Foothill sedge (Carex tumulicola) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

There are about 70 reported patches of Carex tumulicolain Canada, distributed among ten populations (Table 1). A few of these sites have been surveyed on two or more occasions since the late 1990s, when botanists first became alerted to the presence of C. tumulicola in Canada (Ceska 2000); other localities are relatively “new” and have only been visited once (table 1). 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).

Intensive surveys for Carex tumulicola began in 1999, after a student in a summer botany course brought in for identification an unknown sedge that he had collected on the University of Victoria campus grounds. The instructor in the course was A. Ceska, who confirmed the identity of the specimen (Ceska 2000). A follow-up visit to the herbarium of the Royal British Columbia Museum revealed that C. tumulicola had already been collected once before on Vancouver Island, in Oak Bay in 1990. However, the original collector, T.C. Brayshaw, was unaware thatC. tumulicola was rare in British Columbia and therefore did not report his finding (Ceska 2000). Subsequent surveys for the species yielded new records in 1999, 2002, 2003, and 2004 (B.C. Conservation Data Centre 2004).

Approximately ten days of fieldwork, carried out by the authors and others (J. Penny, H. Roemer) in 2003 and 2004 during peak flowering season, focused both on the confirmation of known populations and the search for new ones. As most known populations in B.C. are found in ephemerally moist areas in grassy meadows and open woodlands near the coast, searches for new populations focused on these features. Using aerial photographs and topographic maps, potential habitat areas in the vicinity of known sites at Metchosin, Victoria and Nanaimo were identified and accessed wherever possible (Figure 3). An area of approximately 200 hectares was searched, yielding three new populations and numerous new patches (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, Rocky Point, Uplands Park, East Sooke Park (Aylard Farm), Mt. Tolmie Park, University of Victoria campus, Rithet’s Bog, Albert Head, Christmas Hill, Little Saanich Mt., Mill Hill, Fort Rodd Hill National Historic Site, Joan Point/Harmac, and Harewood Plains.

Carex tumulicola is a highly inconspicuous plant that is difficult to distinguish from other sedges when not in flower. This is presumably one reason why the species has, until recently, escaped notice on southern Vancouver Island. There is thus a chance that additional populations will be found as more habitat is surveyed. Further inventory should be focused in the Southern Gulf Islands, including Sidney Island (subsequent to the completion of this report, a population was reported from Sidney Island), Portland Island, Saturna Island, Pender Island, and Tumbo Island, and other areas around Victoria (subsequent to the completion of this report, a population was also discovered at Becher Bay), including Mt. Douglas, Cedar Hill Golf Course, and Regional Parks (Francis King, Thetis Lake, Elk Lake and Bear Hill). However, it should be noted that search efforts in similar areas have rarely turned up this species.

Survey method. The survey method used to date for Carex tumulicola 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

Because Carex tumulicola is a clonal species that spreads from rhizomes, distinguishing genetic individuals (genets) in the field for the purpose of obtaining population counts is an intractable to impossible undertaking in most cases. In those instances where C. tumulicola plants formed discrete tussocks, each tussock was generally considered a single ramet (or the best representative of a mature plant). Where distinct tussocks could not be distinguished, surveyors have usually provided only a rough estimate of the patch size (i.e., in ). Consequently, there is presently no good estimate available for the total number of C. tumulicola genetic individuals (genets) in Canada. Thus far, about 70 patches ofC. tumulicola have been recorded with a range of about 600 -1500 tussocks in total (Table 2). The number of genetically distinct mature individuals is likely much smaller. The numbers of patches recorded for Population #10, added to the report subsequent to its completion, are based on two observations by different individuals made in 2006 and 2007 (see Table 2).

Table 2. Population summaries for Carex tumulicola
Population No. of patches/tussocks Extent of area occupied
1 ± 24 patches (40-70 tussocks) from< 1 m² to a few m² over an area of 30 ha
2 6 patches(~10 tussocks) most<0.5 m² in extent, distributed among six different sites (subpopulations) over a 0.3 km² area
3 1 patch(possibly a single genet) approx. 5 m²
4 3-5 patches (50-100 tussocks) 1 km stretch of uninhabited coastline
5 13 patches (100-500 tussocks) from<1 m² to several m², with one extensive subpopulation scattered through a mowed, 2 ha meadow; the population extends about 800 m along an uninhabited coastline representing about 16 ha of habitat.
6 5-10 patches (50-200 tussocks) scattered over a 200 m length of coastline.
7 a single tussock (probably one genet) likely < 1 m²
8 a single patch (three tussocks) approx. 2 m²
9 7 patches (280-460 tussocks) covering about 3303 m²
10 about 3 patches with 50-100 tussocks with most in one patch as seen in 2006 (J. Miskelly, pers. comm. 2008) but 6 patches documented in 2007 (J. Fenneman pers. comm. 2007) scattered within an area of about 2.4 ha

Fluctuations and trends

Because Carex tumulicola was only discovered very recently in Canada, it is unclear whether the species has always been rare on southeastern Vancouver Island or whether it has undergone a recent dramatic decline. There is circumstantial evidence to suggest that populations at most sites have been largely displaced during the past century by the spread of trees and shrubs into formerly open meadow habitat, but this is difficult to confirm. There are currently insufficient data to estimate a % rate of decline for the total population, either for the past 10 years or over the last three generations. Long-term trends within populations (i.e., whether they are growing, declining, or stable) are also for the most part unknown.

The number of known Carex tumulicola populations in Canada has increased tenfold since the first collection was made in 1990 (Table 1). This increase can likely be ascribed entirely to increased search effort.

Rescue effect

The nearest populations of Carex tumulicola outside of Canada occur on San Juan Island, where it is found occasionally growing in moist meadows. San Juan Island lies less than 20 km from the nearest recorded population in Canada (Uplands Park), within easy flying range of birds. It is not known whether birds exploit the seeds of C. tumulicola, but it is possible that bird-aided seed interchange occurs between U.S. and Canadian populations on occasion. However, such dispersal events are likely to be extremely rare, if they occur at all.

Similarities in habitat suggest that plants from U.S. populations may be relatively well adapted to site conditions that prevail on southeastern Vancouver Island, so there is some potential for deliberate reintroduction should some or all Canadian populations become extirpated by events that do not reduce habitat suitability.

Page details

Date modified: