Western spiderwort (Tradescantia occidentalis) COSEWIC assessment and status report: chapter 11

Technical Summary

Tradescantia occidentalis (Britt.) Smyth

Western Spiderwort – Tradescantie de l’Ouest

Range of Occurrence in Canada:

Alberta, Saskatchewan, Manitoba

Extent and Area information

extent of occurrence (EO) (km²):

Approximately 500 sq. km

specify trend (decline, stable, increasing, unknown):

stable

are there extreme fluctuations in EO (> 1 order of magnitude)?

no

area of occupancy (AO) (km²):

Approximately 10 sq. km.

specify trend (decline, stable, increasing, unknown):

perhaps a decline due to leafy spurge in MB and SK

are there extreme fluctuations in AO (> 1 order magnitude)?

no

number of extant locations:

4 areas but 5 pops.

specify trend in # locations (decline, stable, increasing, unknown):

stable

are there extreme fluctuations in # locations (>1 order of magnitude)?

no

habitat trend: specify declining, stable, increasing or unknown trend in area, extent or quality of habitat:

a loss of habitat in MB and SK due to leafy spurge infestation

generation time (average age of parents in the population) (indicate years, months, days, etc.):

Unknown, perennial

number of mature individuals (capable of reproduction) in the Canadian population (or, specify a range of plausible values):

about 7,700-54,500, recent number about 22,000 plants

total population trend: specify declining, stable, increasing or unknown trend in number of mature individuals:

no clear trend; pops. fluctuate

if decline, % decline over the last/next 10 years or 3 generations, whichever is greater (or specify if for shorter time period):

census data not adequate

are there extreme fluctuations in number of mature individuals (> 1 order of magnitude)?

yes

is the total population severely fragmented (most individuals found within small and relatively isolated (geographically or otherwise) populations between which there is little exchange, i.e., < 1 successful migrant / year)?

yes

list each population and the number of mature individuals in each

[values in 2001]

  • AB - Manyberries -7450
  • SK - Douglas - 100
  • MB - MHHC - 619
  • MB - Loutit – 4,321
  • MB - Hellman – 9,422

specify trend in number of populations (decline, stable, increasing, unknown):

stable

are there extreme fluctuations in number of populations (>1 order of magnitude)?

no

Threats (actual or imminent threats to populations or habitats)

  • cattle grazing, invasion of leafy spurge, dune stabilization, human disturbance

Rescue Effect (immigration from an outside source)

does species exist elsewhere (in Canada or outside)?

yes

status of the outside population(s)?

Not at risk

is immigration known or possible?

Not possible

would immigrants be adapted to survive here?

probably

is there sufficient habitat for immigrants here?

yes

Quantitative Analysis

[Smith, 11/2001; updated by E. Haber, with new information from AB for 2002 and MB for 2001 provided by these jurisdictions at the time of status designation.]

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