Carla Vázquez-González, University of California, Irvine & Misión Biológica de Galicia (CSIC), discusses her article: Testing the contribution of vertebrate predators and leaf traits to mainland-island differences in insect herbivory on oaks
Insularity research on plant-herbivore interactions
Islands have long fascinated evolutionary ecologists, beginning with the early observational studies of naturalists like Charles Darwin and Alfred Russel Wallace. These investigations laid the groundwork for the formulation of island biogeography theory by Edward O. Wilson and Robert H. MacArthur, whose seminal work, The Theory of Island Biogeography, was published in the 1960s. Even today, islands serve as invaluable natural laboratories for evolutionary ecology. Their unique features—such as geographic isolation, limited size, and relatively recent geological origins—offer exceptional opportunities for in-depth exploration of how ecological interactions, adaptive strategies, and speciation processes drive evolutionary change and species diversity.
In the context of plant-herbivore interactions, ecological theory posits that herbivory pressure should be weaker on islands compared to the mainland, attributed to the reduced abundance and diversity of herbivores. However, empirical evidence supporting this prediction is limited and often contradictory. Indeed, a recent meta-analysis conducted by our research group revealed that herbivory is, in fact, higher on islands than on the mainland, and found no discernible pattern in the case of invertebrate herbivory. This study also highlighted a particular focus on mammalian herbivores in insularity research, while studies on insects remain sparse. Because many insect herbivores are highly specialized and have often coevolved with their host plants, they exert significant selective pressure on plant adaptation. Therefore, these organisms necessitate a greater focus in insularity research.
Moreover, a critical knowledge gap in insularity research is the underestimation of the role played by natural enemies of herbivores, such as predators and parasitoids, in shaping island-mainland patterns of herbivory. This warrants further investigation to understand how these interactions influence plant-herbivore dynamics in island ecosystems.
The study
In our recent study, we examined the effects of insularity on insect herbivory across 12 oak (Quercus) species in three island-mainland systems: Channel Islands vs. mainland California, Balearic Islands vs. mainland Spain, and Bornholm island vs. mainland Sweden. We investigated the role of vertebrate predators, such as birds and bats, and specific leaf traits (chemical and physical defences, and nutrient content) in shaping herbivory patterns. To do so, we excluded vertebrate predators and collected leaves for herbivory assessment and leaf trait analysis from various locations within our study systems.
What we found and why it matters
We found that insect herbivory was lower on islands than on the mainland but only in Mediterranean regions like California and Spain, which have experienced longer periods of isolation (and likely more pronounced differences in herbivore communities) compared to our Boreal system in Sweden-Bornholm. Notably, we found no evidence that predation by bats or birds regulated herbivory across our study systems, indicating that these predators did not influence observed patterns in our systems. Although some leaf traits varied between islands and the mainland—with generally lower defences on islands—these differences did not explain herbivory patterns. Overall, our findings align with ecological theory, suggesting that simplified herbivore communities on islands lead to reduced herbivory and subsequently lower defence mechanisms in plants, providing the first broad-scale test of differences in insect herbivory and anti-herbivore defence between islands and the mainland. Although predator effects were not evident in our research, other groups, such as predatory ants, could still play a role. Our findings open the door to future studies employing methods like model clay caterpillars to identify predation events, or using various predator exclusion techniques to assess predation by different taxonomic groups. Understanding these interactions is crucial for advancing our knowledge of island ecology and the evolutionary processes that shape species in these unique environments.
A case of collaborative research
This project is part of an emerging research line in Dr. Xoaquín Moreira’s laboratory at the Misión Biológica de Galicia (CSIC), aimed at exploring plant-insect herbivore interactions on islands. This particular study was conducted thanks to funding from the Spanish Association of Terrestrial Ecology to young researchers (AEET), granted to Carla Vázquez-González to carry out a research project in the Channel Islands (CA, USA) during her postdoc at Dr. Kailen Mooney’s laboratory at the University of California, Irvine. We expanded our research to include additional systems thanks to collaborations with experts from various institutions, such as Dr. Ayco J.M. Tack (Stockholm University), Dr. Johan A. Stenberg (Swedish University of Agricultural Sciences), Dr. Miquel Capó (Universidad Politécnica de Madrid), and Dr. Raúl de la Mata (Estación Biológica de Doñana – CSIC). This collaboration underscores the project’s collective effort to advance ecological science and identify general patterns across different systems and species.
