The endangered western prairie fringed orchid (Platanthera praeclara) in Manitoba : its associated plant community and an examination of pollinator visitation rates

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Friesen, Christopher P.
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To effectively manage an endangered plant species, managers must have an adequate knowledge of the habitat of the species and its reproductive ecology. This study describes the plant community associated with the endangered western prairie fringed orchid (WPFO) (Platanthera praeclara) in Manitoba and determine whether the plant community differed between sites and between six distances from the centre of orchid beds. In addition, this study exarnines factors which may affect the pollination ecology, specifically pollinator visitation. Assessment of orchid habitat occurred at four sites, separated by at least 800 m, in the Tall Grass Prairie Preserve (TGPP) in southeastern Manitoba in 2005 and 2006. The four sites were broadly similar in that each was dominated by graminoids (sedges, rushes, and grasses) and shared a moderate number of forb and graminoid species. The most abundant forbs included Galium boreale, Fragaria virginiana and Hypoxis hirsuta. The measures of species richness (species richness and rarified richness) varied significantly between distances from the centre of the orchid beds, but not between sites. The diversity measures (the Shannon-Weiner and Simpson indices) and the evenness measures (Shannon-Weiner evenness and rank log abundance slope) were significantly different between sites but not distances from the centre of the orchid beds. Ordination methods (Principal Components Analysis and Multiple Discriminant Analysis) showed the plant community at one site differed from that of the other sites, but the `plant community was not significantly different between distances from orchid bed centre. Pollinator (ie: sphinx moth) visitation rates, measured using seed capsule production, the presence of moth scales, and pollinaria removal, varied considerably between sites and between years. The abundance of nectar sources and larval hosts for the pollinators at the sites was not related to visitation rates. The visibility and proximity of anthropogenic light sources to orchids did not correspond with rates of pollinator visitation. Sphinx moth activity at the TGPP was not significantly correlated with weather conditions. The plant community associated with the WPFO was broadly similar at different sites, though differences in composition and diversity were evident. Controlled experiments would be useful in determining the effect of various factors, including those examined in this study, on the pollinator visitation rates to the WPFO in Manitoba.