Pollination and seed production in Ipomopsis aggregata: differences among and within flower color morphs

The pollination of red, pink, and white color morphs of Ipomopsis aggregata was evaluated to assess whether ethological isolation based on pollinator color discrimination may occur. We observed animal visitors, assessed pollen delivery, seed set per fruit, percentage of flowers setting fruit, nectar production, and timing of flower opening for different color morphs in the Front Range of Colorado. Based on traditional zoophilous flower classifications, we expected hummingbirds to pollinate red-flowered I. aggregata subsp. collina and hawkmoths to pollinate white-flowered I. aggregata subsp. candida. However, ethological isolation does not appear to occur among color morphs of I. aggregata in the Front Range. Hummingbirds visited redflowered plants in excess overall, and, to a lesser extent, so did hawkmoths. Both hummingbirds and hawkmoths visited all color morphs and probably transferred pollen among them. Pollen delivery data and a day-night bagging experiment also suggest that pollinators do not necessarily behave as predicted by flower classifications. In addition, there is little evidence for major differences between red, white, and pink flowers in any aspects of reproductive biology. Indeed, most variation occurs within a given color morph. FLOWER COLOR has long been considered a factor that might influence pollinator behavior (Muller, 1883; Darwin, 1895; Clements and Long, 1923; Kugler, 1959; Stebbins, 1970; Proctor and Yeo, 1973; Kevan and Baker, 1983; Waser, 1983), and zoophilous flower classes ("pollination syndromes") based in part on color have been suggested (Pijl, 1961; Baker and Hurd, 1968; Scogin, 1983). When pollinators discriminate among colors, this may influence patterns of pollen transfer, and thus gene flow and evolutionary dynamics, within and between species (Grant, 1949; Dronamraju, 1958, 1960; Kay, 1978). Studies of pollinator responses to color differences have had inconsistent results. Some have concluded that color discrimination by pollinators is of great importance (e.g., Horovitz, 1969; Levin and Schaal, 1970; Kay, 1978; Brown and Clegg, 1984, and references I Received for publication 7 January 1987; revision accepted 23 November 1987. Many persons helped us during this study. We are especially grateful to Michael Breed, Alcinda Cundiff, Jeff Karron, Meredith Lane, Nickolas Waser and Dieter Wilken for help, advice and information at various stages, including manuscript preparation. We also thank Verne Grant, Mary Price, Ken Paige, and two anonymous reviewers for comments on the manuscript, Michael Grant and Jeffry Mitton for statistical advice, Url Lanham for insect identifications, and Bill Calder, David Inouye, John Capinera, and Paul Opler for comments and information about hummingbird and/or hawkmoth abundances. The study was funded, in part, by the Lichty Fund and the Ramaley Fund of the Department of Biology, University of Colorado. therein; Levin, 1985, and references therein). Other studies have shown either that color is only one of several important factors, or is of little consequence to pollinators (e.g., Mather, 1947; Grant, 1952,1976; Macior, 1966; Lloyd, 1969; Mogford, 1978, and references therein; Miller, 1978, 1981; Hannan, 1981; Ennos and Clegg, 1983; and Stucky, 1984). Results are complicated by the fact that discrimination based on color may have other underlying causes, e.g., differential rewards (Waser and Price, 1981, 1983, 1985). The purpose of this investigation was to determine if flower color is associated with pollinator discrimination by examining visits to red, pink and white colored morphs of Ipomopsis aggregata in the Front Range of Colorado. We wished to answer four major questions: 1) Do hummingbirds, hawkmoths, and other visitors show color fidelity or preference (i.e., are they imposing ethological isolation between redand white-flowered I. aggregata)? 2) Is any color morph more reproductively successful than the others as a result of consistent pollinator behavior? 3) Is morph color accompanied by colorcorrelated plant features such as plant height, nectar amounts, timing of flower opening etc. which might influence preferences of the pollinator? 4) What are the implications of color (in)fidelity on two subspecies of Ipomopsis aggregata?