BEE FLOWERS: A HYPOTHESIS ON FLOWER VARIETY AND BLOOMING TIMES

Abstract

In arctic (Kevan, 1972), temperate (Lovell, 1903), and tropical regions, the flowers of different species of plants in a given habitat at any one time usually exhibit a variety of colors, scents, morphologies, and blooming times. Bees are the primary pollinators of many of them (Baker and Hurd, 1968). Adaptations of given species of plants to specific pollinators (MMuller, 1881; Knuth, 1909; van der Pijl, 1961; van der Pijl and Dodson, 1966; Grant and Grant, 1965; Faegri and van der Pijl, 1971), to an array of generalized pollinators, or to differences in pollinator behavior (Macior, 1970, 1971) probably do not account for all of the extraordinary array of forms, colors and scents of the bee-pollinated flowers in a given habitat. Those commonly visited by bumblebees alone may be white, red, yellow, purple, ultraviolet, green, or blotched and streaked with a variety of colors. The flowers may be zygomorphic (Werth, 1949), platformshape, or equipped with varying numbers of lobed and unlobed petals. The key to the evolution of different types of flowers is usually considered to reside in the preferences and the flowerconstancy of the pollinators. The flowerconstancy of bees, and its probable impact on plant speciation has been discussed (Grant, 1949; Grant, 1950; Manning, 1956a; Free, 1966). However, little is known about the selective pressures producing the variety of flowers upon which flower-constancy is based. It is rarely possible to witness selection, but inferences of differential selection exerted by pollinators can sometimes be made from careful studies of the interactions of flower-visitors and plants in a given area. The studies of Clarkia bees of the western United States (MacSwain et al., 1973) and of bumblebees on Pedicularis in the Rocky Mountains (Macior, 1970, 1971) serve as good examples. It is necessary to determine what affects flower choice and flower fidelity of the foragers in the presence of numerous species of sympatric plants. This paper concerns the functional significance in the variety and in the assortment of flowers visited primarily by bumblebees in one habitat in Maine. The observations from this area may serve as a model applicable to other plant assemblages and pollinators.