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Recovery Strategy for the Western Prairie Fringed-orchid (Platanthera praeclara) in Canada [Proposed]

Background

The following information was extracted from the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) status report (Collicutt 1993) and the COSEWIC status update report (Punter in press), supplemented with new field survey data from 1999 to 2005 (C. Borkowsky, pers. comm.) and other sources as indicated.

 

Description

The western prairie fringed-orchid (also known as the western prairie fringed orchid) (Platanthera praeclara) is a perennial wildflower found in calcareous prairies and wet meadows.

The species is perennial, with thick fleshy roots and a tuber. Flowering plants reach a height of 40–88 cm, with 5–7 leaves per stem. Its flowers are arranged in a spike, 5–15 cm long and 5–9 cm wide, with 4–33 creamy white flowers (Figure 1). The flowers are very showy, with two fan-shaped petals on top and one large petal on the bottom. The bottom petal is deeply divided into three lobes, which are further incised to form a fringe. Flowers are fragrant at night to attract insect pollinators. Peak flowering typically occurs from late June to mid-July. Punter (in press) provides a detailed description of the species and its phenology.

The known history of this species in Canada is short. The first published report of the species was by Catling and Brownell (1987), who collected a specimen of what was then thought to be Platanthera leucophaea in the Vita, Manitoba, area on July 26, 1984. Local residents recall seeing the species much earlier than 1984, and orchid enthusiast Bud Ewacha (pers. comm.) reports having seen the species prior to this time, but no other long-term information exists.

Figure 1. Western prairie fringed-orchid inflorescence

Sheviak and Bowles (1986) then split P. leucophaea into two species, with all records west of the Missouri River being reassigned to the new species, western prairie fringed-orchid (Platanthera praeclara Sheviak and Bowles).

 

Distribution and Abundance

The western prairie fringed-orchid occurs from Manitoba south to Oklahoma, east to Iowa, and west to central Nebraska (Figure 2). It is ranked G2 by NatureServe (2005) and is rare throughout its entire range in central North America (Table 1).

In Canada, there is one extant metapopulation -- defined by Punter (in press) as a population of at least 3000 individuals -- of the western prairie fringed-orchid, found west of Vita, Manitoba, in the Rural Municipality of Stuartburn (Figure 3). The Manitoba metapopulation is the largest of any for the species and represents approximately 50% of the global population.

 

Table 1. NatureServe conservation status ranks (national, provincial, or state) for the western prairie fringed-orchid (NatureServe 2005) and maximum observed number of flowering plants observed there.
JurisdictionNatureServe rankMaximum observed number of flowering plantsa
CanadaN1 
ManitobaS123 530
   
United StatesN2 
IowaS21 100
KansasS154
MinnesotaS113 166
MissouriS1279
NebraskaS22 102
North DakotaS212 911
OklahomaS1Not seen since prior to 1979
South DakotaSHBelieved extirpated

a  In Manitoba, the maximum number of flowering plants observed was in a single season (2003) (C. Borkowsky, pers. comm.). Population figures from the United States are the maximum number of flowering plants observed at each site, from 1979 to the present (P. Delphey, pers. comm.).

 

Figure 2. Global range of the western prairie fringed-orchid (derived from Flora of North America Editorial Committee 2003)

The western prairie fringed-orchid has an area of occupancy in Canada of approximately 670 ha, based on mapping of known patches of the species (Manitoba Conservation Data Centre 2006). The Canadian range of the western prairie fringed-orchid makes up about 0.5% of the global range (Figure 2), although the vast majority of the global range has been altered by anthropogenic activity and no longer provides suitable habitat.

 

Figrure 3: Generalized Canadian range of the western prairie fringed-orchid (information provided by Manitoba Consevation Data Centre 006)

Surveys of the number of flowering plants have been conducted in Manitoba since 1992 (Figure 4). The number of flowering plants fluctuates widely from one year to the next (Punter in press), with a low of 1818 in 1995 to a high of 23 530 in 2003. In any given year, many more plants remain in a vegetative stage, with 1–3 leaves (Punter in press). The difficulty in locating vegetative plants makes counts of flowering plants the easiest way to monitor population status. However, the large fluctuations in numbers of flowering plants from year to year make it difficult to discern any upward or downward trend in the Canadian population.

