Insularity effects on plant-herbivore interactions

Theory and early empirical work posed that herbivore pressure should be weaker on islands than on mainland owing to lower herbivore abundance and diversity in insular systems. Consequently, plant taxa found on islands are expected to be less defended or have even evolved the complete loss of defences. While early observational studies supported the prediction of lowered herbivory and plant defences on islands, recent island-mainland comparisons have shed mixed findings, with some studies finding no differences between islands and mainland or, surprisingly, higher herbivory and plant defences on islands. However, studies remain limited in scope both geographically and taxonomically. In this presentation, I will talk about the knowledge gaps and research opportunities on this topic. In particular, further studies should include (i) a broader geographical extent of island-mainland comparisons with site replication within each system and multiple systems (at both regional and global scales), (ii) a more comprehensive and integrative assessment of plant defensive phenotypes (multiple traits and their co-expression patterns), (iii) measurements of herbivory by vertebrate and invertebrate herbivores species and guilds, and (iv) a consideration of multi-trophic context in which plant-herbivore interactions are embedded, namely how predators and parasitoids respond to insularity and their relative influence on mainland vs. island herbivory and plant defences.