Pollination and plant-defence traits covary in Western Australian Hakeas

Despite the conflicting demands imposed by mutualistic (pollination) and antagonistic (florivory) processes, the relative importance of the key selective pressures influencing floral evolution are not readily apparent. In this study we quantified a range of floral and foliar traits within the genus Hakea to investigate how pollinator and herbivore selection might influence the evolution of floral attraction and defence attributes. Plant material was collected from populations of 51 Australian Hakea species native to southwestern Australia, and measurements were taken of foliage and inflorescence morphology, inflorescence colour and floral chemical defence. Hakeas were separated into bird- vs insect-pollinated species on the basis of stigma-nectary distance. Our results show how the evolution of insect vs bird pollination is closely linked to whether inflorescences are protected by physical (leaf spines, dense foliage) or chemical (floral cyanide) defences, respectively. Rather than being constrained by the necessity to attract pollinators, we suggest that pre-existing adaptations to combat florivore and herbivore attack directed the evolution of floral characteristics employed to attract pollinators and deter florivores. The inter-correlation among bird pollination, red flower colour and floral cyanide indicates floral coloration may signal to vertebrate florivores that the inflorescences are unpalatable despite their high accessibility. New Phytologist (2009) 182: 251-260doi: 10.1111/j.1469-8137.2008.02709.x.