Scouler's catchfly (Silene scouleri ssp. grandis): COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Silene scouleri ssp.grandis has been recorded from up to 12 locations in Canada, all on southeast Vancouver Island or adjacent small islands (Table 1). Two locations, ‘near Victoria’ and Burnside District, are very imprecise and may well refer to other locations described in the table. The Mount Douglas, Beacon Hill, Oak Bay, Uplands and Ten Mile Point areas are frequently visited by botanists and the lack of recent records likely indicates the populations are now extirpated. The author surveyed suitable habitats in each of these locations in 2001.

Table 1. Silene scouleri ssp.grandis Population and Location Data
Population Last Observed Observer Extenttable notea Number of individualstable noteb Apparent Status
Trial Island October 2001 M. Fairbarns 20 x 4 m 5 Extant sub-population
Little Trial Island August 2002 M. Fairbarns 100 x 30 m 23 Extant sub-population
Alpha Islet October 2001 M. Fairbarns 160 x 80 m 250-300 Extant
Mount Tzuhalem July 2002 M. Fairbarns 2 x 2 m No current -year growth Extirpated
Griffin Island August 1991 M.G. Shepard Unknown Unknown Loc. Error
Ten Mile Point August 1966 A.S. Harrison Unknown Unknown Extirpated
Upands, Victoria July 1953 M.C. Melburn Unknown Unknown Extirpated
‘near Victoria’ July 1938 J.W. Eastham Unknown Unknown Location too vague
Burnside District August 1935 E. Cooke Unknown Unknown Location too vague
Oak Bay August 1924 G.A. Hardy Unknown Unknown Extirpated
Beacon Hill 1921 G.V. Copley Unknown Unknown Extirpated
Bare (Mandarte?) Island June 1915 J.R. Anderson Unknown Unknown Extirpated?
Mount Douglas (Cedar Hill) July 1897 J.R. Anderson Unknown Unknown Extirpated

The ‘Bare Island’ location (last observed in 1915) is probably Mandarte Island – the two names appear to be synonymous. This is an unoccupied reserve (Bare Island Indian Reserve 9). It was visited in 2002 and no plants could be found. Mandarte Island is a vast gull and cormorant colony. Gull numbers have apparently increased sharply over the past decades. The enriched soils are now dominated by rank pasture grasses including Holcus lanatus, Dactylis glomerata andAnthoxanthum odoratum – species that probably outcompeteSilene scouleri on rich sites.

The Griffin Island population is problematic. This island is only a few metres from Alpha Islet, where the plant is abundant. Exhaustive surveys of Griffin Island during the peak flowering season in 2001 did not find any plants in flowering or vegetative state. It has only been reported on Griffin Island once, in 1991. The collector was not certain the plants were actually collected on Griffin Island (M.G. Shepard, naturalist, pers. comm., May 2001). It seems likely the collection was made from Alpha Islet and the label is in error.

The Mount Tzuhalem population was first observed ‘many years ago’ by Adolf Ceska (ecologist, British Columbia Conservation Data Centre, pers. com., Oct. 5, 2001). It never consisted of more than 2-3 shoots and was most recently observed in 2000 when it failed to flower. Detailed search of the precise location in 2001 and 2002 did not reveal any evidence of the plant and it is presumed extirpated from the site. This population was very unusual; it occurred well inland and much higher than any other observations or collections.

The three extant observations at Trial Island, Little Trial Island and Alpha Islet span a small area offshore from Oak Bay, a suburb of Victoria. The Little Trial and Trial Island populations are less than 500 metres (m) apart. They likely exchange DNA, through pollination, on a regular basis.

The current extent of occurrence of Silene scoulerissp. grandis in Canada is approximately 0.6 km². The area of occupancy is 1.58 hectare (0.0158 km²). Past observations of the other populations have included estimates of abundance but these appear to be too unreliable to use as a basis for tracking trends. Estimates from 2000 and 2001 suggest a total of between 278-328 individuals. 

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