The pollination ecology of Aquilegia micrantha (Ranunculaceae) in Colorado
Abstract
Aquilegia micrantha occurs in canyons of the Southwestern United States. The pinkish-yellow flowers are nodding or semi-nodding, mildly scented, protandrous, and secrete a concentrated nectar (46% total sugars). Seed set in flowers under pollinator exclosures was 25% compared with 63% for open-pollinated flowers. The most common and effective pollinators of A. micrantha in the upper Crystal River Canyon were nectar-foraging bumblebee queens, Bombus appositus and B. flavifrons, although hummingbirds and hawkmoths also visited the flowers, and hummingbirds are important pollinators at other sites. The absence of nectar-foraging Bombus queens on the red flowers of A. elegantula in the area is probably the result of the narrower spur mouths of A. elegantula that prevent the bees from inserting their heads deeply enough into the flowers to extract the nectar. This mechanical isolaiton may be important in restricting hybridiza- tion between the two species in the narrow elevational range of sympatry. North American columbines may be divided into three groups on the basis of their floral color, morphology, and mode of pollination. Species in the Aquilegia vulgaris group generally have blue or purple flowers with short, hooked spurs and are presumably pollinated by bumblebees. Members of the A. canadensis group have nodding, red flowers with short, straight spurs and are pollinated primarily by hummingbirds. The erect, pale- colored flowers of the members of the A. caerulea group have longer, slightly curved spurs and are adapted for pollination by hawkmoths (Grant, 1952). Members of the latter two groups may also be visited by bumblebees (Miller, 1978, 1981). Aquilegia micrantha occurs in canyons in southern Utah, northern Ari- zona, and southwestern Colorado where it often forms hanging gardens on damp, rocky ledges and seepage slopes. In Colorado it has an elevational range of 1520-2440 m and blooms during May and June (Harrington, 1954). The nodding posture and relatively short spurs of the flowers of A. micran- tha are characteristic of the flowers of the A. canadensis group, while the pale floral color suggests affinities with members of the A. caerulea com- plex. There are no previous records of pollinator activity on flowers of A. micrantha. The observations reported here were undertaken to provide information regarding the pollination ecology of A. micrantha and to enhance our understanding of its relationships to other Aquilegia species in
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