Western spiderwort (Tradescantia occidentalis) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

There are a total of 5 populations located within four sand dune deposits in three provinces in Canada.

Alberta - Pakowki Lake Sand Hills

Manyberries site

Western spiderwort is known to occur in southeastern Alberta at one location near Pakowki Lake. This site was first located in 1986 and has been revisited in 1986, 1987, 1990, 1999, and 2001 by various investigators.

On June 29, 1999, Gould and Cotterill noted 27 plants in the population that were dispersed within a small area in three subpopulations of 3, 19 and 5. Of the 27 plants observed 24 were in bud, 3 were in flower (ANHIC 2001).

The site was investigated by the author (Smith 2001B) on July 14 and again on August 6, 2001. 7 plants were found. The plants were in poor shape. They were less than 15 cm in height and had curled, deformed leaves and were not flowering. All occurred together in a small patch. By the time of the return visit on August 6, no plants were visible.

The population, was apparently always small, varies considerably from year-to-year from a low of 7 to a high of 210 plants (1986 - 50; 1987 - 30; 1990 - 210; 1999 - 27; 2001 - 7). Variation is probably dependent on climatic factors.

[Note: Information, based on surveys in July 2002, after a spring of high precipitation, resulted in a partial count of 1255 clumps (multi-stemmed plants) and an estimate over the entire area of about 7450 plants (pers. com. Sue Peters, Alberta Conservation Association, Edmonton). Clearly the species fluctuates greatly from season to season depending on climatic conditions. E. Haber, Co-chair, Plants and Lichens, COSEWIC]

Saskatchewan - Elbow Sand Hills

DouglasProvincial Parksite

Western spiderwort occurs in southcentral Saskatchewan at only one location. Population work done at this site occurred in 1991 at which time 42 plants were counted. The site was reinvestigated in 2001 by Douglas Provincial Park staff. Plants were found in 15 locations ranging from one to several plants at each location yielding a total population of less than one hundred. The final report on the occurrence of western spiderwort was not available at the time of writing (Douglas Provincial Park, 2001).

Manitoba - Lauder Sand Hills

 Western population (including the MHHC)

This, the first of two Lauder Sand Hills sites was surveyed in 1990, 1992, 1994 , 1996 and 2001. In 1990, 66 plants were counted. During the 1992 and 1994 surveys numbers remained constant at 380. The estimated population in 1996 was 783 plants (Goulet and Kenkel 1997) and in 2001 the count was 619 (pers. com., Elizabeth Reimer, Manitoba Cons. Data Centre).

This site supports comparatively small populations of western spiderwort. Two localized populations have previously been described; both occur in protected 'pockets' of the sand dune (Smith and Bradley 1990, Hohn 1994). In 1996, a small new population was found on a gently rolling meadow west of the main chain of sand dunes. This population occurs in a low-lying area that is sheltered by clumps of low shrubs (Goulet and Kenkel 1997).

Eastern population

This population was first discovered in 1995. A complete search of the area was undertaken in 1996 including areas identified by DeSmet (1995) and the northern portions of the property that had not been previously explored. Western spiderwort was most abundant on the high dunes in the central portion of this private property, where cattle grazing is minimal. Less dense populations also occur at the north end of the property, and on high sand dunes in the southern portion (Goulet and Kenkel 1997). The estimated population in 1996 was 19,540 plants (Goulet and Kenkel 1997). A survey in 2001 by Marjorie Hughes yielded a count of 4,321 (pers. com., Elizabeth Reimer, Manitoba Cons. Data Centre).

Routledge Sand Hills

This population represents the largest Canadian population of western spiderwort. It was examined in 1990, 1992, 1994, 1996 and 2001. The partially stabilized sand dune ridges were found to contain 3278 plants in 1990 (Smith and Bradley 1990). Approximately 7800 plants were counted in the 1992 and 1994 surveys (Hohn 1992, 1994). The estimated population in 1996 was 26,550 plants, 75% of which occur in the ESA (Goulet and Kenkel 1997). Again, climatic factors apparently greatly influence population levels from year-to-year.

