Golden paintbrush (Castilleja levisecta) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

Search effort

Golden paintbrush has distinctive, showy flowers that attract attention and are not easily overlooked. Suitable sites have been surveyed repeatedly since the early 1980s in a series of projects designed to document the distribution of rare plants of maritime meadows on southeast Vancouver Island and the Gulf Islands. The principal investigators included Adolf and Oldriska Ceska, Matt Fairbarns, Hans Roemer, Jenifer Penny, Harvey Janszen, Frank Lomer and the late George Douglas, all of whom are/were familiar with the golden paintbrush. Fairbarns (2005c) conducted a detailed search for a number of rare species including golden paintbrush in forty-one meadow complexes on southeast Vancouver Island and offshore islets and islands between 2002 and 2005. The survey included detailed examination of every meadow complex within the known extent of occurrence in Canada, including the single Indian Reserve within the extent of occurrence. No new populations were detected. Private lands and Indian Reserves peripheral to the known extent of occurrence, which only constitute a small proportion of the apparently suitable habitat, have not been surveyed as thoroughly as public lands.

Abundance

In the absence of information on rates of genetic exchange, occurrences of golden paintbrush more than 1,000 m apart are treated as separate locations. On this basis, it has been reported from eight locations in Canada (Table 1), only two of which have extant populations. The Beacon Hill population has not been seen in annual surveys since 1991. The Trial Island population consists of multiple subpopulations (i.e. patches separated by <1,000 m). Based on the most recent data from each site, there was a total of 3,361 flowering plants in Canada occupying about 3.9 ha (0.039 km²).

Fluctuations and trends

The number of mature, reproductive golden paintbrush plants in a population may fluctuate depending on recruitment, maturation, regression and mortality. Large reproductive plants are quite likely to regress in size to small, non-reproductive individuals in unfavourable years (Wentworth 1994). This may provide a partial explanation for the large apparent fluctuations observed in recent years. High values may also be counts of flowering stems while low values are counts of entire plants (individual plants tend to produce 1-3 flowering stems). Changes in the apparent population size on Trial Island may be largely attributable to these two factors, although there has been an increase in Scotch broom that may have contributed to the recent low numbers. A small component of the reported decline on Alpha Islet may reflect counts of flowering stems vs. entire plants, or regression of mature plants. On the other hand, the magnitude of the change clearly indicates a sharp decline in the population. The magnitude of this decline was so great that the spring count (referred to in Table 1 as 2006a) was followed by an autumn count (referred to in Table 1 as 2006b). Overall, the rate of population decline over the past 10 years was probably about 25% (assuming earlier Alpha Islet counts consisted of plants with an average of 2 flowering stems each).

Rescue effect

Extensive floristic surveys have been conducted in nearby areas of Washington State, including the Olympic Peninsula (Buckingham et al. 1995), the main islands of San Juan County (Atkinson and Sharpe 1993) and small islets in San Juan County (Giblin pers. comm. 2006). From these studies, it appears that golden paintbrush is absent from the Olympic Peninsula and is very rare in the San Juan Islands (three locations). The San Juan Island populations are small and separated from the nearest portion of the Canadian range by over 10 km of open water. The seeds lack any adaptations to facilitate long-distance dispersal by wind, water or animals. For these reasons, there is negligible opportunity for unassisted gene flow (via seed or pollen) from the United States.