Dwarf woolly-heads, specific populations, COSEWIC assessment status report: chapter 7
Population Sizes and Trends
Search effort
The Princeton populations appear to be restricted to non-saline vernal pools, a rare habitat type at low elevations in south-central British Columbia. Experienced searchers can find the plant with relative ease despite its small size, because it has a distinctive form, is visible and exposed on open ground, and can be detected for several months of the year (it was first found in October). All three sites were found by Frank Lomer, who has explored the botany of south-central British Columbia for about 15 years and has searched suitable habitat for new occurrences of Psilocarphus brevissimus several times since originally discovering it in 1996. The site where P. brevissimus occurs also has two plant species not found elsewhere in Canada – Collomia tenella and Antennaria flagellaris – which supports the hypothesis that this site is unique from an ecological and biogeographic perspective (Lomer pers. comm. 2005).
There is no measure of search effort for the Prairie Unit. Most of the records were found in two surveys carried out in exceptionally wet years. It seems likely that other populations exist within the Prairie Unit although they are probably only present in the soil seed bank in most years.
Fluctuations and trends
Population fluctuations are common among annual plants of vernal pools and temporary ponds (Bauder 2000, Griggs and Jain 1983).
The sub-populations of Psilocarphus brevissimus in the Southern Mountain Population fluctuate greatly. The largest of the sub-populations numbered “a few thousand” in 1997 but rose to “1-2 million” in 2002. Repeat counts are only available for one of the other two (much smaller) populations and it fluctuated between 300 and 11,500 plants in an 8-year period.
In the Prairie Population, the majority of the Alberta occurrences were reported in 1996. That year was very wet and the records came from a series of sites that had probably not been flooded for several years, or even decades (D. Bush pers. comm. 2005). Most of the Saskatchewan records came from 1999. During that spring, high runoff and rainfall caused severe flooding in southwest Saskatchewan which took several thousand hectares out of production and probably created ideal growing conditions for Psilocarphus brevissimus. It appears quite likely that many of these sites are too dry to support the species during dry years.
Abundance
Population estimates for the most recent observations are provided in Table 1. At their lowest recorded levels, the aggregate population size of the Southern Mountain Population is as low as 700 individuals. In exceptionally dry years, it may drop even lower. Population counts for most of the Prairie Population sites are imprecise and in most cases may represent peak numbers since negative search results are not recorded. The aggregate total population in peak years may be as low as 9,000 or as high as 27,000 for the Prairie Population, given the uncertainty of individual site counts. The aggregate population in trough years is probably well below 5,000 and perhaps as low as 2,000.
Rescue effect
The two Canadian designatable units are separated by great distances and high mountain barriers so genetic interchange is unlikely. The United States population nearest to the Southern Mountain Population appears to be in Grant County, Washington State, a distance of over 200 km. The population nearest to the Prairie Population appears to be near Dodson, Montana – a distance of over 50 km. Given the species’ limited power of dispersal over great distances, there is little likelihood of regular genetic interchange between Canadian and American populations. Similarities in habitat suggest that seeds from U.S. populations may be relatively well adapted to site conditions that prevail where Canadian populations occur, so there is some potential for deliberate re-introduction if Canadian populations are extirpated by events that do not reduce habitat suitability.
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