Alpine plant-ant interactions

By studying the effects of flower-visiting ants on three plant species, my research combines a species specific approach with a multiple species comparison to understand the role of ants in the alpine. I also address the importance of positive and negative flower visitors in maintaining plant breeding system variation. Using a simple model, I consider how quality, quantity, and consistency affect relationships between interacting species.;I studied three plant species which co-occur on Pennsylvania Mountain and for which ants represent at least 30% of the flower visitors. Ants of the species Formica neorufibarbis gelida pollinate Paronychia pulvinata, are herbivores of the plant species Eritrichum aretioides and appear to have little effect on seed production in the plant species 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, and the interactions represent respectively, a mutualism, a predation, and a commensalism. Floral morphology, the frequency of ant visitation, and the ability of flowers to set seed without insect visitation are key factors determining whether such relationships are mutualistic or antagonistic.;Two of the plant species I have studied are gynodioecious with populations of female (male-sterile) individuals and hermaphroditic (male-fertile) individuals. Theory predicts that, to persist in a population, female individuals must compensate for an inability to acquire fitness through pollen by producing more or better quality seed than individuals of the hermaphroditic morph. The hypothesis that biotic interactions with another species might help explain the higher fecundity of male-sterile plants has been poorly studied. In both gynodioecious species that I have studied, exclusion of ants alters the ratio of average seed set by female plants to seed set by hermaphroditic plants in the direction supporting this hypothesis. This observation is exciting since in Eritrichum aretioides, ants are herbivores (reducing seed set of hermaphrodites more than females) and in Paronychia pulvinata, ants are pollinators (increasing seed set of females more than hermaphrodites). Thus, I have shown that ants are likely to help maintain the observed sex ratios of the two species by very different mechanisms.