Recovery Strategy for Pitcher's Thistle in Canada [Final Version]: Populations and Distribution

The global range of Pitcher's Thistle is shown in Figure 1. There are 30 extant populations in Canada, all in Ontario. Three populations are on southern Lake Huron, two are on Lake Superior in Pukaskwa National Park, and the remaining 25 are in the Manitoulin Region (on Manitoulin Island, Cockburn Island, and the Duck Islands). The bulk of the global range of the species is in Michigan, where there are approximately 128 sites in 18 counties. There are also 9 sites in Wisconsin and 7 in Indiana (NatureServe 2010). The species was apparently extirpated from Illinois but has been successfully reintroduced at one site. Based on current information (Jones 2009, NatureServe 2010), a cautious estimate would give Canada approximately 15% of the global population if all Canadian occurrences are as big or bigger than U.S. occurrences.

The historic extent of Pitcher's Thistle in Canada is poorly known. Records show the species once occurred at Sauble Beach in Bruce County and at Kettle Point in Lambton County (Guire and Voss 1963, NHIC 2010), but has been extirpated from both areas. A population at Crescent Beach in Pukaskwa N.P. has not been seen since 2006 and is also presumed extirpated. Pitcher's Thistle was planted in 1999 at Chantry Dunes, Southampton, Bruce County, a site where the species was not historically known to have occurred. Apparently the reintroduction was unsuccessful as no Pitcher's Thistle was found in 2002 (Jones 2002). The Middle Beach population at Pukaskwa N.P. (included in the tally of 30 extant populations) is also an artificial introduction, but the species is assumed to be native within the park at Oiseau Bay. The introduction at Hattie Cove has been quite successful, and the population size has increased greatly.

Survey and monitoring work since 2001 has discovered several previously unknown populations of Pitcher's Thistle in the Manitoulin Region. At the same time, monitoring has documented large increases in numbers of individuals in many known populations in that region. The current total Canadian population is around 55,000 individuals (2008 monitoring data) of which roughly 15,000 were mature plants that died after flowering, leaving approximately 40,000 rosettes in 2009. This calculation did not take into account new seedlings established in 2008 or 2009. Site by site abundance of Pitcher's Thistle is given in Appendix B.

Figure 1. Global range of Pitcher's Thistle (sources: Bowles and McBride 1994, NHIC 2009, Jones 2009, NatureServe 2010).

Nearly all major dune sites on Lake Huron have now been searched for Pitcher's Thistle, so there is little likelihood of finding additional new populations. Jones (2002, 2003a) found no Pitcher's Thistle at Sauble Beach, at 27 other sites on southern Lake Huron, or at 66 sites on the North Channel (Jones 2006), although much suitable habitat is still present. Many large areas of excellent dune grassland habitat (not occupied by Pitcher's Thistle) exist on the Lake Huron - Georgian Bay shoreline, and many of these sites support at-risk, rare, or endemic species. Although most of these sites are not known to have supported Pitcher's Thistle historically, most are thought to be suitable and could support Pitcher's Thistle if the species should get established there or be introduced. As well, large dunes such as these are subject to the same threats outlined here for Pitcher's Thistle, so recovery actions developed for this species may also benefit these sites and the associated rare species.

Monitoring data from 2001 to 2009 show a steady, multi-year increase in overall numbers in 15 populations. Some populations have increased as much as 200-800% while others have had more gentle increases. For 9 populations, multi-year data show natural fluctuations in numbers due to flowering and die-off. Six populations have undergone serious declines due to threats-5 of these are small populations affected by succession as well as at least one other factor such as browse or ATV use. These populations are: Belanger Bay, Crescent Beach, Christina Bay, Fisher Bay, and Michael's Bay. The decline at Pinery Provincial Park may be the result of recreational pressure or perhaps from ecological processes (MacKenzie, pers. com. 2010).

Pitcher's Thistle clearly has the reproductive capacity to recover and increase in numbers over just a few years, so it remains unclear why numbers of individuals were so low when monitoring began in 2001-2003. It is likely there are longer time-frame cycles of increase and decline that have not yet been observed in their entirety. As well, it is possible that heavy, synchronous flowering and massive die-off (caused by a series of hot, dry years) without concurrent seedling establishment could cause periodic crashes in population numbers (Jones, pers. com. 2010).

