Scouler's catchfly (Silene scouleri ssp. grandis): COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

Habitat loss presents a serious and urgent threat to Silene scouleri ssp. grandis in Canada. The specialized coastline habitats in Victoria and surrounding areas have been extensively developed for residential and commercial purposes and recreation facilities.

Habitat degradation compounds this threat. Both populations are threatened by the encroachment of exotic grasses and shrubs, most notably Cytisus scoparius, Hedera helix, Ulex europaeus, Dactylis glomerata, Anthoxanthum odoratum, Lolium perenne, Bromus sterilis and B. hordeaceus. No weed control strategies have been developed on the islands.

The warm dry sites that support Silene scouleri ssp. grandis were probably burned frequently by First Nations groups seeking to improve camas production on the adjacent uplands. Fire has been almost completely suppressed on coastal sites for several decades, which has favoured growth by introduced shrubs as well as native species including Rosa nutkana, Symphoricarpos albus, Populus tremuloides and Pteridium aquilinum. S. scouleri ssp. grandis was not found within dense patches of native or exotic shrubs or thick swards of introduced grasses.

 Seed dispersal and rescue effects present a complex problem. At the broad scale, seed dispersal over distances greater than 10 metres is probably extremely rare. The widely separated populations have no potential for unassisted re-colonization of former sites.

Kayakers frequently visit Trial Island, Little Trial Island and Alpha Islet. They may cause some mortality as they walk about the maritime meadows and their beach fires could escape and burn the uplands. In July 2000, an escaped beach fire intensely burned a significant portion of Little Trial Island adjacent to the area occupied by Silene scouleri ssp. grandis and may have killed some plants. While Silene scouleri ssp. grandis may benefit from light fires, heavy fuel build-ups due to the introduction of exotic shrubs and grasses may lead to intense fires that could kill the plant caudices.

The Trial Island population faces additional pressure from trampling by resident staff from both the federal coast guard lighthouse a few hundred metres away and workers associated with the radio-communications towers and associated facilities. Two of the five plants on Trial Island are within a metre of a maintained path and are susceptible to impacts from trail maintenance activities including mowing and herbicide application.

Both populations are also at risk from potential marine pollution as they grow close to the intertidal zone of one of the most active oil shipping lanes in North America.