Aim: Plants on islands are often subjected to lower levels of herbivory relative to those at mainland sites. As a consequence, island plants are predicted to exhibit lower levels of physical and chemical defences, which renders them more susceptible to introduced herbivores. Yet, instances of high pressure by superabundant herbivores native to islands have been reported in many insular systems, which presumably would result in heightened plant defences. To date, no quantitative review has been conducted to determine how common these contrasting patterns are and their implications for the evolution of plant defences.

Location: Islands worldwide

Taxon: Plants, insects, mammals

Methods: We conducted a meta-analysis of insularity effects on herbivory and plant defences by including studies that conducted island-mainland comparisons of the same plant species in both environments (90% of cases), or insular endemics vs. mainland congeners (10% of cases). We tested for differences between mammalian and invertebrate herbivory as well as between plant chemical and physical defences by specifying comparisons based on the type of herbivore included in the study.

Results: Mammalian herbivory was significantly higher on islands than on mainlands. In contrast, no significant effect was observed on invertebrate herbivory. In addition, we found no significant difference in either plant physical or chemical defences between insular and mainland plants, though physical defences tended to be higher for plants on islands.

Main conclusions: All analysed mammal studies focused on species introduced to islands, suggesting greater susceptibility of insular plants to exotic mammals, whereas the lack of effects in the case of invertebrate herbivory suggests no difference in susceptibility to molluscs and insects between insular and mainland plants. Interestingly, plant trait patterns suggest a trend for increased physical defences by insular plants, possibly due to heightened pressure by exotic mammalian herbivores on islands, whereas chemical defences appear uncorrelated to differences in herbivory. These findings call for further experimental and observational studies measuring defences and herbivory for multiple sympatric plant species occurring at both mainland and island sites within a system, or comparing insular endemics to congeneric mainland species.