In the United States, where information on the species dates back to the 19th century, there is a long list of sites where the species has either been extirpated or not observed for more than 30 years (U.S. Fish and Wildlife Service 1996). The significant declines throughout its range are attributed to conversion of habitat to cropland, overgrazing, intensive hay mowing, drainage, and fire suppression (U.S. Fish and Wildlife Service 1996).

In Canada, the relatively short documented history of the western prairie fringed-orchid makes it difficult to describe similar declines in extent of occurrence or population size. The species has never been observed outside of its current distribution in Canada, and monitoring of flowering plants has taken place only since 1992. While there have been documented cases of habitat conversion that resulted in the loss of western prairie fringed-orchid habitat and plants (Collicutt 1993; Punter in press), these losses took place in the same general area where plants are still found. The area in which plants have been observed increased slightly over the last 10 years, as a few additional patches of plants have been located, but this is more likely to be attributable to increased search effort from year to year, rather than to an expanding range.

The western prairie fringed-orchid is listed as Endangered by regulation under Manitoba’s Endangered Species Act. It is listed as Threatened under the U.S. Endangered Species Act and is listed under three state Endangered Species Acts (Endangered in Minnesota, Threatened in Iowa and Nebraska).

Figure 4. Number of flowering western prairie fringed-orchids observed in Manitoba, 1992-2005

Figure 4. Number of flowering western prairie fringed-orchids observed in Manitoba, 1992– 2005.

 

Species’ Needs

Biological

Two pollinator species for the western prairie fringed-orchid have been observed in Manitoba: the wild cherry sphinx (Sphinx drupiferarum) and the galium sphinx (Hyles gallii) (Westwood and Borkowsky 2004). In Manitoba, peak flying activity for the two moths does not overlap completely with peak flowering activity for the western prairie fringed-orchid. This may contribute to low levels of pollination and seed production (Westwood and Borkowsky 2004). Additional moth species have been observed as pollinators in the United States, but it appears that most of these species are unable to survive in Canada (C. Borkowsky, pers. comm.). The leafy spurge hawkmoth (Hyles euphorbiae) was recently documented as a pollinator of the western prairie fringed-orchid in North Dakota (Ralston et al. in press). This moth is a non-native species from Europe, introduced as a biocontrol for leafy spurge. It has been observed in southwestern and south-central Manitoba and in time may expand its range to include southeastern Manitoba.

The needs of the pollinator species at all stages in their life cycles should be considered when managing for the western prairie fringed-orchid or other tallgrass prairie species. Mixed aspen forest habitat is required by both moth species for their larval stages; host plants are chokecherry or pincherry (Prunus spp.) for the wild cherry sphinx and bedstraws (Galium spp.) for the galium sphinx (C. Borkowsky, pers. comm.).

Habitat

The western prairie fringed-orchid is found on wet to mesic tallgrass prairies, sedge meadows, and wet brush prairies. Punter (in press) provides a detailed list of associated species.

The Vita area is underlain by highly calcareous glacial till, with a ridge and swale topography (Punter in press). Soils are wet to mesic, imperfectly drained, extremely calcareous, Dark Gray Chernozemic sandy loams to loams. Lands supporting the western prairie fringed-orchid are generally too wet and stony for cultivation.

Encroachment of trees and shrubs such as trembling aspen (Populus tremuloides) on upland ridges and bog birch (Betula pumila), shrubby cinquefoil (Dasiphora fruticosa), and willow species (Salix spp.) in wetter swales is common in the absence of disturbances such as fire (Punter in press). Landowners use grazing, haying, and spring burning as tools to control woody species encroachment. These habitat management techniques could have positive or negative effects on recruitment and survival of the western prairie fringed-orchid, depending on frequency, intensity, and timing (U.S. Fish and Wildlife Service 1996). A variety of management regimes have been assessed in the United States, with varying results (U.S. Fish and Wildlife Service 1996). The highly variable abundance of the species from one year to the next, even in the absence of management techniques, may make it difficult to assess the success of various management techniques. The western prairie fringed-orchid Recovery Team in the United States recommended that additional research be undertaken to document the influence of various management techniques in all parts of the species’ range (U.S. Fish and Wildlife Service 1996).

