Open Access

Jean Claessens, Bernhard Seifert

• Ants were shown to be significant pollinators of two orchid species in the alpine zone of the Alps. Repeated observations from several localities confirm the ant Formica lemani as pollinator of Chamorchis alpina whereas Formica exsecta is reported here for the first time as pollinator of Dactylorhiza viridis. These findings appear of great interest, as significant ant pollination of orchids is unknown so far from any other region or habitat type in the Holarctic. This raises the question if there are specific adaptations. The observations do not provide suggestions to adaptations of the Formica ants for pollinating orchids – they simply followed their normal foraging behavior shown in any type of habitat. Yet, special adaptations are given by the two orchid species in developing pollination mechanisms more strongly involving ground-moving insects which are not inactivated by increased wind velocity and lower temperatures in the alpine zone. These are mainly beetles and ants. The pollination mechanisms and interactions with ants of both orchid species are described. Dactylorhiza viridis is outstanding among orchids in needing 20 to 30 minutes for the 90-degree forward bending of the pollinium after attachment to the insect’s forehead whereas the same process takes place between 15 seconds and 3 to 5 minutes in other orchids. Forward bending of pollinia is required for precise placement of the pollen at the stigmatic surface of another flower. The very long bending time is an adaptation to the longer presence time of ground-moving insects at the same plant and aims to reduce the frequency of geitonogamy (self-fertilization). The high frequency of ant pollination in these orchids is a consequence of the high activity density of aggressive, predatory worker ants leading to a displacement of other pollinators. Attempts of ants to remove the fresh, strongly adhesive pollinia from their foreheads failed and a single ant head could carry up to eight pollinia.
• Aus der Sicht der Pflanzen sind Ameisen unerwünschte Nektardiebe, die zudem als räuberische Insekten durch ihre Anwesenheit in Blüten eine Gefahr für andere Bestäuber darstellen. Der Wert der Ameisen selbst als Bestäuber von Gefäßpflanzen ist zudem nahe Null, was unter anderem dadurch begründet wird, dass das auf der ganzen Körperoberfläche verteilte Sekret der Metapleuraldrüse antibiotisch wirkt und die Befruchtungsfähigkeit von Pollen stark einschränkt (BEATTIE et al. 1984, DUTTON & FREDERICKSON 2012, SANDERSON & WRIGHT 1989). Bestäubung von Orchideen durch Ameisen ist extrem selten: bei nur zwei tropischen Orchideenarten sind Ameisen exklusive Bestäuber (JONES 1975, PEAKALL & BEATTIE 1989, PEAKALL et al. 1987, PEAKALL et al. 1990, PEAKALL & BEATTIE 1991, SIEGEL 2014) und aus der Holarktis war Ameisenbestäubung bisher nur von der alpinen Orchidee Chamorchis alpina bekannt (BAUMANN & BAUMANN 2010, CLAESSENS & KLEYNEN 2011, 2016, SCHIESTL & GLASER 2012).