Eastern prairie fringed-orchid COSEWIC assessment and status report: chapter 9

Population Sizes and Trends

Global Population Size

Based on data in Bowles (1983), which includes information from the 1970s, the largest global population estimate was 2907, of which 2000 (68.8%) were in Ontario. Maximum populations are currently about 3708 (Bowles 1991, Engel 1992 and this report), of which 1053 (28.4%) are in Ontario. Based solely on these numbers, the proportion of the global population in Ontario has thus declined by more than half since the 1970s. This may be misleading as the US populations are undoubtedly more thoroughly documented than those in Ontario. In addition, the US population numbers included a very large population in Ohio which has now substantially declined.

Changes in Canadian Population Size

In 1984, there were 12 populations believed to be extant in Ontario out of a total of 19 previously documented occurrences (Brownell 1984). Currently 20 populations are considered to be extant out of a total of 34. Fourteen populations are therefore considered non-extant (see definition in section on methods). The increase in numbers of both total and extant occurrences can be largely attributed to the increased interest in fen and prairie habitats in recent years. It is to be noted that although the number of occurrences has increased, many of the populations are extremely small and possibly not viable (Table 1).

Since Brownell (1984), 13 new stations have been discovered. Of the new stations, 6 are in Lambton County (Woodliffe and Allen 1996), 2 are in the adjacent St. Clair River delta of Kent County (A. Woodliffe pers. com. 1997; J. Haggeman pers. com. 1997), 3 are in southwestern Essex County (V. Brownell pers. obs. 1997, P. Pratt pers. com.1997, 2000), 1 is in Lennox and Addington County (T. Norris pers. com. 1997) and 1 is in the United Counties of Stormont, Dundas and Glengarry (D. Cuddy pers. com. 2000, P. Catling pers. obs. 2000).

Although comparative data is not available for all sites, trends in population sizes may be evaluated through reference to Table 1. There have been substantial variations in population sizes over time. The current area of occupation and total number of individuals, based on maximums for 1990s data, are ~8.75 kmĀ² and 1053 plants.

Declines in populations have generally involved the disappearance of habitat -- due to conversion, succession or other factors. The population at site 31 is of particular concern. Over a long period there has been a substantial decline in the population. Where 1000-1500 existed in the late 1960s and early 1970s, only a few hundred could be found in the 1990s. Where hundreds occurred in more northern parts of the swamp, none have been seen for two decades. This site contained one of the largest recorded in North America in recent times. Its decline may be related to changes in ground water flow (Bradford 1999, Bradford and Watt 2000).

The old field at site 2 in Essex County once contained about 150 robust, flowering plants, but through natural succession became a dogwood (Cornus drummundii) thicket which, despite some cutting by the Essex Region Conservation Authority, resulted in a major change in vegetation cover. At the two sub-populations, the population size had decreased by 60-80% (Table 1). The population of 40 plants (Haggeman pers. com. 1997) at site 11 has steadily declined over a period of just 5 years and the habitat has become strongly dominated by Common Reed Grass (Phragmites australis).

Excluded Reports

  1. Morley Lake, St. Edmunds Township, Bruce County. Reported in Cuddy, Lindsay and Macdonald (1976) on the basis of a personal communication; however, it was not seen on separate visits by K. Lindsay and J. Johnson (pers. comm. 1997).
  2. Conroy Marsh, Renfrew County. The report from Conroy Marsh on the York River, Raglan Twp. (Lewis and Tae 1994) has not been included based on lack of material evidence and insufficient information.

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