Hill’s thistle (Cirsium hillii) COSEWIC assessment and status report: chapter 2

Executive Summary

Hill’s Thistle
Cirsium Hillii

Species information

Hill’s thistle is a short (25 to 60 cm tall) perennial thistle with a deep, hollowed, clustered root system. The leafy stems are soft, ridged, and either sparsely hairy or with woolly hairs; with 1 or 2 short branches near the top terminating in one to several large heads (3.5 to 5 cm) with deep pink-purple, or occasionally white, flowers.

Distribution

Hill’s thistle 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. The Canadian range is restricted to southern Ontario, with a total of 64 extant stations supported, primarily along the shores of Manitoulin Island and the west side of the Bruce Peninsula.

Habitat

Throughout its range, Hill’s thistle is found in a variety of open, dry, sandy, fire-prone habitats, including such communities as gravel hill or bluff prairies, dry mesic to mesic sand prairies, pine barrens, oak barrens, sand dunes, oak savannah, and open woods. In Michigan, Wisconsin, and Ontario, it is also known from alvar grasslands. One essential aspect of the habitat of Hill’s thistle is its need for open, or relatively open conditions. From 41 Ontario stations with habitat information cited, the most commonly noted dominant species in the ground layer with Hill’s thistle is povery oat grass (Danthonia spicata), with bearberry (Arctostaphyllos uva-ursi) the most commonly noted associate, followed by the lichen Cladina rangiferina, bracken fern (Pteridium aquilinum), and little bluestem (Schizachyrium scoparium). Jack pine (Pinus banksiana), white spruce (Picea glauca), and white cedar (Thuja occidentalis) are the trees most often noted as dominant in the habitat, and common juniper (Juniperus communis) is the most noted dominant shrub, followed by creeping juniper (Juniperus horizontalis). Shoreline areas of woodland alvar on Manitoulin Island and the associated islands, and the west shore of the Bruce Peninsula, are critical to the long-term maintenance of Hill’s thistle in Canada. These woodland alvars have existed historically in a landscape shaped by natural disturbances such as drought and fire.

Biology

Hill’s thistle is a relatively short-lived perennial, generally persisting two or three years but usually no more than four or five. Flowers are produced one or two seasons after the establishment of the basal rosette, most typically in three-year-old plants. Reproduction also occurs vegetatively by adventitous buds that form along the lateral roots and give rise to basal rosettes. Seed production generally is abundant and the seeds are wind-dispersed, but excessive litter accumulation is thought to interfere with successful germination, and seedlings may be poor competitors for the available light and space. Loss of the historic, natural fire regime has also enhanced the degree of litter accumulation on historic habitats and has resulted in canopy closure of the openings that the species requires. Both flowers and seeds are vulnerable to insects and perhaps fungi. In Ontario, flowering occurs from mid-July through August.

Population sizes and trends

A total of 70 sites have been recorded for Hill’s thistle; six of these are now believed to be extirpated. Of the 64 extant sites, 36 have been recorded with quantitative data. For these, the total number of plants counted is at least about 4000, with about 250 of these flowering, about 3700 as vegetative rosettes, and 86 with no distinction made between reproducing plants and basal rosettes. Forty-five (113) of the flowering plants recorded during the 2002 field surveys occurred at one station, Site 35, which is licenced for aggregate extraction, and where there was evidence of site preparation for this activity at the time of the field work. Nearly all of these population counts are from 1995 to 2003, with only three prior to this, thus rendering long-term trend analysis impossible. Of the 36 sites with good population data, only 13 of these support populations with greater than 100 plants. The largest documented population is Site 57, with 1175 plants counted in 2003, including 35 plants that had flowered. It was estimated that this site may have had as many as 1500 plants. The largest populations are likely known; however some significant populations may still be undetected at the other 28 stations. The Canadian population likely consists of at least 5000 plants (flowering plus vegetative). The total number of plants in Canada capable of reproducing in any year is estimated at 10% of the total, about 500 plants. The total area of occupancy is estimated at 30 square km and the extent of occurrence for the 64 extant sites is estimated at 3,000 square km.

Limiting factors and threats

The species is subject to several limiting factors, the most important of which are the fact that its prime habitat, alvar, is very restricted and in decline, and the increasing degree to which these shoreline areas are being converted to permanent estate residences. What remains of this specialized alvar habitat is being lost to succession, in the absence of natural disturbance regimes, in particular fire, which served to maintain the open character of these sites. The site with the largest number of mature plants is under licence for aggregate extraction.

Existing Protection or Other Status Designations

The species is recognized as Globally Vulnerable (G3); it is Critically Imperiled (S1) in Illinois, Indiana, and Iowa; Vulnerable (S3) in Michigan, Minnesota, Wisconsin and Ontario; with a national rank in Canada of Vulnerable (N3).

COSEWIC History

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5th 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal agencies (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government members and the co-chairs of the species specialist and the Aboriginal Traditional Knowledge subcommittees. The Committee meets to consider status reports on candidate species.

Definitions (November 2004)

Wildlife Species
A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and it is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for atleast 50 years.

Extinct (X)
A wildlife species that no longer exists.

Extirpated (XT)
A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)
A wildlife species facing imminent extirpation or extinction.

Threatened (T)
A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)Footnotea
A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)Footnoteb
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)Footnotec
A wildlife species for which there is inadequate information to make a direct, or indirect, assessment of its risk of extinction.

 

Canadian Wildlife Service

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

 

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