Globally the Little Bluestem - Long-leaved Reed Grass - Great Lakes Wheat Grass Dune Grassland vegetation where Pitcher's Thistle is found is in decline. It has been given a global conservation rank of G3G5 and thus may be of global conservation concern. In Ontario this vegetation type is ranked Imperilled (S2) (Bakowsky, 1996; NHIC 2010).

A number of threats affect Pitcher's Thistle and dune grasslands (Table 1). They derive from both natural and human sources. Presence and severity of threats vary by region and by ownership since dunes in protected areas may experience different usage than dunes on private lands.

During monitoring of Pitcher's Thistle and its habitat, the threats listed below were assessed at each site using a standardized protocol and specific criteria for each threat (Dune Grasslands Recovery Team 2004).

Table 1: Threats to Pitcher's Thistle and habitat at individual sites. Severity is ranked as H - High, M - Medium, or L - Low. (Source: monitoring data and observations by recovery team members). A blank box indicates that no evidence of that threat was detectable at the site.

Off-road Vehicles: Off-road vehicle use is a serious concern, especially as all-terrain vehicles (ATVs) are nearly unrestricted in their movements and do not require trails or roads. ATVs disturb or destroy vegetation, displacing the grasses and shrubs that stabilize sand and allowing erosion and blow-outs. They are also vectors for weeds. Because ATV use is an increasingly popular recreational pastime, the threat is widespread. Damage to habitat from ATVs is prevalent on Manitoulin Island especially where shoreline areas contain a public right-of-way.

Browse: Pitcher's Thistle is subject to browsing by White-tailed Deer (Odocoileus virginianus), the Plume Moth (family Pterophoridae), rabbits, and Canada Geese (Branta canadensis). Leaves in the rosette stage may be eaten as well as the young flowering heads. In some cases, browsing can be so severe that only the small central nub of the plant remains. Browsing may contribute to over-wintering mortality by reducing the quantity of energy being stored in reserve. Browsing has contributed to a drastic crash in the population at Western Duck Island, and is an especially serious issue where it is combined with other threats such as in very small populations with marginal habitat (Jones 2009).

Browsing is considered a threat, rather than a natural limitation because the effect can be very serious and the high number and densities of browsers is not natural where Pitcher's Thistle occurs. The deer population on Manitoulin Island peaked in 2003 at a level higher than the carrying capacity of their habitat. Numbers were reduced in the last few years by increasing the number of tags issued to hunters (Wayne Selinger, pers. comm). Browsing by deer continues to be a problem observed during monitoring. The huge population explosion of Canada Geese in recent years contributes to the problem. The results of heavy browsing seriously threaten the continued survival of Pitcher's Thistle at several sites.

Trampling by Pedestrians: High visitor use and the resulting foot traffic, especially in protected areas, can damage vegetation and remove stabilizing grasses and shrubs. Foot traffic creates paths through dunes that then encourage more pedestrian use causing further increases in the severity of this threat.

In recent years, this threat has been addressed at Inverhuron Provincial Park, Pinery Provincial Park, Providence Bay, and Carter Bay by constructing boardwalks, stairways, or designated pathways through dunes to keep pedestrians off the vegetation. The severity of trampling, although still a concern, is now somewhat reduced at Inverhuron Provincial Park, Providence Bay, and Carter Bay. At Pinery Provincial Park, this threat remains a concern and may be one factor in the continuing decline of the population there. At Pukaskwa National Park, trampling has been reduced at Hattie Cove by cordoning off the Pitcher's Thistle population with a railing.

Succession: Pitcher's Thistle requires some level of natural environmental disturbance to keep sand open, loose, and sparsely-vegetated. In natural dune ecosystems, the actions of wind, wave-wash, and storms accomplish this, counteracting the natural trend for land to grow up with shrubs and trees. However, in the absence of natural dune processes, sand may become too vegetated for dispersal, germination, and establishment of Pitcher's Thistle seedlings (although rosettes may survive for a number of years in heavy vegetation). Low water levels in Lake Huron have moved the wave-wash zone away from the foredune, allowing heavy growth of vegetation to occur at some sites.

Human Structures: This threat encompasses both new construction on dunes and the presence of existing structures that change or prevent dune processes. Most sandy bays on Manitoulin Island are subdivided and have cottages present in the back dune or forest. Some landowners have placed fire pits, volleyball courts, boat storage, decks or stone patios, or even fill dirt and grass lawns on dunes. Depending on how they are constructed and the intensity of use, these structures may damage or destroy Pitcher's Thistle and areas of habitat. Human structures on dunes remain an on-going issue.

