THE POLLINATION ECOLOGY OF AQUILEGIA ELEGANTULA AND A. CAERULEA (RANUNCULACEAE) IN COLORADO

Aquilegia elegantula Greene and A. caerulea James occur in montane and subalpine habitats in the southern Rocky Mountains of western North America. The red and yellow flowers of A. elegantula are nodding, odorless, protogynous, and secrete a concentrated (44%) sucrose nectar in the floral spurs. Seed set in flowers under pollinator exclosures was 12% while seed set in open-pollinated flowers was 65%. The flowers of A. elegantula are pollinated primarily by the Broad-tailed Hummingbird (Selasphorus platycercus [Swainson]) and by at least three species of pollen-foraging bumblebees, of which Bombus occidentalis Greene is the most common. The blue and white flowers of A. caerulea are erect, mildly fragrant, protandrous, and secrete a 26% sucrose nectar. Seed set in caged flowers in the field averaged 39%, in uncaged flowers 54%. The most important pollinators of A. caerulea are the crepuscular hawkmoth, Hyles (=Celerio) lineata (Fabricius) and ten species of pollen-foraging Bombus. The most abundant bumblebee species, B. occidentalis, is also a frequent nectar thief. Differences in pollination systems alone probably do not constitute an effective antihydridization mechanism between A. elegantula and A. caerulea, but do serve to reinforce differences in habitat and flowering time that distinguish the two species. AQUILEGIA is a genus of perennial herbs, widely distributed in the Northern Hemisphere, and characterized by a high degree of floral diversity. Grant (1952) recognized five major species complexes, largely on the basis of differences in floral color and morphology, and suggested that the differences between the complexes reflect adaptation to different groups of pollinators. He also argued that differences in pollination systems are effective anti-hybridization mechanisms within the genus. Thus, although representatives of the two species groups are interfertile and often grow in close sympatry in western North America, members of the red-flowered (and presumably hummingbird-pollinated) A. canadensis complex rarely hybridize with pale-flowered (and presumably hawkmoth-pollinated) members of the A. caerulea complex. Chase and Raven (1975) studied populations of A. formosa Fischer (A. canadensis group) and A. pubescens Coville l Received for publication 6 April 1977; revision accepted 29 November 1977. Portions of this study were included in a doctoral dissertation submitted to the Graduate School of Yale University. Funds were provided by the Department of Biology, Yale University, a Sigma Xi Grant-in-Aid of Research, and NSF Grant GB-30135. I wish to thank C. L. Remington for his assistance throughout the course of the study, and gratefully acknowledge the use of facilities of the Rocky Mountain Biological Laboratory. Personnel at the Smithsonian Institute in Washington, D.C., identified the Diptera, and Lola Schoenrich provided able field assistance during the 1976 season. G. J. Anderson and L. W. Macior kindly read and commented on an earlier draft of the manuscript. (A. caerulea group) in the Sierra Nevada of California, however, and concluded there was no evidence of differential visitation to either species by any major group of pollinators. The present investigation of the pollination systems of the red-flowered A. elegantula Greene and the blueand white-flowered A. caerulea James in Colorado is an attempt to further our understanding of the relationships between Aquilegia species and their pollinators. In North America the red-flowered species such as A. elegantula have long been recognized as being attractive to hummingbirds. Numerous authors have noted the Ruby-throated Hummingbird, Archilochus colubris Linnaeus of eastern North America, feeding at the flowers of A. canadensis Linnaeus (Todd, 1880; Trelease, 1880; Robertson, 1895; Schneck, 1901; Graenicher, 1910; Pickens, 1931;Bene, 1946; James, 1948; Macior, 1966). In addition to hummingbirds, sphinx moths, solitary bees, bumblebees, and syrphid flies have also been observed visiting flowers of A. canadensis (Schneck, 1901; Macior, 1966; Miller, unpublished). Aquilegia formosa, a red-flowered western species, is visited by hummingbirds, bees, and under some conditions, hawkmoths, in the mountains of California (Merritt, 1896; Grant, 1952, 1976; Chase and Raven, 1975). Red-flowered A. skinneri Hooker, from the Sierra Madre of northern Mexico, attracted bumblebees in an experimental garden in Wisconsin (Macior, 1966). The only previously published record of floral visitors to