Eastern prairie fringed-orchid COSEWIC assessment and status report: chapter 4

Methods

Information Sources

In order to update the original status report prepared for Platanthera leucophaea (Brownell 1984), we gathered information from a variety of sources. A literature search was conducted using AGRICOLA, BIOSIS and Biological Abstracts. Numerous field biologists were contacted in Ontario over a three-year period, and historical and current data were requested from various government and non-government sources [e.g. Natural Heritage Information Centre (Ontario Ministry of Natural Resources); Field Botanists of Ontario].

Site Definition

A site was defined as populations that are separated by more than 1 km of largely unsuitable habitat or populations that are separated by more than 1.5 km with suitable habitat between them.

Calculation of Area of Occurrence

Some movement over time has to be assumed even with a sessile organism (i.e. it moves around within its habitat). Plant habitats of course vary greatly in size, but for application of the IUCN criteria, a general rule needs to be applied to account for colonies of 1 or 2 plants, to reduce subjectivity and to allow for this movement. A minimum area of 0.5 ha seems appropriate since it is on the large size (occupied habitat may be a few square metres in a ditch or portion of a bog mat), but is not excessively large. A circle of 0.5 ha in extent (radius of 39.89 m) for each plant in each population (see site definition above) seems reasonable. A line was drawn around the edge of all of the circles where they overlap in a population or by adding the circle areas where they did not overlap. The total area thus identified was determined to be the "area of occupancy" for the population.

The term "extent of occurrence" as used by IUCN is different. For a species such as Platanthera leucophaea with a number of small but widely separated populations, the term may not be very meaningful. Nevertheless, if a convex polygon is drawn to include all of the populations in Ontario an area of about 114,000 kmĀ² would be enclosed.

Non-extant Populations

A population was considered no longer extant if:

  1. not seen or reported for over 20 years; or
  2. known to be destroyed by urbanization, conversion to agriculture, loss due to succession etc.; or
  3. not seen in the 1990s despite at least 4 searches during the flowering period.

Calculation of Population Size

Where two reliable observers recorded different numbers in the same year, we used the higher number in calculations of maximum population size. Unless habitat has been destroyed, in the absence of specific information to the contrary, we assumed no change in population size between the 1980s and 1990s. In a few cases, observers have seen a number of plants in a portion of habitat at a site, then extrapolated that number to the entire area of habitat. In the absence of a carefully laid out, statistically defendable, sampling procedure, and because what appears to be suitable habitat for this species is often found upon closer inspection to be unoccupied, we have chosen not to use these "estimates" as a basis for status recommendation. At the same time, we recognize that some of the habitat areas are difficult to access and inventory thoroughly, and that there may well have been more plants present than were counted during some visits to the sites.

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