Coast Microseris (Microseris bigelovii) COSEWIC assessment and status report: chapter 7

• Previous page
• Table of contents
• Next page

Population Sizes and Trends

Search effort

Suitable sites have been surveyed repeatedly since the early 1980s in a series of projects designed to document the distribution of rare plants in open meadows in southeast Vancouver Island and the Gulf Islands. The principal investigators included Adolf and Oldriska Ceska, Matt Fairbarns, Hans Roemer, Jenifer Penny, Chris Brayshaw, Harvey Janszen, Frank Lomer and George Douglas, all of whom are familiar with the species.

Over 1,000 hectare of suitable habitat in over 80 sites have been investigated and much of it has been surveyed more than once during this period. During the past decade alone, over 500 person-days have been spent searching for rare species in suitable habitats.

While Microseris bigelovii is a small plant easily overlooked during casual botanical inventories, this status report was only prepared after a directed survey by botanists familiar with the species. This survey effort includes three annual searches specifically for it (approximately 14 person-days in 2002, 8 person-days in 2003 and 4 person-days in 2004). Despite the concentrated effort, only two new populations and four previously reported populations were documented. All three years appear to have been suitable years for surveying M. bigelovii, based on the number and vigour of plants at known, extant populations. These efforts included unsuccessful surveys of the many sites that had other rare plants that have been associated with M. bigelovii (Figure 4).

Abundance

Records from 2002, 2003 and 2004 indicate there were between 5,000 and 7,000 individuals in Canada. Flowering is asynchronous and the flowers are short-lived. It is impossible to determine what proportion of these individuals actually flowered rather than perishing in the summer drought before they had a chance to reproduce (a common problem in inventorying this and other annual species).

The number of populations and subpopulations, defined by the rate of genetic interchange via seed movement and pollen exchange, is difficult to establish. Seed dispersal between patches is probably restricted to fairly short distances. The species relies primarily upon self-pollination but out-crossing occurs and it is not clear how frequently pollen is exchanged and over what distances (Chambers 1955).

Table 1 presumes that two occurrences at Hornby Island function as subpopulations (exchanging genetic material more than once in an average year) rather than separate populations. Several patches of Microseris bigelovii are known from Oak Bay #1 and they appear to constitute subpopulations of a single population. The Oak Bay #2 population consists of a single discrete population. Three occurrences at Esquimalt are best considered subpopulations belonging to a common population.

The report of Microseris bigelovii from Thetis Lake may be erroneous. It appears to be based on a herbarium specimen (V 10117) that has two place names on its label. One place name corresponds to Oak Bay population 1, while the other is Thetis Lake. It may be that the collector visited both locations in one day and was not able to determine which species were collected in which location.  Thetis Lake is an unusual location for M. bigelovii because it lacks seashore habitats.

The plants at Rocky Point #1 are confined to a small area and almost certainly function as a single population. The two patches at Rocky Point #2 are treated as subpopulations but further analysis may show that they are actually different populations. Certainly, they are well separated and the intervening terrain is likely inimical to both seed and pollen dispersal. Nevertheless, they are only about 400 metres apart and the British Columbia Conservation Data Centre, by convention, treats them as a single population^[1].

Fluctuations and trends

There is no reliable information on past population sizes so fluctuations and trends in the size of extant populations cannot be determined. This species, like many other annuals (Harper 1977), may experience significant natural fluctuations in population sizes.

Rescue effect

Microseris bigelovii is extirpated in Washington State and there is negligible opportunity for unassisted genetic exchange (seed or pollen) with US populations in Oregon or California. Genetic evidence and greenhouse studies (see above) suggest that populations in Oregon and California may be poorly adapted to conditions in British Columbia. These differences may restrict the potential for successful re-introductions should Canadian populations be lost.