Dense spike-primrose (Epilobium densiflorum) COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
Climatic limitations
In Canada, Epilobium densiflorum is restricted to a small area of southeastern Vancouver Island. This area, which is highly correlated with the distribution of Garry oak, has mild winters and dry, cool summers. The four extant populations and most or all of the historical populations occur close to the Pacific Ocean. Winter temperatures are moderated by proximity to the ocean, which is dominated by mild waters of the California Current all year long. Summer temperatures are lower near the ocean and daily maximum temperatures often increase by several degrees as one moves inland on a warm summer day.
Edaphic limitations
Edaphic factors strongly limit the distribution of Epilobium densiflorum within southeastern Vancouver Island and the adjacent Gulf Islands. The low-elevation band of coastal environments is narrow and quickly gives way to uplands that lack the necessary mesoclimatic conditions. Broken terrain over much of the lowland band creates many cool north- and east-facing slopes where forest plants have a comparative advantage. Well to moderately-well drained soils favour forests. Shallow, outcropping rock is common in the coastal fringe and provides too little moisture to sustain Epilobium densiflorum.
It appears that the suitable soil conditions only occur in a small fraction of the landscape where the soil is too wet in the winter to allow woody species to flourish, and yet too dry in the summer to support wetland species.
In summary, edaphic factors limit Epilobium densiflorum to a small portion of a climatic region which itself is restricted to a very small area of Canada.
Habitat loss
The loss of habitat (described earlier) is likely to continue into the future. The distribution of Epilobium densiflorum in Canada lies at the heart of one of North America’s fastest growing regions. The Victoria Metropolitan Area includes two of the four extant populations of Epilobium densiflorum and many of the apparently extirpated populations. The population of metropolitan Victoria has increased from approximately 180,000 in 1966 to 318,000 in 1996 and is projected to increase to more than 400,000 by 2026. The Capital Regional Plan adopted in 1959 led to expansion into rural areas surrounding Victoria over the past 44 years and this pattern of expansion may continue in order to accommodate population increases (Capital Regional District Regional Growth Strategy 2003a). During past the 18 years, average prices for single family residences in Metropolitan Victoria have increased 330% from $94,000 to $313,000, reflecting the pent-up demand for property (Capital Regional District Regional Growth Strategy 2003b).
Similar projections apply to the Nanaimo Regional Districts, where Epilobium densiflorum also occurs. The population of the Regional District of Nanaimo increased from 77,624 residents in 1981 to 127,016 residents in 2001, and is projected to increase to 219,321 residents by 2026 representing an average growth rate of approximately 2.9% per year and a total increase of 73% over 1981 to 2026 (British Columbia Statistics Population Section, 2003).
The real estate market within the Regional District of Nanaimo saw above-average growth during 2002-2003, due to low mortgage rates and consumer confidence in the prospects of the provincial economy. According to the Canada Mortgage and Housing Corporation (2003), housing starts in Nanaimo were 57.7% higher in the first quarter of 2003, compared to the first quarter in 2002. The momentum of starts is expected to continue through 2003 as low housing inventories in British Columbia and strong consumer demand continue to support sales activity (Canada Mortgage and Housing Corporation 2003). Despite the rocky terrain, the site where population No. 1 occurs has high potential for residential development due to the broad panoramas provided of the surrounding landscape.
The most pressing threat to Epilobium densiflorum in Canada comes from the potential development of site 1 supporting the largest of the remaining populations. As of July 28, 2004, there were no current development applications on the property where population #1 occurs (Kevin Brydges pers. comm. 2004). There have been, however, two recent attempts to develop the site for residential purposes and it remains the property of a real estate developer. Consequently, site development remains a potential threat to this population.
Threats associated with altered fire regimes
Pre-European fire regimes in the dry coastal belt of southeast Vancouver Island are probably more complex than is generally reported. There is no doubt that First Nations in the area used fire extensively to stimulate the growth of food species -- particularly Camassia bulbs that provided a storable form of starch (Turner and Bell 1971). Fire may also have been used to improve forage for game species (elk and deer).
