Soapweed (Yucca glauca) COSEWIC assessment and status report 2013: chapter 4

Technical Summary

Yucca glauca

Soapweed

Yucca glauque

Range of occurrence in Canada (province/territory/ocean):
Alberta, Saskatchewan

Demographic Information

Generation time
Base on time to maturity for a sexual recruit.
 ~25 yrs; perhaps longer
Is there an [observed, inferred, or projected] continuing decline in number of mature individuals? No
Estimated percent of continuing decline in total number of mature individuals within [5 years or 2 generations] No decline
[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last [10 years, or 3 generations].
A slight increase has been observed at Onefour over the last generation, based on newly reproducing clones; longer-term trends are uncertain.
Unknown
[Projected  or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations].
A slight increase (0.4%) projected under current conditions, based on stage based modelling. However, conditions are unlikely to remain stable over 3 generations (75 years).
Unknown
[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over any [10 years, or 3 generations] period, over a time period including both the past and the future. Unknown
Are the causes of the decline clearly reversible and understood and ceased? Not applicable
Are there extreme fluctuations in number of mature individuals?
Not in Soapweed, but there are extreme fluctuations in its obligate pollinator, which affect seed set in Soapweed
No

Extent and Occupancy Information

Estimated extent of occurrence
Includes the two previously known Alberta sites and the Rockglen, SK site. Omission of Rockglen reduced the EO to 32 km2.
2040 km2
Index of area of occupancy (IAO)
Based on number of 2 x 2 km squares that are occupied by the species on 1:50,000 scale topographic maps.
Biological area of occupancy is 185 ha based on critical habitat designation in Environment Canada (2011)

16 km2

1.85 km2

Is the total population severely fragmented? No
Number of locations[1] 3
Is there an [observed, inferred, or projected] continuing decline in extent of occurrence? No
Is there an [observed, inferred, or projected] continuing decline in index of area of occupancy? No
Is there an [observed, inferred, or projected] continuing decline in number of populations? No
Is there an [observed, inferred, or projected] continuing decline in number of locations*? No
Is there an [observed, inferred, or projected] continuing decline in [area, extent and/or quality] of habitat?
Area and extent are unchanged in recent years, and there are no data to support recent changes in habitat quality. Fire suppression likely limits sexual recruitment, but does not appear to impact established clones, which can live at least 50 years.
Unknown
Are there extreme fluctuations in number of populations? No
Are there extreme fluctuations in number of locations[2]? No
Are there extreme fluctuations in extent of occurrence? No
Are there extreme fluctuations in index of area of occupancy? No

Number of Mature Individuals (in each population)

Population N Clones (index of Mature Individuals)
1. Onefour Research Station, AB 46,000 – 72,000
2. Pinhorn Grazing Reserve, AB 423
3. Near Rockglen, SK 6
Total
Note that the number of clones is used here as an index of the number of mature individuals, but likely underestimates the true value. The number of mature individuals is probably best measured as the number of rosettes above a certain age or size, but these data are not available.
46,500 – 72,500 clones

Quantitative Analysis

Probability of extinction in the wild is at least [20% within 20 years or 5 generations, or 10% within 100 years]. Not applicable

Threats (actual or imminent, to populations or habitats)

Lack of disturbance resulting from fire suppression and lack of grazing
Wild ungulate herbivory
Oil and gas development
Extreme weather events
Additional threats and limiting factors have been previously identified, but due to changes in management, their effects are thought to be negligible at present.

Rescue Effect (immigration from outside Canada)

Status of outside population(s)?
Stable and secure throughout most of the range
Is immigration known or possible? No
Would immigrants be adapted to survive in Canada ? Yes
Is there sufficient habitat for immigrants in Canada ? Yes
Is rescue from outside populations likely? No

Status History

COSEWIC: Designated Special Concern in April 1985. Status re-examined and designated Threatened in May 2000 and May 2013.

Status and Reasons for Designation

Status:
Threatened

Alpha-numeric code:
Does not strictly meet any criteria, but assessed Threatened because of interdependency with its Endangered sole pollinator.

Reasons for designation:
This long-lived perennial is known from only three sites in southeastern Alberta and southwestern Saskatchewan, where it occurs mainly on south-facing coulee slopes. Plants are pollinated exclusively by Yucca Moth, which lays eggs in the flowers. In Canada, the species reproduces almost exclusively by vegetative spread. Seed production is limited by low numbers of Yucca Moth pollinators, while seedling establishment is negatively impacted by lack of natural disturbance, including fire suppression. Herbivory of flowering stalks by native ungulates also limits seed set. Despite improved management to limit the impact of threats, this perennial is designated Threatened. It is the only host for Yucca Moth, Non-pollinating Yucca Moth and Five-spotted Bogus Yucca Moth, all of which were assessed as Endangered.

Applicability of Criteria

Criterion A
(Decline in Total Number of Mature Individuals): Not applicable. No evidence of decline in mature individuals; projections suggest slight increase over time.

Criterion B
(Small Distribution Range and Decline or Fluctuation): Not met. EO and IAO are below the thresholds for Endangered, and the species is known from fewer than 5 locations, but there is no documented decline and extreme fluctuations do not occur.

Criterion C (Small and Declining Number of Mature Individuals):
Not met. Total population exceeds the threshold of 10,000 mature individuals.

Criterion D (Very Small or Restricted Total Population):
Not met. Index of area of occupancy and number of locations below thresholds for Threatened D2, but factors that could rapidly lead to endangerment or extirpation are not known.

Criterion E(Quantitative Analysis):
Not done.

1 See Definitions and Abbreviations on COSEWIC website and IUCN 2010 for more information on this term.

2 See Definitions and Abbreviations on COSEWIC website and IUCN 2010 for more information on this term.

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