Mast fruiting in a hawkmoth-pollinated orchid Habenaria glaucifolia: an 8-year survey

Abstract

The hypothesis of predator satiation has been proposed to explain mast fruiting in various flowering plants. It considers that the simultaneous production of large numbers of seeds by a plant population reduces the risk of seed predation for each individual. Orchids produce huge numbers of seeds per fruit and rarely experience seed predation. It remains unclear which factors may affect fluctuating fruit production in orchids, which generally suffer a widespread pollen limitation. To explore the temporal pattern of fruiting and potential factors related to fluctuation in fruit production, we investigated reproductive success of a long-spurred orchid (Habenaria glaucifolia) in an alpine meadow with thousands of individuals over 8 years. To estimate reproductive success, pollinator observation was conducted by day and at night, and pollinia removal and receipt were recorded in the field population for 8 years. To examine whether fruit set and seed set are pollen limited, we conducted supplementary pollination experiments and compared fruit set, seed set and pollinia movement of open-pollinated flowers from 2011 to 2013. We measured lengths of spurs and pollinator proboscises, and nectar volume and concentration, to identify potential pollinators. Hawkmoths were seen to be effective pollinators for H. glaucifolia in 3 years, whereas in the remaining 5 years no pollinators were observed, and consequently pollinia were rarely transferred. Numerous pollinia movements were observed in 2012, 2013 and 2014 (pollinia removal: 48, 59 and 85%; pollinia receipt 51, 70 and 80%), and correspondingly fruit set was significantly higher in 2012 and 2013 (59 and 46%) than in 2011 (25%). It was fruit set, rather than seed set, that was pollen limited in this orchid in the 3 years, in that supplementary pollination increased fruit set but did not increase seed set per fruit compared to natural. Three species of hawkmoths had proboscis lengths that matched the spur length of H. glaucifolia. Fruit set in this long spurred orchid depends on the activity of long-tongued hawkmoths, resulting in significant temporal variation in fruit production. Mast fruiting in this alpine orchid could be attributed to a ‘sit and wait’ strategy, awaiting an abundance of effective pollinators.