Frequent low-intensity burns killed young red alder and Douglas-fir and checked the growth of trembling aspen and most shrub species -- notably Symphoricarpos albus and Rosa nutkana. The resulting increase in light levels and decrease in competition favours the growth of herbaceous plants such as Epilobium densiflorum. Even the composition of the herb layer was altered, since many highly competitive plants decrease under a regime of frequent burning (Turner and Bell 1971, Roemer 1972, and pers. obs.).
First Nations’ fire management practices also played a significant role in the development (and therefore fertility) of soils. The organic component of the upper mineral horizon would not be greatly reduced by low-intensity fires because it accumulated through the in-situ decomposition of root material. In contrast, the surface organic materials did burn, rather than accumulate, releasing nutrients. Since the main inputs of organic matter came from herbs rather than coniferous trees, the upper mineral horizon also had a relatively neutral reaction in sharp contrast to the acidic nature of soils under Douglas-fir forests. As well, the frequent fires provided a continuous supply of “safe sites” where the small seeds of Epilobium densiflorum may have been able to germinate and grow without the stifling influences of surface organic horizons.
Threats associated with livestock grazing
In some respects, livestock grazing during the early 20th century may have offset the impacts of altered fire regimes. They convert recalcitrant foliage litter into labile forms (primarily dung), releasing nutrients in a form available to plant growth (Stoddart et al. 1975). The persistence of Epilobium densiflorum at many sites (e.g., Uplands Park) until the late 20th century may have been partly related to the lingering affects of livestock activity earlier in the century. Livestock owners may have created or maintained open habitats such as is found at the site for population No 4. Such open habitats are now shrinking as the cover of forest trees increases (pers. obs.).
More significantly, livestock grazing also played a major role in the establishment and eventual dominance of exotic forage species, which appear to have pre-empted sites where Epilobium densiflorum formerly flourished.
Threats associated with invasive alien plants
Invasive plants pose the most striking threat in most open habitats in southwestern British Columbia. Extant populations of Epilobium densiflorum were seen growing in a matrix dominated by invasive alien shrubs (e.g., Crataegus monogyna, Cytisus scoparius, Rubus armeniacus and R. laciniatus), grasses (e.g., Agrostis gigantea, A. stolonifera, Aira caryophyllea, A. praecox, Anthoxanthum odoratum, Bromus hordeaceus, Cynosurus echinatus, Holcus lanatus, Phalaris arundinacea and Vulpia bromoides) and herbs (e.g., Barbarea vulgaris, Hypericum perforatum, Hypochaeris radicata, Leucanthemum vulgare, Lythrum salicaria, Plantago lanceolata, Ranunculus repens, Rumex acetosella and Vicia sativa).
Invasive species threaten Epilobium densiflorum in many ways. Taller plants may suppress it by shading it out. Many alien species may out-compete it for moisture (particularly in the late spring and early summer) and nutrients. The most serious threat posed by alien annual species is their ability to pre-empt “safe sites” -- recent soil disturbances that would be favoured by Epilobium densiflorum were it not for the ubiquity of annual invasives. Perennial aliens appear to establish permanent cover in sites that formerly provided a constant supply of bare mineral soil.
Limitations on a potential rescue effect
The nearest United States populations of Epilobium densiflorum occur on San Juan Island, where it is rare and is only known from a few sites. These populations are about 20 km from the nearest recorded Canadian population in Victoria. It does not occur elsewhere in the San Juan Islands (Atkinson and Sharpe 1993).
There is little likelihood of interchange between Canadian and American populations because of the species’ limited powers for unassisted dispersal (unlike most species of Epilobium, it lacks a coma to assist in wind dispersal). Similarities in habitat suggest that seeds from U.S. populations may be relatively well adapted to site conditions that prevail where Canadian populations occur, so there is some potential for deliberate re-introduction if Canadian populations are extirpated by events that do not reduce habitat suitability.
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