Coast Microseris (Microseris bigelovii) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Habitat loss

The loss of habitat is likely to continue into the future. The distribution ofMicroseris bigelovii in Canada lies at the heart of one of North America’s fastest growing regions. The Victoria Metropolitan Area includes five of the six extant populations of M. bigelovii and four of the five apparently extirpated populations.

Threat associated with recreational activities

One major threat to remaining populations of Microseris bigelovii comes from recreational use and outdoor recreation development. The populations at Hornby Island, Oak Bay #1, Oak Bay #2 and Esquimalt all occur in popular walking areas. Light foot traffic likely favours M. bigelovii by discouraging the growth of competitive species. The plants were not observed directly on footpaths so it appears that heavy trampling threatens the species. Picnickers were observed lunching on a blanket placed directly over a major portion of the population at Oak Bay #2 during seed maturation in 2004. Recreational use is likely to increase in all of these locations as the number of people in southeast Vancouver Island increases (see above). Recreational use also poses an indirect threat as new structures such as park benches and interpretive displays are developed. Several park benches have already been established on or adjacent to subpopulations of M. bigelovii at Oak Bay #1 and Esquimalt, as well as near populations of other rare plants (including Triphysaria versicolor, Limnanthes macounii and Callitriche marginata) elsewhere in the Victoria area. Species that favour small, level areas such as M. bigelovii are at greatest risk because these are the easiest areas to build benches and kiosks and are much less extensive than the rugged rock outcrops that typify many coastal parks on southeast Vancouver Island.

Threat 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 (Turner and Bell 1971) – particularly camas bulbs, which provided a storable form of starch. 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 low herbaceous plants such as Microseris bigelovii. Even the composition of the herb layer is altered, since many highly competitive plants decrease under a regime of frequent burning.

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 was not greatly reduced by low-intensity fires because it accumulated below the surface 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 (Broersma 1973). As well, the frequent fires provided a continuous supply of ‘safe sites’ where the small seeds of Microseris bigelovii may have been able to germinate and grow without the stifling influences of litter and surface organic horizons.

First Nations’ burning has ended and all fires are now suppressed throughout the range of Microseris bigelovii. In the absence of burning, the supply of suitable habitats has likely diminished.

Threat 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 (more easily decomposed) forms (primarily dung), releasing nutrients in a form available to plant growth. The persistence of Microseris bigeloviiat many sites (e.g. Uplands Park) until the late 20th century may have been partly related to the lingering effects of livestock activity earlier in the century.

More significantly, livestock grazing also played a major role in the establishment and eventual dominance of exotic forage species, which have pre-empted sites where Microseris bigelovii may have formerly flourished.

Threat associated with invasive alien plants

Invasive plants pose the most striking threat in most open habitats in southwestern British Columbia. Extant populations of Microseris bigelovii were seen growing in a matrix dominated by many invasive alien shrubs, grasses and herbs.

Invasive species threaten Microseris bigelovii in many ways. Some species (e.g. Trifolium subterraneum, T. dubium, Silene gallica) are capable of growing in the drought-stressed environments where M. bigelovii occurs and present a direct threat. Such 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 which would be favoured by M. bigelovii were it not for the ubiquity of annual invasives. Perennial aliens may have established permanent cover in sites that formerly provided a constant supply of bare mineral soil. Taller plants may not be able to survive in the drought-stressed microhabitats where M. bigelovii grows, but in finely grained mosaics they may occupy adjacent habitat and shade it out. 

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