Flowers and enemies:: predation by nectar-thieving ants in relation to variation in floral form of an alpine wildflower, Polemonium viscosum

Flowers of the alpine skypilot, Polemonium viscosum, are attacked by nectar-thieving ants of Formica neorufibarbus. Plants in this species display continuous variation in floral form from long, broadly-flared flowers to short, narrow flowers. This study addresses three aspects of the potential For ants to exert selection on floral traits in P. viscosum: the effect of ant visitation on seed production, the relationship between ant visitation patterns and variation in floral form among co-occurring individuals, and the behavioral mechanisms underlying nonrandom flower;er choice by ants. Ants visiting flowers of P. viscosum interfere with seed production by dislodging the style from the ovary. In 1982 and 1997 plants protected from ant visits set significantly more seeds per flower than open controls. Plants with broadly-flared flowers incurred significantly higher rates of ant damage than individuals with narrower flowers in 1995. Similarly, in 1997, damage increased significantly with plant score on a principal component positively correlated to corolla flare and length. Ant abundance declines with elevation above timberline, and, in 1997, damage associated with corolla flare and length was significantly greater for plants in the timberline (krummholz) habitat than fur plants higher on the alpine tundra. Laboratory choice trials showed that ants prefer flowers with lung broadly-flared corollas over flowers with short narrow corollas, Ant preference may reflect differences in floral reward: broadly-flared flowers contain more nectar sugar than narrow ones in P. viscosum. Additionally energetic costs associated with nectar collection depend in part on flower form. In choice experiments, handling time, or the time required for an ant to move from the corolla opening to the nectary decreased significantly with corolla flare and was also less variable for ants choosing broadly-flared flowers than those choosing narrow ones. Ants have the potential to exert selection on flower form in P. viscosum, as their activity negatively affects the female component of reproductive fitness and is nonrandomly associated with different floral phenotypes. In this system, enemies show similar flower choice patterns to bumblebee pollinators, and their behavior may help maintain heritable variation in newer form within populations of P. viscosum.