A meta‐analysis of insularity effects on herbivory and plant defences

Abstract

AbstractAimPlants on islands are often subjected to lower levels of herbivory relative to those found at mainland sites. As a consequence, island plants are predicted to exhibit lower levels of physical or 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‐herbivore interactions.LocationIslands worldwide.TaxonPlants, insects, molluscs, mammals.MethodsWe conducted a meta‐analysis of insularity effects on herbivory and plant defences by including studies that involved island‐mainland comparisons of the same plant species in both environments (90% of cases), or insular endemics versus mainland congeners (10% of cases). We tested for differences between mammalian and invertebrate (mollusc or insect) herbivory as well as between plant chemical and physical defences by specifying comparisons based on the type of herbivore (vertebrate or invertebrate) or plant trait included in the study.ResultsMammalian 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 conclusionsAll 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 or 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.