A roadmap for future research on insularity effects on plant-herbivore interactions

State of the art Theory predicts that herbivore pressure should be weaker on islands than on the mainland, owing to lower herbivore abundance and diversity because of dispersal constraints and environmental filtering. As a result, plants on islands should invest less in defences against herbivory. Although early empirical studies supported these predictions, recent systematic island-mainland comparisons have questioned this paradigm, with some studies reporting either no difference between islands and mainland or higher herbivory and plant defences on islands. Current data therefore appear to be unsupportive of predictions on insularity effects on plant-herbivore interactions, calling for more research to reassess predictions and to test underlying mechanisms for observed patterns. Research opportunities To meet this challenge, a renewed research programme based on the accrual of studies with specific features is needed. These should include more robust experimental designs with replication within and across systems, integrative and more nuanced assessments of plant defensive phenotypes and herbivory, a food web approach that considers the multi-trophic context in which plant-herbivore interactions are embedded, and a consideration of historical factors (e.g., island origin and biogeographical factors, defensive anachronisms). Outlook This new research programme will require integration of evolutionary ecology research on plant-herbivore interactions with island biogeography, palaeoecology and community ecology to understand the influence of factors acting at different scales, from local factors driving herbivory and plant defences to historical processes and regional drivers of species composition determining species traits and their interactions.

Automated summary

State of the art theory predicts weaker herbivore pressure on islands than on the mainland, leading to lower investment in plant defenses against herbivory. While early studies supported this prediction, recent research questions this paradigm, suggesting that islands may actually experience higher herbivory and plant defenses. Further research is needed to reassess predictions and investigate underlying mechanisms for observed patterns.