Understanding the impact of plant–arthropod interactions, pollination, and canopy light on the rare orchid, small whorled pogonia (Isotria medeoloides)

Abstract

Recent work has warned of a global crisis for terrestrial orchids and an urgent need for conservation. Pollinator declines have been documented globally, which may exacerbate the pollination limitations orchids experience. Small whorled pogonia (Isotria medeoloides) (Pursh) Raf. is a North American terrestrial orchid considered vulnerable by the International Union for the Conservation of Nature, threatened in Canada and USA, and endangered in 20 states. Like many orchids, it has specialized habitat needs and fragmented populations. Here, plants were monitored diurnally and nocturnally throughout the flowering period to evaluate arthropod activity that might impact pollination. Ultraviolet photographs and spectrophotometric data were collected on flowers. Canopy light penetrance was measured at plants. Seed capsule production was evaluated over time. In sum, 41 flowers were monitored for > 300 h, resulting in > 140,000 photographic frames of compiled video. In this extensive monitoring, 33 plant–arthropod interactions were detected, only one likely to facilitate self-pollination, and none that facilitated outcrossing. UV absorbance was identified on petals and sepals, which contrasted starkly with whorl UV reflectance, a previously undescribed form of floral display visible to pollinators. Flowering plants grew in higher light than vegetative ones. Capsule production was highly variable over time, making the species vulnerable to 'bad years' and low effective population size, given the low genetic diversity previously identified, and lack of outcrossing detected here. Imminent attention is warranted for species-level protection, consistent with needs of other terrestrial orchids.