The evolution and ecology of floral colour in the biological partnership of flowering plants and pollinators

Plant-pollinator interactions have a very important role in the reproductive cycle of flowering plants. Plants typically provide small amounts of nutrition for animal pollinators, and pollinators incidentally transfer pollen between flowers to enable sexual reproduction. Despite the apparent simplicity, this type of interaction is very complex. Colour is a major cue used for signalling by flowers. I investigated floral spectral signals, as perceived as colour by a pollinator’s visual perception, the phylogenetic pattern of colour signal evolution, and the signal-receiver relationship in two natural communities in Victoria, Australia, and over an altitudinal gradient in Nepal. From this study, I found that bird-visited flowers and insect-visited flowers differ significantly from each other in their respective chromatic cues. These differences are concentrated near wavelengths of optimal colour discrimination for the respective pollinators. Further investigation considering a steep altitudinal gradient of Nepal Himalaya provides evidence that species in subtropical vegetation had colours significantly less dispersed in a bee-vision colour space than expected by chance, while subalpine species had significantly greater colour dispersion than expected by chance. I found that at both low altitude sub-tropical and high altitude sub-alpine regions of Nepal, flowers had spectral signals that closely matched the regions of best colour discrimination by hymenopteran trichromatic pollinators. This finding is somewhat surprising because hymenopteran pollinators, especially bees, tend to decline in abundance with increasing altitude, while flies become relatively more abundant and are thought to be important pollinators at high elevation. Next I investigated the similarity of floral colours in orchids and non-orchids species as perceived by insect pollinators. Surprisingly, there was no congruency between phylogeny and position in a colour space, showing that floral colours were highly labile traits in the evolution for both orchids and non-orchids. Finally, I addressed the question about role of flower colour in two natural communities in Australia and found that fruit sets of focal plant species increased in the presence of neighbouring flowering plants. Thus the presence of co-flowering plants appears to facilitate the pollination of other species in the communities studied