Shoreline development itself may destroy dune habitat; however, in many cases development occurs in the adjacent forest directly behind dunes rather than on the dunes themselves. Maintaining an adequate set-back distance from active dunes during development planning can reduce impacts to Pitcher's Thistle habitat and allow dune processes to continue. However, the aftermath of shoreline development has generally resulted in increased human use of dunes which has increased the incidence of other threats discussed in this section.

Erosion and Blow-outs: Areas of open, loose sand are beneficial to Pitcher's Thistle, but actively eroding or suddenly shifting sand can mean a loss of plants and beneficial substrate as well as the potential of mortality from burial. Once an area begins to erode or blow out, there tends to be a sequential effect as the hole becomes bigger and bigger allowing more and more sand to blow and the effect to increase in severity. Erosion and blow-outs are more often the result of human activities although they occasionally occur naturally. Newly exposed sand may later be re-colonized by Pitcher's Thistle, but the net result of this threat is usually a loss of large areas of stabilized vegetation and a loss of thistles.

Invasive Species: Any species that spreads rapidly and takes over habitat to the exclusion of native species may be a threat on dunes. On southern Lake Huron, the invasive race of the Common Reed (Phragmites australis) has taken over huge areas of shoreline, eliminating the natural vegetation. So far, Pitcher's Thistle populations in the southern Lake Huron region remain unaffected, although the Port Franks site has other invasives present. However, monitoring has now picked up the presence of invasive Common Reed at several sites on Manitoulin. At Pukaskwa National Park, one site is becoming heavily overgrown with Silver-berry (Elaeagnus commutata) following a major natural disturbance. While this is a native species, it appears to have the ability to spread rapidly and reproduce heavily, and it is reducing habitat quality for Pitcher's Thistle there.

Genetic Isolation of Small Populations: Coleman (2007a) examined genetic diversity among Pitcher's Thistle populations and found the species had a low genetic diversity with heterozygote deficiencies within nearly all populations. This confirms earlier studies (Loveless 1984, Loveless and Hamrick 1988) showing that Pitcher's Thistle has a genetic diversity lower than other species of the genus Cirsium. Pitcher's Thistle is restricted to a very specific habitat within just the Great Lakes region. If a change in conditions or a disease were to occur, it is unknown whether the species would have the genetic diversity to adapt to a new situation. As well, the effects of small populations (fewer than 50 individuals) are many. With a monocarpic life strategy the plants do not flower every year, and with few individuals there is decreased likelihood of outcrossing because only a few plants may flower at the same time. The distribution of Pitcher's Thistle range-wide consists of many widely separated populations, several of which contain fewer than 50 individuals. These small populations face greater risk of extirpation. Lack of connectivity and genetic interchange between populations may limit Pitcher's Thistle recovery. Even within geographic regions, populations are separated by hundreds of kilometres. Large tracts of suitable habitat are no longer present between current populations, due to development and recreational uses. The effects of this potential threat needs further study.

Changes in Lake Levels: Flooding, wave-wash, and ice-scour are dynamic processes vital in maintaining the habitat of Pitcher's Thistle. The recent period of low water level in Lake Huron greatly increased the distance between the wave-wash zone and the dunes, removing active processes from the area where Pitcher's Thistle is found. This has resulted in a great increase in successional vegetation cover, with the habitat becoming nearly unsuitable at several sites. Historically, this has probably been one of the natural limitations on Pitcher's Thistle. However, with human controlled out-flow rates in the Great Lakes (Derecki 1985) and potential diversion of water from Lakes Huron and Michigan, it is not certain that the natural lake level cycles will continue.

Changes in Climate: Changes in temperature and rainfall patterns may affect Pitcher's Thistle. Monitoring data showed a great increase in the percentage of plants that flowered in 2006 and 2007, years which were exceptionally dry and hot. Drought has also been observed as a major cause of mortality in seedlings (McEachern 1992, D'Ulisse 1995, D'Ulisse and Maun 1996, Jones 2001, Weller pers. comm. in USFWS 2002). Since plants of Pitcher's Thistle die after flowering, any factor that changes the proportion of maturing plants may affect the overall population.