Island Features and Abiotic Factors as Drivers of Insect Leaf Herbivory on Islands

Herbivory is a dominant interaction across the globe and has shaped within- and among-species variation in both plant and herbivore traits. Islands are not the exception, and several studies have documented cases where native herbivores such as mammals, tortoises, crabs, birds and to a lesser extent insects exert strong effects on insular plant traits and abundance, and in some cases drive speciation. Nonetheless, we still know little about the drivers of herbivory on islands, in particular the influence of classically studied geographical or physical island features and the virtually unstudied influence of abiotic factors. In particular, more isolated (i.e. distance to mainland), geologically older and smaller islands are predicted to lead to decreases in herbivore abundance and richness which would in turn lead to lower herbivory. In addition, islands found in more humid or warmer climates and with soil types that promote productivity should exhibit higher herbivory. In this chapter, we aim at investigating the effects of island physical factors such as island size, isolation and geological age, as well as abiotic factors (namely, climate and soil), on insect herbivory for 76 plant species found across 6 archipelagos (17 islands in total) in Europe. Contrary to expectations, we found no significant relationship between herbivory and island features (area, isolation and geological age), as well as no association between herbivory and climate. The only exception was a significant relationship between herbivory and soil characteristics where plants growing under silty soils exhibited lower levels of leaf damage, indicating that soil physical features could play a bottom-up role in shaping herbivory patterns on island. Overall, our findings provide little support for predicted effects of island features and climate on insect leaf herbivory levels but some evidence for soil abiotic controls. These findings call for further studies involving larger taxa and island sample sizes combined with robust controls on plant traits, phylogeny and insect feeding guilds across broader abiotic gradients.