Coast Microseris (Microseris bigelovii) COSEWIC assessment and status report: chapter 4
Distribution
Global range
Microseris bigelovii ranges from Vancouver Island south along the coast to California (Hitchcock et al. 1955; see Figure 2). The nearest United States record was from San Juan County in North Puget Sound. The species disappeared from San Juan County, where it formerly occurred at a number of locations, mostly on San Juan Island. The most recent San Juan County report was made in the mid-1980s (Atkinson and Sharpe 1993). It has not been found elsewhere in Washington State, where it is now listed as extirpated (Florence Caplow pers. comm. 2003). The nearest United States records are from Yachats, Oregon about 450 kilometres (km) south of the nearest Canadian populations. M. bigelovii has been extirpated from mainland Oregon, apparently as a result of competition from introduced weeds. It persists on some offshore islands, where bird guano appears to limit growth by weedy competitors (K. Chambers, pers. comm. 2004).
Canadian range
In Canada, Microseris bigelovii is restricted to southwestern British Columbia from Hornby Island to Victoria and vicinity (Figure 3). It occupies a coastal strip, no more than 50 metres wide, along southeast Vancouver Island. The extent of occurrence of this strip was estimated to cover approximately 20 km² (using GIS tools to calculate the length of the shoreline between Comox and Rocky Point).
The Canadian populations tend to be widely separated. The northern population is approximately 150 km from its nearest neighbour. The other populations are all separated by at least 2, and up to 8 km. The Canadian populations tend to occupy quite small areas (Table 1) and have a total area of occupancy approximating 3,000 m².
Four lines of evidence suggest it is a native element of the Canadian flora. The first line of evidence relates to the presence of many other disjunct species with similar distributions. The sub-Mediterranean climate of Victoria and the Georgia Basin (including San Juan County, Washington State) is anomalous along the Pacific Northwest coast and may account for the pattern of disjunct distributions of many “semi-desert species” (e.g. Allium amplectens, Crassula connata, Clarkia purpurea ssp.quadrivulnera, Dryopteris arguta, Isoetes nuttallii, Juncus kelloggii, Minuartia pusilla, Lupinus densiflorus, Montia howellii, Ranunculus californicus, Trifolium depauperatum, Triphysaria versicolor and Woodwardia fimbriata) as noted by Hitchcock et al. (1961 p. 321).
Secondly, Microseris bigelovii is locally abundant and broadly (if thinly) distributed in southeast Vancouver Island and, formerly, on San Juan island, despite the poor dispersal abilities of its seeds. It sometimes occurs on very remote sites that are rarely if ever visited by people. This pattern of distribution suggests a relictual origin rather than recent immigration.
Thirdly, it was collected in Victoria as far back as in 1875 (Macoun collection #14986 at CAN account number 111,350), early in the European settlement of Vancouver Island and at the very beginning of botanical studies in the area. In conclusion there is little evidence to suggest it is an introduced taxon.
Fourthly, there are significant genetic differences between British Columbia plants and most populations in the United States (see genetic description) which may reflect the fact that Canadian M. bigelovii populations have been isolated from other populations long enough for genetic and morphological differences to evolve.
There have been either 11 or 12 reported populations of Microseris bigelovii (depending on whether the 1947 Beacon Hill Park and 1910 Dallas Road collections came from a common population) in Canada (Table 1). Considering the degree of habitat fragmentation, the limited dispersal ability of the species and its diminutive stature, it seems reasonable to conclude that recently discovered populations were probably previously overlooked. Since there are now only six extant populations, it appears there has been a long-term decline in the number of populations. There is no reliable evidence on changes in the number of populations over the past decade.
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