The pollination syndrome hypothesis suggests that there is a link between flower color and morphology, floral rewards for animal visitors, and the taxonomic identity of animal visitors and pollinators to flowers based on color and morphology. The value of the pollination syndrome approach has been questioned in the literature. In order to test the validity of predictions of the pollination syndrome approach we identified four congeneric species pairs in which the flowers of each species differed in color in order to study floral traits and animal visitors to flowers of these four species pairs. In the field we collected nectar to examine in the lab total sugar concentration and the proportion of sucrose, glucose, and fructose in the nectar from flowers of each species, measured morphological traits of flowers, and conducted timed surveys of insect and vertebrate visitors to flowers. Insect visitors were classified into larger taxonomic categories such as bumblebees, butterflies, etc. Species pairs included Impatiens capensis with orange flowers and Impatiens pallida with yellow flowers, Lobelia siphilitica with purple flowers and Lobelia cardinalis with red flowers, Lonicera japonica with white flowers and Lonicera sempervirens with red flowers, and Monarda fistulosa with lavender flowers and Monarda didyma with red flowers.
Results/Conclusions
Results provide indirect support for the pollination syndrome hypothesis. For congeneric species pairs, there were differences in nectar sugar concentrations when flowers differed both in color and morphology. Consistent with the pollination syndrome approach red tubular flowers of three species (L. cardinalis, L. sempervirens, and M. didyma) produced nectar with relatively low sugar concentration (~20% sucrose equivalents), while the purple flowers of L. siphilitca produced a nectar with relatively high sugar content (30-35% sucrose equivalents); nectar samples from M. fistulosa were small and were not analyzed. White tubular flowers of L. japonica had sugar concentrations similar to those of the red-tubular flowered L. sempervirens. While flowers of I. capensis and I. pallida differ in color they are similar in morphology and both produced nectar with high sugar concentration (~35% sucrose equivalents). All flowers with nectar with high sucrose concentrations were preferentially visited by bees, particularly bumblebees, while observations of visitors to red flowers led to infrequent visits by hummingbirds, and evidence of nectar robbing by bees. These preliminary results suggest that consistent with the pollination syndrome approach, there is some relationship between floral color, morphology, sugar composition of nectar, and the animals that visit these flowers