From mainland to islands: the evolution of resistance and tolerance to herbivory in long‐lived oaks

Island biogeography theory predicts that island plants should exhibit reduced defences compared with their mainland relatives due to relaxed herbivory pressure. However, growing empirical evidence challenges this prediction, revealing substantial variation among systems, plant lineages and defence types. These inconsistencies suggest that simple expectations of reduced defence on islands overlook the diversity of selective agents and strategies plants use to cope with herbivores. A more integrative perspective proposes that island – mainland comparisons should simultaneously consider multiple defensive mechanisms, including resistance – both constitutive and inducible – and tolerance‐related traits. In this study, we conducted two complementary greenhouse experiments to examine resistance and tolerance responses in seven island–mainland species pairs of oaks Quercus across three biogeographical regions: Bornholm Island versus mainland Sweden, the Balearic Islands versus mainland Spain, and Lesbos Island versus mainland Greece. Seedlings were exposed to controlled foliar herbivory by the generalist Lymantria dispar , with undamaged plants serving as controls. Resistance was quantified by measuring key chemical defences – namely total phenolic content and volatile organic compounds – assessed both at constitutive levels and in terms of inducibility following herbivore damage. Tolerance, in contrast, was evaluated as the plant's capacity for growth compensation, quantified through height regrowth after herbivory. Our results show that, contrary to traditional expectations, island and mainland oak seedlings did not differ significantly in either chemical resistance or growth‐based tolerance to herbivory. These results suggest that the evolution of plant defences on islands may not be universally reduced and that both island and mainland oaks maintain comparable strategies to cope with herbivory, highlighting the importance of considering multiple defence mechanisms and local ecological contexts when assessing insularity effects.