Hydrology is an important factor in determining the suitability of orchid habitat. Generally, the water table is high during the spring and summer in areas where the western prairie fringed-orchid occurs. Wolken et al. (2001) observed plants growing in swales with no standing water and in swales with as much as 80 cm of standing water in the same year and found that swales containing orchids had significantly higher surface soil moisture and water depths than swales that did not contain orchids.

Limiting Factors

The western prairie fringed-orchid is at the northern limit of its range in southeastern Manitoba. Populations at the limits of a species’ range often occupy poorer habitat and are more fragmented, less dense, and more variable than those at the core of its range (Channell and Lomolino 2000; Vucetich and Waite 2003). Peripheral populations are therefore more vulnerable to extinction due to low immigration rate, disrupted pollinator relationships, and other density-related factors. Genetic diversity is sometimes, but not always, lower in peripheral populations, although these populations may possess unique genetic characteristics (Vucetich and Waite 2003). Pollination is a potential limiting factor, particularly at the periphery of the range, where both plant and pollinator populations may be fragmented.

The period of peak orchid flowering does not overlap completely with the period of peak flight activity for the two species of sphinx moth that have been observed pollinating the western prairie fringed-orchid in Manitoba (C. Borkowsky, pers. comm.). The diversity of orchid pollinators also appears to be lower in Manitoba than farther south in the species’ range. This may lead to lower rates of seed production than elsewhere in its range, although seed set is arguably low for the species range-wide and may simply be a characteristic of the species.

Manitoba’s cooler climate, compared with that of other places in the species’ range, is almost certainly a factor that limits seed production in some years. Punter (in press) notes that frost damage to flowers has been observed in two of the last 10 years and also suggests that low temperatures at flowering time may reduce the flight activity of insect pollinators, resulting in a reduced seed set.

 

Protection

The western prairie fringed-orchid has been listed as Endangered under Schedule 1 of the federal Species at Risk Act since June 2003. It was also listed as Endangered by regulation in 1996 under The Endangered Species Act in Manitoba, which specifically prohibits acts that “destroy, disturb or interfere with the habitat of an endangered species.”

 

Threats

The following list of threats to the western prairie fringed-orchid is summarized from Punter (in press) unless otherwise stated and can be viewed in more detail in that report. Broad strategies to address these threats are outlined in section 2.4 and in Table 2.

Habitat Loss and Degradation

Habitat Conversion

Range-wide, the most significant threats to the western prairie fringed-orchid are considered to be human activities that permanently alter its habitat (U.S. Fish and Wildlife Service 1996; Punter in press). Conversion of native prairie habitat to cropland, tame pasture, or hayland has resulted in significant decreases in abundance of the western prairie fringed-orchid in the United States. However, habitat conversion as a factor in the decline of this species is less well documented in Canada, as described in section 1.2. Although some examples of habitat conversion have been documented in the last 10–15 years (Collicutt 1993; Punter in press), much of the land supporting the western prairie fringed-orchid today is already part of the Manitoba Tall Grass Prairie Preserve (Appendix A), and conversion is now an unlikely threat for most of the population.

Agricultural Impacts

Intensive long-term grazing may affect plants by trampling or herbivory. Pasture “enhancement” through introduction of tame forage grasses could result in increased competition.

Haying in mid- or late summer, prior to seed set, may remove the inflorescence and photosynthetic tissue, weakening the plant and reducing seed production.

A relatively new potential threat to the western prairie fringed-orchid is the application of liquid hog manure to lands supporting populations of the orchid or on lands near orchid populations, since overapplication or improper application of hog manure can lead to transport of nutrients into the groundwater through leaching or overland flow (Saskatchewan Soil Conservation Association undated). Changes in species composition due to nutrient additions are well documented in the scientific literature. Wedin and Tilman (1996) observed a greater than 50% decrease in species richness after 12 years of nitrogen addition to Minnesota grasslands, with native C4 (warm season) grass species declining in favour of weedy C3 (cool season) grass species. Little is known, however, about the specific effects, if any, of nutrient loading as a result of manure application on the western prairie fringed-orchid. It is possible that higher nutrient levels may simply not be tolerated by the orchid, or the orchid could be outcompeted by other plants for nutrients, light, or water as nutrient levels and species composition change. To date, however, no research has been conducted to answer these specific questions on this potential threat.

