Floral and pollination biology of the critically endangered insular orchid Calanthe amamiana: Implications for in situ conservation

Abstract

AbstractCalanthe amamiana is a terrestrial orchid that thrives on the shady forest floors of broad‐leaved forests on Amami Island, central Ryukyus, Japan. Here, I report on the floral and pollination biology of this critically endangered orchid based on a 7‐year study conducted in the species' natural habitats. As the nectarless flowers are probably self‐compatible but are unable to autonomously self‐pollinate, they need to attract pollen vectors by deceit for pollination. To advertise themselves, the flowering inflorescences, which comprise a few to dozens of flowers, use whitish floral colors that are comparatively conspicuous against the shaded background. The exclusive pollinator was the long‐horned bee Eucera okinawae, which nests in soils of the forest floor, although bee abundance varied considerably among study years. Natural fruit‐set ratios were generally low but showed interannual variation. The annual fruit‐set ratios were positively correlated with annual bee abundance, thereby suggesting that under natural conditions, the orchid is pollinator‐limited. Given that C. amamiana is dependent exclusively on E. okinawae for pollination and is also probably pollinator‐limited, conservation plans for this orchid should take into consideration the welfare of long‐horned bees. In particular, as nectariferous flowers were unavailable within broad‐leaved forests, to satisfy the foraging requirements of long‐horned bees, conservation managers should be cognizant of conservation values of flowering plants distributed in sun‐lit sites near orchid habitats, and manage those in an appropriate manner.