wild ungulate herbivory can severely impact reproductive output of Soapweed, and consequently limit populations of yucca moths. Under such conditions, herbivory is best thought of as a threat to Soapweed.
Oil and Gas Development
Both the Onefour and Pinhorn sites are on public land and could experience degradation of habitat through oil and gas activity (ASYMRT 2006). There has been no oil and gas development at Onefour, but there was some development and new road construction near the Pinhorn site around 2003. Since the last assessment, Alberta Fish and Wildlife has applied to place protective notations on quarter sections containing Soapweed at Pinhorn and Onefour; designation is pending (Environment Canada 2011). At Onefour, all oil and gas dispositions must obtain consent from the Minister of Community Development, given that the Soapweed occur within the boundaries of the Onefour Heritage Rangelands Natural Area (Environment Canada 2011).
Grazing by Cattle
In both Alberta sites, Soapweed and their moths coexist with cattle grazing. Most Soapweed occur on steep, rocky slopes that are not preferred by cattle, but cattle have been known to consume flowering stalks of Soapweed (COSEWIC 2002). Other than using some travel corridors along select slopes to reach the coulee bottom for shelter and water, cattle generally make little use of the steep slopes.
Some Soapweed at the tops of slopes and on prairie in Onefour have been susceptible to cattle grazing during drought, such as the one that occurred in 2001. Typically, the Onefour Research Substation has not pastured cattle in the area of the Soapweed during flowering and fruiting; however, during periods of drought, such as in 2001, feed for cattle can be in short supply and there was need to use the pasture (COSEWIC 2002). Grazing has not been a problem in Pinhorn since 1998 (no information is available prior to 1998), even though cattle have access to the area during the flowering and fruiting season. There are few observations of cow manure within the Soapweed patch. That said, Mule Deer usually consume all of the stalks shortly after the initiation of flowering (D. Hurlburt, pers. obs.), so there may be little incentive for cattle to forage for Soapweed at this site. Destruction of Yucca inflorescences by grazing cattle is common in the United States, and it is plausible that grazing could become a substantial threat in Alberta should the Soapweed expand its habitat (COSEWIC 2002).
Agricultural Crop Production
Most areas inhabited by Soapweed are not ideal for cultivation and are in no immediate threat of such activity. It is possible that cropland conversion and associated activities, like pesticide application, could take place in the future, especially on the prairie upland areas at Onefour, as they do occur immediately across the Lost River coulee in Montana. However, existing protections and land ownership make this unlikely (see Protection, Status and Ranks).
Although herbicides have only been used to spot-kill individual weedy plants near the Onefour Soapweed site, widespread use of herbicides and insecticides could cause widespread plant and moth mortality and reduce reproductive success. In Montana, Soapweed plants along roadsides have fewer ovipositions and produce fewer fruits in areas sprayed for weed control, which could impact plants, especially at the Rockglen site in the future.
Off-road Vehicle Use
Prior to 2003, Soapweed sites at Onefour and Pinhorn were well known and highly accessible by passenger vehicles, and the sites were regularly visited. Crushed plants had been observed at both locations. Since 2003, the impact of off-road vehicle use has noticeably declined (Hurlburt 2011) and these activities do not appear to be a major current threat.
Agriculture and Agri-Food Canada has imposed some conditions on access to the lands they manage at Onefour, partly as means to address concerns regarding liability and fire hazard. In 2011, Hurlburt (2011) observed considerable recovery of the vegetation along prairie trails and did not observe any sign of off-trail vehicle use. General access to the Pinhorn Grazing Reserve is more regulated than in the past with restrictions on accessing pastures with cattle and using ungravelled trails.
Harvesting for Horticultural and Medicinal Uses
Prior to 2003, Soapweed plants were routinely dug up at Onefour and Pinhorn and transplanted to home gardens (COSEWIC 2000, 2002), but no such activity has been observed since that time (D. Hurlburt, pers. obs.).
Three locations are identified, corresponding to each of the three known populations. The population at Onefour is threatened by lack of disturbance, which limits natural recruitment. The most serious threats to the Pinhorn population is grazing of flowers and inflorescences, which severely limits sexual reproduction. While parts of this population are currently protected by an exclosure, this structure is unlikely to be permanent, and a breach by a single deer during the flowering season could eliminate fruit production for the entire population. The population at Rockglen has no clearly identified threats at this time, but it does not support a population of Yucca Moths, and therefore may lack the ability to reproduce sexually, which threatens its long-term persistence.
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