Hill’s thistle (Cirsium hillii) COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
1) Limited habitat
The species is restricted to alvar and savannah habitats along the shore of Lake Huron and Georgian Bay. These vegetation types constitute some of the most threatened in North America, and in Ontario. More widespread and extensive historically, the prime habitat in southern Ontario today faces a multitude of stressors, some of which are virtually impossible to reverse, such as population growth.
2) Successional change and lack of disturbance:
All of the extant stations are subject to increased levels of succession in the absence of natural fire, and this trend toward more shaded habitats has been particularly noted in the last few decades by observers, e.g., Morton at Site 9, Beecroft at Site 60. With the exception of Fisher Harbour, the number of flowering plants did not exceed 15 at any site. Although this is a snapshot, it nonetheless raises serious questions regarding the current reproductive potential of populations and the factors that may be contributing to the low flowering rate. Some of the stations were composed entirely of basal rosettes when last surveyed, e.g. Sites 4 or had a very low number of flowering plants, such as at Site 44, a Parks Canada site, where less than one percent of the population was observed by the author as reproducing. There appears to be only a limited program to restore natural fire to these systems. It is likely that a much more ambitious program would be required to maintain the current distribution and abundance of Hill’s thistle.
3) Shoreline development:
The majority of the stations, 35, are in private hands, in one of the most scenic shoreline areas in the province. Development is encouraged by the municipalities in these areas; development has occurred on a scale that would not have been easily envisioned just a decade ago. Carter Bay on Manitoulin Island is a case in point (6,000 acres on 17 kms of shoreline, encompassing the best quality dune system in the province), and Gauley Bay on the west side of the Bruce Peninsula, are realities today. With the projected growth in southern Ontario, people will continue to demand permanent estate residences in the same shoreline areas supporting Hill’s Thistle. Five of the stations have been noted as threatened with residential development (Sites 3, 6, 51, 52, & 55).
4) Aggregate demand:
The alvar habitats supporting Cirsium hillii, are also favoured by the aggregate industry for limestone extraction. The station discovered in 2002 at Site 35, supporting about 45% of the known reproducing plants in the province, will probably be extirpated in the next couple of years for crushed limestone extraction. Of the three stations on Great La Cloche Island and Little La Cloche Island, Site 33 and 35 are under licence for extraction of the limestone (La Cloche Quarry Licence #3863 issued 29 May 1990). Both islands are owned by the same individual, and his licence applies to those areas of Great La Cloche and Little La Cloche south and southeast of Highway #6 (pers. comm. Hallaiken, 2004). This constitutes an area of some 20 square kms, much of which is prime alvar.
5) Invasive species:
Some invasive species may have a potential impact on Cirsium hillii in Ontario. The following invasive species were noted by the author in or very proximal to the populations during 2002 fieldwork: white sweet clover, Melilotus alba (at Great La Cloche Island); spotted knapweed, Centaurea maculosa (at Little Eagle Harbour-Coal Oil Point. These invasives could be especially troublesome in alvar, savannah, and woodland habitats on sandy soils.
6) ATVs and trail bikes:
Problems with ATV and trail bike use have been noted at several of the stations, including Site 60, where ATV use was believed to be the cause of extirpation of a sub-population. On the other hand, some of the sites on Manitoulin have not yet been affected but this is likely to change as more people take up residence. The alvars lend themselves well to this activity, and it will likely be a major stressor at many of the sites in the next few years.
7) Conversion to agriculture:
This is a minor threat to Hill’s Thistle, and unless the current agricultural practices change in southern Ontario, the alvar habitats should not become prime agricultural land in the forseeable future. The population of Hill’s Thistle recorded by Dodge in the early part of the last century on Squirrel Island was probably extirpated by conversion to agriculture, as was nearly all of the natural vegetation on the island outside of the marshes.
8) Deer browse:
The overall palatability and impact of deer browse on mature plants of Cirsium pitcheri, observed at Site 44 by the writer in 2002, are unknown. However, Manitoulin Island is well known for its high populations of White-tailed Deer. In the case of another Ontario thistle, the endangered Pitcher’s thistle, Cirsium pitcheri, deer browse was identified as a significant limiting factor in Pinery Provincial Park, Ontario (Maun 2000).
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