Ecological theory predicts that herbivory should be weaker on islands than on mainland based on the assumption that islands have lower herbivore abundance and diversity. However, empirical tests of this prediction are rare, especially for insect herbivores, and those few tests often fail to address the mechanisms behind island-mainland divergence in herbivory. In particular, past studies have not addressed the relative contribution of top-down (i.e. predator-driven) and bottom-up (i.e. plant-driven) factors to these dynamics. To address this, we experimentally excluded insectivorous vertebrate predators (e.g. birds, bats) and measured leaf traits associated with herbivory in 52 populations of 12 oak (Quercus) species in three island-mainland sites: The Channel Islands of California vs. mainland California, Balearic Islands vs. mainland Spain, and the island Bornholm vs. mainland Sweden (N = 204 trees). In each site, at the end of the growing season, we measured leaf damage by insect herbivores on control vs. predator-excluded branches and measured leaf traits, namely: phenolic compounds, specific leaf area, and nitrogen and phosphorous content. In addition, we obtained climatic and soil data for island and mainland populations using global databases. Specifically, we tested for island-mainland differences in herbivory, and whether differences in vertebrate predator effects or leaf traits between islands and mainland contributed to explaining the observed herbivory patterns. Supporting predictions, herbivory was lower on islands than on mainland, but only in the case of Mediterranean sites (California and Spain). We found no evidence for vertebrate predator effects on herbivory on either islands or mainland in any study site. In addition, while insularity affected leaf traits in some of the study sites (Sweden-Bornholm and California), these effects were seemingly unrelated to differences in herbivory. Synthesis. Our results suggest that vertebrate predation and the studied leaf traits did not contribute to island-mainland variation patterns in herbivory, calling for more nuanced and comprehensive investigations of predator and plant trait effects, including measurements of other plant traits and assessments of predation by different groups of natural enemies. La teor & iacute;a ecol & oacute;gica predice que la herbivor & iacute;a ha de ser m & aacute;s d & eacute;bil en las islas que en el continente, ya que las islas tienen una menor abundancia y diversidad de herb & iacute;voros. Sin embargo, todav & iacute;a no contamos con suficiente evidencia emp & iacute;rica que apoye estas predicciones, especialmente en lo que se refiere a la herbivor & iacute;a por insectos, y los pocos estudios que existen a menudo no abordan los mecanismos que generan estos patrones de divergencia entre islas y continente en los niveles de herbivor & iacute;a. En particular, las investigaciones previas no han examinado la contribuci & oacute;n relativa de las fuerzas top-down (es decir, efectos mediados por los depredadores) y bottom-up (es decir, efectos mediados por los rasgos funcionales de las plantas) en estas din & aacute;micas. En este trabajo, excluimos experimentalmente a depredadores insect & iacute;voros vertebrados (p. ej., aves, murci & eacute;lagos) y medimos rasgos foliares asociados con la herbivor & iacute;a en 52 poblaciones de 12 especies de robles (Quercus) en tres sitios insulares y continentales: las Islas del Canal de California vs. California continental, las Islas Baleares vs. Espa & ntilde;a continental, y la isla de Bornholm vs. Suecia continental (N = 204 & aacute;rboles). En cada sitio, al final de la & eacute;poca de crecimiento, medimos el da & ntilde;o foliar causado por insectos herb & iacute;voros en ramas control vs. ramas con exclusi & oacute;n de depredadores, y medimos diferentes rasgos foliares, en particular, la concentraci & oacute;n de compuestos fen & oacute;licos, el & aacute;rea foliar espec & iacute;fica y el contenido de nitr & oacute;geno y f & oacute;sforo. Adem & aacute;s, obtuvimos datos clim & aacute;ticos y de suelo de las poblaciones insulares y continentales utilizando bases de datos globales. Espec & iacute;ficamente, evaluamos los efectos de la insularidad sobre la herbivor & iacute;a y si exist & iacute;an patrones contrastados de los efectos de depredaci & oacute;n y expresi & oacute;n de rasgos foliares entre islas y continentes que contribuyesen a explicar los patrones observados en la herbivor & iacute;a. De acuerdo con la teor & iacute;a ecol & oacute;gica, la herbivor & iacute;a fue menor en las islas en comparaci & oacute;n con el continente, pero solo en el caso de los sitios mediterr & aacute;neos (California y Espa & ntilde;a). No encontramos evidencia de efectos de los depredadores sobre la herbivor & iacute;a en ninguno de los sitios de estudio, ya sea en las islas o en el continente. Adem & aacute;s, aunque la insularidad afect & oacute; a la expresi & oacute;n de rasgos foliares en algunos de los sitios de estudio (Suecia-Bornholm y California), estos efectos no estuvieron aparentemente relacionados con las diferencias observadas en la herbivor & iacute;a. S & iacute;ntesis. Nuestros resultados sugieren que la depredaci & oacute;n por vertebrados y los rasgos foliares estudiados no contribuyeron a los patrones de variaci & oacute;n entre islas y continente observados en los niveles de herbivor & iacute;a, lo que plantea la necesidad de investigaciones m & aacute;s exhaustivas que incluyan la evaluaci & oacute;n de otros rasgos funcionales y evaluaciones de la depredaci & oacute;n por otros grupos de enemigos naturales de los herb & iacute;voros.

As primary producers, plants play a central role in mediating interactions across trophic levels. Although plants are the primary food source for herbivorous insects, they can protect themselves from herbivore damage. Many plants produce toxic compounds that directly reduce herbivore feeding, but plants also protect themselves indirectly by attracting natural enemies of the attacking herbivore through volatile signaling. These so-called tritrophic interactions have historically been documented aboveground in aerial plant parts but are also known to occur belowground in root systems. In addition to herbivores, plants directly interact with other organisms, which can influence the outcomes of tri-trophic interactions. Arbuscular mycorrhizal fungi (AMF) are symbiotic soil microbes that colonize the roots of plants and facilitate nutrient uptake. These microbes can alter plant chemistry and subsequent resistance to herbivores. Few studies, however, have shown how AMF affect tri-trophic interactions above- or belowground. This study examines how AMF colonization affects the emission of root volatiles when plants are under attack by western corn rootworm, a problematic pest of corn, and subsequent attraction of entomopathogenic nematodes, a natural enemy of western corn rootworm. Mycorrhizal fungi increased rootworm survival but decreased larval weight. Differences were detected across root volatile profiles, but there was not a clear link between volatile signaling and nematode behavior. Nematodes were more attracted to non-mycorrhizal plants without rootworms and AMF alone in soil, suggesting that AMF may interfere with cues that are used in combination with volatiles which nematodes use to locate prey.