The majority of western spiderwort plants at this site are found in the ESA, which is on private property. Adjacent Crown land supports a dense growth of western spiderwort, but these plants only occupy a small area extending about 5-20 m beyond the fence line of the private property. In 1996, additional plants were discovered on two other private properties adjacent to the ESA. The extension into the one property continues only about 100 m past the fence line, even though the sand dunes extend much further. The second property was not fully explored as permission to search the land was not obtained (Goulet and Kenkel 1997).

In 1996, two new sub-populations of western spiderwort were discovered in the ESA. A small sub-population was found in a grass-shrub meadow on stabilized, gently rolling sand hills. The plants at this site were most abundant in low-lying, protected areas. A second, larger sub-population was found on the Crowfoot sand dunes that extended into adjoining private lands. This second sub-population contains a large number of pink-flowered plants (Goulet and Kenkel 1997).

Western spiderwort at the Routledge Sand Hills site were reported to have remained stable to the present day (Hellman, pers. com 2001). Actual counts, however, made by Marjorie Hughes in 2001 (pers. com., Elizabeth Reimer, Manitoba Cons. Data Centre) give a total of 9,422 plants, a considerable drop in numbers from the 1996 high of 26, 550.

Overview

Western spiderwort is limited in distribution in Canada to one moderately large population in Alberta (Pakowki Lake Sand Hills) that appears to fluctuate considerably with seasonal rainfall, two small populations, with one in Saskatchewan (Douglas Provincial Park) and one in Manitoba (Western Lauder Sand Hills) and two fairly substantial populations in Manitoba (Eastern Lauder Sand Hills and Routledge Sand Hills). Population sizes are summarized in Table 1.

The two largest populations of western spiderwort both occur in southwestern Manitoba. The Routledge Sand Hills likely contains the largest Canadian population consisting of a high of over 26,000 plants as estimated in 1996. This population, like others, fluctuates considerably in size, with the most recent count representing a drop in numbers to just over 9,400. This site also supports a large number of the pink-flowered form of the plant. Over the past twelve years, total population numbers, in Canada, have remained in the thousands of plants with considerable fluctuations in numbers. Minimal human disturbance and limited presence of leafy spurge may account for the relatively large numbers of western spiderwort at the Routledge Sand Hills. It seems likely that favourable site conditions also play a role. The Eastern Population in the Lauder Sand Hills contains the second largest Canadian population with the most recent count being in 2001 at 4,321 plants, a considerable drop from the 1996 count of 19,540. This population, first discovered in 1995 covers both high and low relief dunes (Goulet and Kenkel 1997).

Population estimates in 1996 are much higher than previous estimates. A number of factors, including the following, likely have contributed to these higher values: 1. The Eastern Population at the Lauder Sand Hills, which includes over 40% of the known Manitoba population, was not included in earlier surveys. 2. In the 1996 survey, the number of non-flowering plants was included in the estimate of total population size. 3. The 1996 census was more complete than previous surveys. It included areas in both the Routledge Sand Hills and MHHC site that were not previously enumerated (e.g., Crowfoot dunes, and two other private properties). 4. The observed increase in western spiderwort population size between 1990 and 1992 may have been attributable to increased levels of precipitation over the same period (Goulet and Kenkel 1997, Hohn 1994).

Information based on fieldwork in July of 2002 commissioned by Alberta Sustainable Resource Development, Fish and Wildlife Division, has provided new insights into population fluctuations at the single site in Alberta. About 7,450 plants are estimated to have been present in the summer of 2002 after a spring of high rainfall. Similarly, new data from the province of Manitoba for its three sites provided by the Manitoba Conservation Data Centre were particularly informative. The Manitoba data (Table 1) indicates a considerable drop in population sizes during 2001 from a previous high in 1996 for all three sites. These low values were also reflected in the extremely low number documented for the single Alberta population for the same year. For the current year of 2002, the ample rainfall in southern Alberta provided good growing conditions for the resurgence of the Alberta population at the Pakowki Lake Sand Hills to high numbers. Clearly population sizes fluctuate considerably in this species.

The total Canadian population is estimated to fluctuate in size in recent years (see table 1) from a low of about 7,714 (low values from all of the known sites) to 54,423 plants (high values at all the sites).