Floral traits and pollinator constancy: Foraging by native bees among three sympatric legumes

Abstract The foraging constancy of the solitary bee, Trichocolletes sp., was documented in a semi-arid ecosystem where three morphologically similar legumes co-occur. To quantify the degree of floral similarity among the forage plants, Dillwynia hispida, Dillwynia uncinata and Pultenaea densifolia, discriminant function analysis of petal dimensions and a spectral analysis of corolla colour was undertaken. Petal size varied moderately among species resulting in 2–14% of specimens being misclassified, usually as D. uncinata. Subtle variation in corolla colour in the ultraviolet wavelengths contributed to the distinctiveness of each species. All three species exhibit an UV-absorbing region at the base of the standard petal which contrasts sharply with the rest of the UV-rerlecting petal. Between 450 nm and 470 nm, per cent reflectance drops for D. hispida and P. densifolia, but remains steady for D. uncinata. Above 480 nm, parts of the standard petal in all species reflect similar amounts of light with little variation exhibited among species. Nectar was only offered by D. hispida and D. uncinata, whereas P. densifolia produced an excess of pollen which may serve as a reward. Despite the subtle differences in floral morphology and rewards among these sympatric legumes, Trichocolletes bees only remained faithful to a given species in 43% of 183 interplant flights. Inconstant foraging by Trichocolletes sp. increases the pool of resources because rewards in all three legume species can be obtained in exactly the same manner.