Effects of insularity on insect leaf herbivory and chemical defences in a Mediterranean oak species

Abstract

Aim Research on plant–herbivore interactions has shown that islands typically have low abundances and diversity of herbivores because of barriers to dispersal, isolation and reduced land area. Islands commonly have lower levels of herbivory relative to mainland regions, and, as a consequence, insular plants should exhibit lower levels of defences than their mainland counterparts. Despite these predictions, there are significant gaps in our understanding of insularity effects on plant–herbivore interactions. For instance, most work addressing the effects of insularity on plant–herbivore interactions have compared one or a few islands with a single mainland site. In addition, studies have measured herbivory or plant defences but not both, and the influence of abiotic factors has been neglected. Location Mediterranean Basin (from Spain to Greece). Taxon Quercus ilex L. Methods We conducted a large‐scale study to investigate whether insect leaf herbivory and plant chemical defences in holm oak (Quercus ilex L.) differ between insular versus mainland populations. We further investigated mechanisms by which insularity effects on herbivory may take place by assessing the influence of defences and climatic variables on herbivory. Results We found that insular populations exhibited lower herbivory and higher defences (condensed tannins) than their mainland counterparts. Our analyses, however, suggest that these concomitant patterns of insect herbivory and plant defences were seemingly unrelated as island versus mainland differences in defences did not account for the observed pattern in herbivory. Furthermore, climatic factors did not explain insularity effects on either herbivory or plant defences. Main conclusions Overall, this study provides one of the most robust assessments to date on insularity effects on herbivory and builds towards a better understanding of the ecology and evolution of plant–insect interactions in insular ecosystems.

Search for top-down and bottom-up drivers of latitudinal trends in insect herbivory in oak trees in Europe

“…We quantified C:N ratio, N:P ratio, cellulose and soluble sugars as proxies for leaf nutritional quality to herbivores (Moreira et al, 2019). We measured these traits on leaves collected by both professional scientists and schoolchildren.…”

“…This result is in line with Zverev et al (2020) study performed in Arctic ecosystems, that concluded that birds are unlikely to shape the spatial patterns of insect herbivory. Associations between bird insectivory and insect herbivore can be positive (Mäntylä et al, 2014;Gunnarsson et al, 2018), negative (Maguire et al, 2015;Kozlov et al, 2017) or non-significant (Moreira et al, 2019;Valdés-Correcher et al, 2019), depending on the study and methods used. Arthropod predators (e.g.…”

“…leaf N, phenolic compounds) (Chen et al, 2013;Holopainen et al, 2018;Gely et al, 2019) and herbivory (Jamieson et al, 2015;Gely et al, 2019). However, many regions deviate from the global trend in temperature and precipitation toward higher latitudes due to their proximity to oceans or the presence of mountains (De Frenne et al, 2013), which can markedly change the relationship between latitude and plant-herbivore-predator interactions (Roslin et al, 2017;Loughnan & Williams, 2019;Moreira et al, 2019). Thus, further studies should not only rely on latitudinal clines to infer the effect of climate on plant-herbivore interactions, but should also stretch latitudinal gradients longitudinally to better capture the diversity of climatic conditions in which plant-herbivore interactions are embedded and use climatic factors such as temperature and precipitation rather than latitude per se as predictors (Anstett et al, 2016).…”

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

“…These studies have reported mixed findings, including no difference between islands and mainland sites to higher herbivory and/or plant defences on islands (e.g. Burns, 2019; Monroy & García‐Verdugo, 2019; Moreira et al., 2019; Pardo et al., 2018). While capitalizing on controlling plant species identity by using populations of the same species found in each environment.…”

Leaf functional traits and insular colonization: Subtropical islands as a melting pot of trait diversity in a widespread plant lineage

“…Lineages distributed at both insular and continental regions are valuable in that they allow for straightforward tests of phenotypic differentiation associated with colonization of novel (insular) environments (e.g. Martén-Rodríguez, Quesada, Castro, Lopezaraiza-Mikel, & Fenster, 2015;Moreira et al, 2019;Ottaviani et al, 2020). Furthermore, by shifting the typical focus of functional biogeography on communities to intra-specific variation, we could test predictions at finer biodiversity scales (Taylor et al, 2019;Violle et al, 2007), while controlling for the stochastic biogeographical patterns of island colonization (Clegg et al, 2002;García-Verdugo, Mairal, Monroy, Sajeva, & Caujapé-Castells, 2017).…”