Showy phlox (Phlox speciosa): COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

Population sizes

The number of populations, population sizes, and area of occupancy of Phlox speciosa in Canada is greater than previously suspected. Table 2 summarizes the available information on Phlox speciosa populations. The populations known to be extant are located within a 57 km² area.

All of the current information on Phlox speciosa population sizes in Canada is based on a field survey completed over a 5-day period in June 2003 by Ksenia Barton and Jan Teversham. Only two locations have previous population size estimates available, from 1994 (BC CDC 2002). No census information or results from monitoring studies are available.

Populations in Table 2 were defined by mapping all extant locations in relation to one another. Gene flow is assumed to be significantly reduced between locations with significant intervening distances. As no data from gene flow studies are available for Phlox speciosa, the NatureServe’s Element Occurrence Data Standard was applied to determine how locations should be grouped into populations. For species Element Occurrences, the recommended minimum separation distance is ≥1 km where the type of separation is unsuitable habitat or apparently suitable habitat (NatureServe 2002). Locations less than 1 km apart were therefore considered sub-populations of a single population.

Population and sub-population sizes were based on field estimations. For calibration, individuals (flowering and non-flowering) were counted within large defined areas to determine exact densities. Densities were typically higher than expected due to the less noticeable appearance of non-flowering individuals. Then sub-populations were surveyed on foot as qualitative changes in density were noted. The number of individuals was calculated based on the estimated area of occupancy and the estimated average density. Area of occupancy was estimated visually or by pacing for small areas. For large areas, area of occupancy was estimated by recording GPS points around the boundaries of the area. The points were then mapped onto a topographic map and a boundary was drawn around the area, taking into account GPS points and topography.

The estimated ranges of population sizes span up to an order of magnitude, due to the crude field population size estimation methods employed. Due to time limitations, field surveying emphasized finding new occurrences of Showy Phlox and defining the extent of occurrence of the species, as opposed to carrying out detailed population size estimates.

Unless otherwise indicated, data above are based on fieldwork conducted by Ksenia Barton and Jan Teversham (June 2003). Where a single value is given, that is the exact value that was measured or counted in the field. Where a range of values is given, the values have been estimated.

In summary, the Ford Lake and Yellowlake Creek populations are very small (<100), the McKay Creek, Park Rill, and Yellow Lake East populations are of medium size (100s to 1,000s), and the Twin Lakes, White Lake, and Yellow Lake West populations are abundant (1,000s to 10,000s). The Yellow Lake South population size is unknown. The three Yellow Lake populations may actually form a single population along a large slope – additional surveying would be required to determine this.

Number of individuals in Canada

Population estimates detailed in Table 2 indicate an approximate range of 25,000-220,000 individuals of Phlox speciosa spp. occidentalis in Canada. Of those, approximately 6,400-57,000 individuals are reproductive, and flowered in 2003. The full extent of every population identified has not necessarily been surveyed, and population estimates may be low in some cases. It is likely that additional surveying within habitats that meet Phlox speciosa habitat requirements will also result in discoveries of new populations and sub-populations.

Population trends

Little information is available on the long-term or recent changes in Phlox speciosa numbers and range. Before the June 2003 field survey conducted by Ksenia Barton and Jan Teversham, most of the information available was from herbarium labels, with relatively vague location information in some cases.

Population sizes and areas of occupancy were estimated for two Twin Lake locations in 1994 (BC CDC 2002). It is likely, however, that those estimations were based on incomplete surveys of what are now considered sub-populations of the Twin Lake population. The current population estimate for Twin Lakes is two orders of magnitude greater than the 1994 estimates, a change that does not represent a realistic population increase.

Two or three historical populations have not been relocated. Their original sizes or exact locations are unknown. If they have been extirpated, it would appear that Phlox speciosa range within Canada has substantially contracted. It is possible, however, that the historical locations are still extant.

The population sizes of Phlox speciosa are not expected to fluctuate greatly from year to year as the plant is a perennial.

Based on herbarium records, it can be assumed that Phlox speciosa has always been of limited distribution in Canada. The taxon’s limited distribution can be attributed, in part, to its habitat requirements.

Although Phlox speciosa is rare in southcentral British Columbia, the plant is apparently widespread in the Pacific Northwest (Douglas et al. 1999; Hitchcock and Cronquist 1973).