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 con tinuous variation in floral form from long, broadly-flared flowers to short , narrow flowers. This study addresses three aspects of the potential For a nts to exert selection on floral traits in P. viscosum: the effect of ant v isitation on seed production, the relationship between ant visitation patte rns and variation in floral form among co-occurring individuals, and the be havioral mechanisms underlying nonrandom flower;er choice by ants. Ants vis iting flowers of P. viscosum interfere with seed production by dislodging t he style from the ovary. In 1982 and 1997 plants protected from ant visits set significantly more seeds per flower than open controls. Plants with bro adly-flared flowers incurred significantly higher rates of ant damage than individuals with narrower flowers in 1995. Similarly, in 1997, damage incre ased significantly with plant score on a principal component positively cor related to corolla flare and length. Ant abundance declines with elevation above timberline, and, in 1997, damage associated with corolla flare and le ngth was significantly greater for plants in the timberline (krummholz) hab itat than fur plants higher on the alpine tundra. Laboratory choice trials showed that ants prefer flowers with lung broadly-flared corollas over flow ers with short narrow corollas, Ant preference may reflect differences in f loral reward: broadly-flared flowers contain more nectar sugar than narrow ones in P. viscosum. Additionally energetic costs associated with nectar co llection depend in part on flower form. In choice experiments, handling tim e, 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 variab le 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 polli nators, and their behavior may help maintain heritable variation in newer f orm within populations of P. viscosum.