Rosy owl-clover (Orthocarpus bracteosus) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Habitat degradation poses a critical and urgent threat to Orthocarpus bracteosus in Canada. The single remaining population occupies an area of less than 300 which already contains a number of alien species including Plantago lanceolata, Hypochaeris radicata, Holcus lanata, Vulpia bromoides and Geranium molle. In addition, English ivy (Hedera helix) has reached the margins of the population and may continue to spread in phalanx fashion across the site as a ground cover. Several introduced shrubs including Scotch broom (Cytisus scoparius), gorse (Ulex europaeus), and spurge-laurel (Daphne laureola) also occur in the vicinity of the site and may pose a longer-term threat if they prove capable of invading the site or altering it by shading and litter deposition.

The second threat to the existing population comes from foot traffic. Trial Island is relatively inaccessible except to Coast Guard staff and recreational boaters from nearby Victoria. Nevertheless, the population is situated in a constriction between two rocky areas and attracts some foot traffic, likely attributable to kayakers.

The Trial Island population is also potentially threatened by marine pollution because it grows only a few metres above the high tide line along one of the most active shipping lanes in North America.

The Trial Island population has little potential for dispersal into suitable habitats. It appears to have little capability for long-distance dispersal. Many waterfront sites on nearby areas of Vancouver Island that were formerly capable of supporting the species have been irreversibly impacted by urbanization. Sites elsewhere on Trial Island and nearby areas of Vancouver Island which remain capable of supporting the species, are currently unsuitable due to encroachment by highly competitive, non-native shrubs and grasses.

The very fact that the species is limited to a single, small site in Canada presents a serious limitation because stochastic forces could quickly eliminate the population. There is no opportunity for a rescue effect from elsewhere. All other populations in the Victoria area and Puget Sound appear to have been extirpated. The nearest known, extant populations are approximately 300 km away.

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