Golden paintbrush (Castilleja levisecta) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Habitat related threats including direct habitat loss and habitat deterioration due to invasion by exotic species and changes resulting from altered fire regimes are discussed above. The following section deals with direct threats to golden paintbrush plants where it currently occurs or existed in the past. The sharp decline on Alpha Islet is unusual because the site is almost free of invasive plants, has not experienced forest or shrub ingrowth, and was not affected by the threats described below (mowing, herbivory, trampling and marine pollution) during the period of decline.

Mowing

The decline and eventual extirpation of golden paintbrush at Beacon Hill Park appears to have been hastened by a summer mowing regime that removed inflorescences before they matured. The last remaining plants occurred in what is still an open meadow apparently capable of supporting golden paintbrush (F. Hook pers. comm. 2005). In contrast, fall mowing may actually favour golden paintbrush by reducing the invasive grasses that make up a significant component in meadow communities (Wilson and Clark 2001) and by increasing seed dispersal within suitable habitat (Caplow 2004).

Herbivory

Herbivory poses a minor threat to extant Canadian populations of golden paintbrush because they occur on small islands with no mammalian herbivores. Invertebrates may feed on tissue but damage is usually slight and localized (see above). Herbivory is a more significant threat to populations in areas with deer, rabbits and voles (Caplow 2004) and may have contributed to the loss of Canadian populations in the past.

Trampling

Trampling and related activities may have presented a minor threat to populations in the past, but the persistence of a population at Beacon Hill Park throughout several decades of intensive use suggests that the species was not particularly susceptible to trampling damage. Regardless, there is little foot traffic in either of the extant populations of golden paintbrush because they occur on islands where visitation is discouraged.

Marine pollution

Douglas and Ryan (1999) suggested that marine pollution may pose a threat to golden paintbrush in Canada because both extant populations occur just above sea level along the shipping lanes that pass through Haro and Juan de Fuca Straits. These are the most crowded and heavily used shipping lanes in North America north of San Francisco. Despite their arguments, it is not clear that marine pollution poses a significant threat to golden paintbrush.

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