Scouler's catchfly (Silene scouleri ssp. grandis): COSEWIC assessment and status report: chapter 5

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Habitat

Habitat requirements

There is no detailed information on habitat relationships of Silene scouleri ssp. grandis but parallels can be drawn to similar taxa: S. douglasii var. oraria and S. spaldingii. Kephart and Paladino (1997) found that shallow rocky soils were not optimal for S. douglasii var. oraria and that its abundance is also negatively correlated with vegetation height, cover and soil depth. Silene spaldingii is tolerant of light to moderate grazing (Schassberger 1988) and may benefit from prescribed burning (Lesica 1992).

In Oregon, Silene scouleri ssp. grandis occurs on seaward slopes along the coast (Peck 1941). In California, it occurs on rocky slopes and coastal bluffs under 300 metres (m) elevation (Hickman 1993). In Canada, Silene scouleri ssp. grandis is restricted to the lowland Coastal Douglas-fir biogeoclimatic zone. It prefers winter wet/summer dry grassy openings on gently-sloping sites, generally under 30 m elevation (locally referred to as maritime meadows). Common associates include Rosa nutkana, Symphoricarposalbus, Pteridium aquilinum, Festuca rubra, Achillaea millefolium, Grindelia integrifolia, Fragaria chiloenesis, Piperia maritima, Lomatium nudicaule, Hypochaeris radicata, Camassia sp. and Aira praecox. It is absent from shrub thickets and from patches of robust introduced grasses within both populations. The Mount Tzuhalem population, now presumed extirpated, was atypical with respect to elevation (over 200 m); otherwise the site and vegetation characteristics of that location are similar to conditions at Trial Island, Little Trial Island and Alpha Islet.

Trends

The loss of Garry oak ecosystems in Victoria parallels the loss of suitable habitat for Silene scouleri ssp. grandis. The extent of these ecosystems decreased 95% from 10,510 hectare (ha) in 1800 to 512 ha in 1997 (Lea 2002). This loss is almost entirely attributable to agricultural conversion followed by residential and recreational development.

Silene scouleri ssp. grandis is now restricted to open meadows a few metres above sea level on offshore islets. Habitat quality has decreased on these islets as several introduced species of shrubs, grasses and forbs have formed thick swards at many locations. Aboriginal burning to improve camas crops may have maintained habitat in the past. If so, fire suppression may now favour the development of dense patches of native shrubs where Silene scouleri ssp. grandis cannot survive.

Protection/ownership

Silene scouleri ssp. grandis populations are restricted to areas that are provincially owned. No populations are known to occur on private lands. The Alpha Islet population occurs in Oak Bay Islands Ecological Reserve and the Little Trial Island sub-population occurs in Trial Islands Ecological Reserve while the Trial Island sub-population occurs on lands leased to Seacoast Communications, who maintain radio-communication towers on the site.