The Roles of Ants as Flower Visitors: Experimental Analysis in Three Alpine Plant Species

Abstract

I investigated three co-occurring North American alpine plant species for which ants comprise at least 20% of the flower visitors. Ants of the species Formica neorufibarbis gelida pollinate gynodioecious Paronychia pulvinata, are herbivores of the gynodioecious Eritrichum aretioides and appear to have little effect on the hermaphroditic Oreoxis alpina. Given that floral nectar and lipids are important resources for alpine ants, the effect of all three plant species on the ants is positive. In E. aretioides, ants chew on the coronal ring of flowers but are unlikely to contact the stigmas, explaining their negative effects on seed production. In ant-pollinated P. pulvinata, ants affect pollination in females more than in hermaphrodites. Single ant visits to unpollinated pistillate flowers show that ants are inefficient pollinators, transferring no pollen (viable or inviable) to stigmas 55% of the time. This result is probably explained by low pollen loads and infrequent movement between plants. In contrast, single ant visits to previously unpollinated perfect flowers on hermaphrodites produce seed-set similar to that in flowers open to pollination. In O. alpina, the lack of an effect of ants on seed production is probably explained by autogamous seed-set and lack of pollen limitation. In both gynodioecious species, E. aretioides and P. pulvinata, female plants produce more seeds than hermaphrodites in the absence of ants, but this difference is magnified when ants are allowed access to plants, suggesting that ants can influence the evolutionary maintenance of breeding system dimorphism in these species.