Hill’s thistle (Cirsium hillii) COSEWIC assessment and status report: chapter 4

Distribution

Global range

Cirsium hillii is endemic to North America, with a range that is centred in the Great Lakes region. Its distribution ranges from southern Ontario, through Michigan, Wisconsin, Minnesota, Iowa, Illinois, and Indiana (Penskar 2001) (see Figure 1). References to the occurrence of the species in Ohio were determined to be in error as a result of field searches and herbarium investigations by Allison Cusick (1995), and earlier references to its distribution in Pennsylvania appear to have all been revised to C. pumilum (e.g. NatureServe 2001, Penskar 2001).

Figure 1. Global distribution of Cirsium hillii (updated from White & Maher, 1983).

Figure 1.  Global distribution of Cirsium hillii (updated from White & Maher, 1983).

Canadian range

The Canadian range is restricted to southern Ontario (Figure 2), where it is found only in Bruce and Simcoe Counties, and Manitoulin District. A total of 64 extant stations have been recorded, primarily along the shores of Manitoulin Island and the west side of the Bruce Peninsula. Other extant stations are known from Cockburn Island (1), Barrie Island (1), Clapperton Island (2), Amedroz Island (1), Great La Cloche Island (2), Little La Cloche Island (1), La Cloche Peninsula (1), Squaw Island (1), Club Island (1), Fitzwilliam Island (1), and Simcoe County (1). Despite the fact some of these stations are technically “historical” according to the NHIC standard of 20 years, they have been retained in this report as extant, for the reason that to the best of our knowledge no one has revisited these remote sites, and there is no reason to believe the populations have been extirpated. Morton & Venn’s records for Clapperton, Amedroz, Squaw, and Club, for example, from the 1970s and early 1980s, would fall in this category. A total of six stations are either known or believed to be extirpated, with four of these from the Lake Huron Shore of Bruce County. One station is treated as a historical population of unknown status: Cove Island. An additional five stations are considered to be erroneous: Cape Crocker, Flowerpot Island, and the Fishing Islands in Bruce County, and Johnson’s Harbour in Grey County, and reports from Manitoba.

Figure 2. Canadian distribution of Cirsium hillii.

Figure 2. Canadian distribution of Cirsium hillii.

The extent of occurrence in Canada is estimated at 3,000 square km, and there has been a decline in this area over the past one hundred years with cessation of natural fire and development in shoreline alvars and open woodlands (and loss of the Walpole Island population). The area of occupancy is difficult to assess, as the 64 known stations for the species have not been fully documented as yet. However, using the recent figure of 112 square km of alvar of reasonable quality remaining across the entire Great Lakes basin (The Nature Conservancy, 1999), it seems appropriate to use 30 square km as the area of occupancy for Cirsium hillii in Ontario. The area of occupancy is in decline, due to reasons noted for decline in the extent of occurrence, and as described in the Limiting Factors and Threats section. Declines in area of habitat (area of occupancy) have been noted at Sites, 32, 33, 35, 44, 46 and 55. No extreme fluctuations in area of occupancy are known.

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