Muhlenberg's centaury (Centaurium muehlenbergii COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

There are three extant localities of Centaurium muehlenbergii in Canada. Only one of these (Oak Bay) has been surveyed regularly in the past decade. The Gulf Island population occurs on First Nations land with restricted access, while the Nanaimo population was not discovered until 2003 (Table 1).

In June 2002, the lead author conducted a week-long survey of potential Centaurium muehlenbergii habitat on southeastern Vancouver Island.The surveys were undertaken as part of a series of projects designed to document the distribution of rare plants in open meadows on southeast Vancouver Island and the Gulf Islands. Over the past decade, >500 person-days have been spent searching for rare species in suitable habitats, spanning 1000 ha of suitable habitat in >80 sites (Fairbarns et al. 2003). Using aerial photographs and topographic maps, potential habitat areas were identified and accessed wherever possible. An area of approximately 200 hectares was searched, without yielding any new localities (Table 1). Some potential territory was under private ownership with limited access, and was not searched. The search sites included: all of Trial Island and portions of Chatham Island, Discovery Island, Rocky Point, Uplands Park, Government House, Harling Point, Griffon Island, Little Saanich Mt., Mill Hill, Fort Rodd Hill National Historic Site, Witty’s Lagoon, Sidney Island, Somenos Garry Oak Protected Area, Mt. Tzuhalem Ecological Reserve, and Harewood Plains (Fig. 3).

It is possible that Centaurium muehlenbergii continues to be overlooked elsewhere on southern Vancouver Island and the Gulf Islands, especially in some of the under-collected tidal marshes found along the coast. Continued surveys for this species are therefore recommended.

Survey method. The survey method used to date for Centaurium muehlenbergii is the “directed search.” In this approach, surveyors familiar with the taxon in question rely on a combination of expertise and intuition to target areas most likely to support suitable habitat. They then sample the area for species presence/absence by walking it repeatedly until they are satisfied the habitat has been sufficiently well searched. This is generally considered by rare plant specialists to be the most efficient and cost-effective method of surveying for rare plants, and is the most common approach taken to date by botanists in British Columbia. However, this approach does not lend itself to statistical evaluations, making it difficult to assign confidence levels to past search efforts.

 

Abundance

The Oak Bay population is the largest known population with an estimated 800 (500-1000) flowering individuals in 2002. This represents a rough estimate only, based on a one-time count in 10 randomly placed 1-m2 quadrats. It does not take into account the somewhat staggered flowering phenology of the plants or the possible presence of a dormant seed bank, either of which may have resulted in an underestimate of population size.

The Gulf Islands site contained 30 flowering plants in 2003. The Nanaimo population is a very small patch, with only seven plants tallied in 2003 (A. Ceska, pers. comm. 2004).

Fluctuations and trends

Early collection notes for Centaurium muehlenbergii on Vancouver Island do not specify population sizes, thus long-term population trends are unknown. However, both the Oak Bay and Gulf Island populations have persisted in small areas for at least 40 and 70 years, respectively, suggesting that population dynamics at these locations have remained relatively stable. On the other hand, it is well known that annual plants commonly undergo major fluctuations in population size from year to year (Harper 1977). The same probably holds true for this species. Considering the amount of habitat lost to development over the past century, it is conceivable that there has been a historical decline in the populations in the region.

Rescue effect

The nearest extant populations of Centaurium muehlenbergii outside Canada occur in Oregon State (NatureServe 2005). The status of plants represented by the 1992 collection on San Juan Island is unknown since the species is still recognized as of historic occurrence in the state. The distance between the Canadian and U.S. populations is such that natural interchanges between them, genetic or otherwise, are highly unlikely. Environmental differences between regions may have resulted in the evolution of distinct ecotypyes ill-adapted for survival in other environments. Such ecological or genetic divergence could seriously limit the feasibility of relying on sources of propagules from elsewhere for purposes of reintroduction should the Canadian population become extirpated.

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