Eastern prairie fringed-orchid COSEWIC assessment and status report: chapter 10

Limiting Factors and Threats

Limited Habitat

Only a very small percentage of the total North American area of occurrence can be inhabited by Platanthera leucophaea due to its narrow habitat preference. Most of the long-term populations are found in fen and prairie habitats. Within Ontario, fens constitute only 0.48% of the total wetland area off the Canadian Shield (Riley 1988). In southern Ontario, off the Canadian Shield, there are only 11 fens more than 75 ha in size. Only some of these fens and only portions of the larger sites are appropriate community types for Platanthera leucophaea (e.g., Carex lasiocarpa fen and open Phragmites fen).

Bakowsky and Riley (1994) have estimated that less than 21 km² (2100 ha) of prairie and savanna remain in Ontario and much of this is dry and not appropriate for Platanthera leucophaea. It is likely that the area of suitable mesic and wet mesic prairie is less than 3 km² and less than 0.1% of presettlement extent.

Both prairie and fen habitat continue to be lost in Ontario, mostly due to direct and indirect human impacts.

Pollinators

The Eastern Prairie Fringed-orchid requires pollination by hawk moths for seed development (Sheviak and Bowles 1986, Bowles 1991). These large moths in turn require large areas and a diversity of nectar sources.

Successional Changes

Old field sites in Ohio, Michigan and Ontario have been lost to succession to woody cover.

Invasive Species

Invasive, especially alien plants, such as Glossy Buckthorn (Rhamnus frangula) are contributing to the loss of fen habitat, as evidenced by changes to fens near Ottawa (e.g., site 24). Glossy Buckthorn may have contributed to the loss of sites near London, Ontario, where it has dominated some wet meadow and bog margin communities. Bowles (1991) indicates competition with 3 exotic species in the U.S. portion of the range, including Cut-leaved Teasel (Dipsacus laciniatus), Glossy Buckthorn (Rhamnus frangula) and Purple Loosestrife (Lythrum salicaria). Purple Loosestrife is present in varying amounts in the fen complex at site 30 in eastern Ontario (Cuddy, pers. com.).  Common Reed Grass (Phragmites australis), while a native species, may become aggressively dominant in fens, especially if water tables are lowered, thereby shading out lower vegetation. Monitoring of succession and invasive species is critical if we are to understand the role of habitat change for this species.

Human Impacts

Impacts on water table at site 31 (Bradford 1999) have been suggested as possible causes for an order of magnitude decline in that population since the 1960s. Loss of mesic and wet mesic prairie to agriculture particularly in Lambton County has continued over the past two decades. Loss of the gradient of fluctuating water to cropland in dry years is also a factor in Ontario (see 4) and elsewhere (Case 1987). Collection for gardening could potentially pose a threat and thus make the ongoing monitoring of populations important.

Hybridization

On the one hand hybridization may be a mechanism of plant adaptation and speciation, but hybrids may compete with parents for pollinators and lead to an increasingly diluted genome (e.g., Husband and Burgess 2000). Hybrids have been documented at sites 31 and 32 in Ontario (Catling and Brownell, 1999, Catling et. al. 1999).

Grazing

White-tailed Deer graze the flowering stems (Bender 1988 and pers. obs.) and deer have become very abundant in some of the fens occupied by the Eastern Prairie Fringed-orchid in Ontario. No assessment of impact of deer on Ontario populations is available.

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