The predictability of traits and ecological interactions on 17 different crosses of hybrid oaks

Herbivory on hybrid plants has the potential to affect patterns of plant evolution, such as limiting gene-flow through hybrids, and can also affect herbivore biodiversity. However, few studies have surveyed multiple hybrid species to identify phylogenetic patterns in the inheritance of plant traits that may drive herbivory. We surveyed 15 leaf traits and patterns of chewing, mining, and galling herbivory in a common garden of 17 artificially crossed hybrid oak species and each of their parental species over a 2-year period. Using a phylogeny of oaks, we tested whether hybrids that resulted from more divergent parents received more herbivory than those derived from closely related parents (as would be predicted by a build-up of incompatibilities in defensive systems over evolutionary time) and found only marginal evidence in support of this. We found that chewing damage to hybrids was weakly predicted by the relatedness of a parental species to the single native oak. The levels of chewing and mining herbivory on hybrids were typically intermediate to those of their parental species, though less than the parental mean for chewing damage in 2008. Most leaf traits of hybrids were also intermediate to those of their parental species. There was no clear pattern in terms of an association between 11 species of cynipid gall wasps and hybrids. The patterns of (1) intermediate levels of herbivory on hybrids and (2) no trend in herbivory on hybrids based on the phylogenetic relatedness of parental species suggest that herbivory may not play a general role in limiting hybrid fitness (and thus gene-flow through hybrids) in oaks.