White meconella (Meconella oregana) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

In the Canadian populations the two main threats to the long-term survival of Meconella populations are loss of habitat and habitat degradation.

Loss of habitat to development is a major factor in the decline of Meconella. Eleven out of the 15 locations listed in Table 1 are in the Capital Regional District (CRD). The human population of the CRD has been growing rapidly and is forecast to increase by nearly 75,000 by the year 2020 (CRD Regional Growth Strategy 2003). As much of the easily developed land has been used up, present and future residential growth is now beginning to occupy the remaining rocky hillsides outside of parks, the general environment within which Meconella oregana microhabitats are embedded. Only one of the five extant Meconella occurrences is inside a park.

Unfortunately, the above-described trend of urban growth is impacting the two largest Canadian Meconella occurrences and has placed these under imminent threat of development. Both of these occurrences are on private land: The largest population (Port Alberni) was recently acquired by a new owner and is now under application for residential subdivision. The second-largest population (Skirt Mountain), part of a large golf course and residential development, is on the verge of being accessed by a new road. Subdivision application for the latter area is expected “within a year” (D. Fraser 2005, pers. comm., after enquiries with municipal planning department). Threats on these sites include complete obliteration by construction of roads and residences, landscaping, soil disturbance and hydrological changes, to name a few. Part of another population (Little Saanich Mountain) is also on private property. These three private sites together hold nearly 85% of the total Canadian Meconella population. All these sites have in common that they are highly desirable real estate on south-facing hillsides with mountain and sea views.

Meconella oregana is a species strongly associated with the Garry oak ecosystem. Lea (2002) mapped the decrease in Garry oak areas on the Saanich Peninsula for the period from 1800 to 1997. The Saanich Peninsula includes nine of the original 15 and three of the five extant Meconella sites (Table 1). Lea found that only 512 ha of Garry oak communities remained in 1997, less than 5% of the extent in the year 1800. A corresponding decline, possibly slightly less due to the more rocky terrain, will have occurred in Meconella habitat.

Habitat degradation can be initiated by disturbances such as domestic grazing or soil injury that favour invasion by exotic species (see “Habitat trends’). However, exotic species invasion also proceeds on its own and causes habitat change and degradation itself. As already mentioned, invasive species, in this case mostly annual grasses and herbs, form stands that are taller than Meconella and compete with it for light and soil moisture.

Seventeen sub populations within the five extant Meconella populations were examined for associated species by tabulating all vascular plants and bryophytes growing in the same microhabitat within 10 cm from the nearest Meconella plants (vegetation table available upon on request). Among 82 vascular plants, 25 were introduced species and the proportion of these species in the 17 subpopulations sampled ranged from 19 to 56%. Much higher percentages (up to 85%) of introduced species are found on rocky and grassy knolls within Greater Victoria (Roemer 1995) where some of the older, now lost Meconella sites were located.

Previous to the subdivision application tree harvesting occurred surrounding the natural opening of the Port Alberni population. This led not only to direct disturbance of the site by machinery and scattered wood debris, but also to increased access from all sides and new damage by motorcycles (Figure 4).

Fire suppression may also negatively impact Meconella oregana habitat. Regular fires would have maintained an open landscape and limited competition from other species (Rush et al. 1999; California Native Plant Society 2003; Washington Natural Heritage Program2004). Meconella oregana is vulnerable to competition from exotic, weedy grasses (Rush et al. 1999; NatureServe 2003; Washington Natural Heritage Program 2004).

Degradation of habitat from grazing or soil compaction from recreational use will negatively affect this species (Rush et al. 1999; Washington Natural Heritage Program2004). Changes to hydrology either of the site or upslope from populations will alter the habitat and will threaten this species (Rush et al. 1999; Washington Natural Heritage Program 2004).

As discussed under “Dispersal”, the extremely small patch size of Meconella subpopulations can be considered an indication that dispersal is an intrinsic limiting factor for the species.

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