Brook spike-primrose (Epilobium torreyi) COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

Epilobium torreyi is limited by its need for a summer-dry sub-Mediterranean climate, which only occurs on southeast Vancouver Island and some of the adjacent Gulf Islands in Canada. In British Columbia, the species was found in moist grasslands and on open slopes. Weak seed dispersal mechanisms limit its ability to spread to suitable, unoccupied habitats. It is further limited by its apparently weak competitive ability as evidenced by its decline in weedy areas.

Threats to the species are the degradation/loss of habitat mainly through alteration by residential and agricultural development. Habitat has also been altered by physical disturbance of sites by heavy equipment, by the introduction of alien invasive plants and natural encroachment of native Douglas-fir trees into open areas (due to fire suppression).

The extirpation of Epilobium torreyi in Canada appears to be the result of its inherent rarity (i.e., small disjunct populations at the northern extent of its range in North America) and the degradation/loss of habitats where it formerly occurred. The main sources of habitat degradation have been invasion by alien plant species and physical disturbance of sites by humans.

The Thetis Lake Park population originally occupied a small area within Craigflower Meadows. Prior to designation of the area as a park, the City of Victoria planted Abies grandis (Grand fir) on this site. As well, the site has been invaded by Crataegus monogyna (English hawthorn) and many alien invasive forbs and grasses. This has a serious impact on the habitat and may be responsible for the extirpation of the species.

The North Saanich occurrence was in an open, grassy meadow along the eastern end of McTavish Road. Much of this stretch has been converted to residential use and the remainder is either forested, used for intensive agriculture or has suffered from ditching and invasion by highly competitive non-native grasses (e.g., Agrostis capillaris) and shrubs (e.g., Rubus armeniacus). These alien invasive species have directly affected many native plant species by reducing light levels and competing for water and nutrients. Alien invasive plants have also had an indirect impact by gradually increasing levels of soil organic matter and nitrogen which may facilitate site invasion by other competitors including native species (D’Antonio and Vitousek 1992).