Leaf Traits Linked to Herbivory in Lineages with Mediterranean-Macaronesian Distributions: Does an Island Syndrome in Plant Defence Exist?

Island-mainland comparisons offer an excellent framework to improve our understanding about phenotypic evolution, including particular plant traits associated with defence against herbivores. In this chapter, we provide an overview of leaf phenotypic traits putatively related to plant defence on islands, with a focus on lineages spanning mainland Mediterranean and Macaronesian island distributions. The information available to date shows that Macaronesian plants inhabiting lowland island habitats typically display large leaves, high concentrations of phenolic compounds in leaves and low photosynthetic rates as compared to their mainland counterparts. Such phenotypic convergence provides strong evidence of an island syndrome, but does not support the prediction that, due to the evolutionary absence of large herbivores, Macaronesian plant endemics have systematically lost or lowered their defences as compared to closely related mainland species. We argue that biogeographical and climatic factors should also be considered to achieve a more complete understanding of insularity effects on the evolution of defence-related traits in plants.