Alterations to Hydrology

Deepening of roadside ditches and construction of drains to remove surface water from agricultural land have the potential to lower the water table of the area, to the detriment of the western prairie fringed-orchid and its habitat. The presence of the western prairie fringed-orchid may be dependent on the moisture content of the topmost 10 cm of soil in swales (Wolken et al. 2001). A decrease in flowering and an increase in mortality observed in the United States during an extreme drought led the Western Prairie Fringed-orchid Recovery Team there to state that hydrological alterations resulting in a drawdown of the water table near the root zone of the orchid could have serious adverse impacts (U.S. Fish and Wildlife Service 1996).

Roadway Maintenance

Undeveloped road allowances and ditches adjacent to roads support a limited number of plants in most years. Road maintenance activities such as mowing, herbicide use, and ditch clearing and deepening have resulted in the loss of some plants.

Exclusion by Other Plants

Woody Species Encroachment

Encroachment by woody vegetation is a threat to the western prairie fringed-orchid and other shade-intolerant grassland species.

Invasion of Non-Native Plants

Non-native plants could displace the western prairie fringed-orchid and other native plant species through competition. Invasive plants such as leafy spurge (Euphorbia esula), St. John’s wort (Hypericum perforatum), smooth brome (Bromus inermis), Kentucky bluegrass (Poa pratensis) and clovers (Trifolium spp.) are present at the Tall Grass Prairie Preserve and have the potential to affect the western prairie fringed-orchid. Wolken et al. (2001) identified both Kentucky bluegrass and leafy spurge as threats to orchid habitat, with both species observed growing within swales that also contained the western prairie fringed-orchid.

Genetic Isolation

The Canadian metapopulation of the western prairie fringed-orchid is about 45 km from the nearest orchid population in Minnesota. This appears to be beyond the feeding flight range of pollinator species. This genetic isolation creates the potential for genetic drift and inbreeding.

Illegal Removal of Plants

Because the western prairie fringed-orchid is an extremely showy plant, digging or picking of the orchid, especially near roads, is a threat. Evidence of illegal removal of plants (i.e., holes where plants have been dug up) has been observed.

 

Actions Already Completed or Under Way

In 1995, the Canadian Nature Federation’s Endangered Plants and Invertebrates in Canada Program produced a National Recovery Plan for the Western Prairie Fringed-orchid (Davis 1995). The plan focused largely on actions required for recovery. Although never formally adopted by the governments of Manitoba and Canada, many of these actions have been undertaken at the Manitoba Tall Grass Prairie Preserve (Appendix A). Specifically:

  • More than 3000 ha (5,000 acres)  of land supporting tallgrass prairie and other native plant communities have been purchased and protected by the various partners in the Preserve. Many of the largest patches of plants are found on Preserve land -- more than 80% of flowering plants observed in 2005 were found there.
  • Preserve lands are managed to conserve the native flora and fauna using a variety of techniques, such as burning, haying, rotational grazing, and exotic species control.
  • Long-term monitoring has been undertaken.
  • A number of research projects have been undertaken to better understand the relationship between the western prairie-fringed orchid and its insect pollinators.
  • Extension activities have been undertaken to educate both locals and ecotourists, including a school program, interpretive trails and signage, guided tours, and a yearly Prairie Day.

 

Knowledge Gaps

Knowledge gaps that, if filled, would assist with recovery of the western prairie fringed-orchid include:

  • the ability to accurately determine (or estimate) total population size and thereby set quantitative population objectives for recovery;
  • standardized guidelines for inventory and monitoring of existing populations;
  • knowledge of the full extent of population and distribution (e.g., undiscovered populations, particularly on private land);
  • the impacts of isolation and population size on population viability;
  • the extent of factors affecting survival and reproductive success (e.g., habitat, weather, pathogens, grazers, invasive species, woody species encroachment, pollinators, grazing regimes, burning regimes, hydrological regimes, and nutrient loading); and
  • a model for the potential impact of climate change.