Genetic diversity and connectivity shape herbivore load within an oak population at its range limit

Host genetic diversity and genotypic identity have been reported to affect the abundance, species richness and species diversity of associated herbivore communities. Recent work, however, suggests that these effects are highly context-dependent and that the magnitude and direction of the effects may vary with e.g., spatial factors and the amount of genetic variation present in the host population. Here, we use observational data on a Finnish oak (Quercus robur) population to examine whether low genetic diversity within peripheral populations reduces the impact of host genotype on associated herbivore communities. We first compared measures of genetic variation within Finnish oak populations with those recorded in more central parts of the species' range, confirming that genetic variation within the Finnish populations is comparatively low. Despite this result, we found consistent imprints of host genetic diversity on herbivore communities: herbivore load, but not the species richness, increased with host genetic diversity in both years and both spatial scales examined. Spatial connectivity of hosts increased herbivore diversity as well as abundance. While the similarity of herbivore communities increased with the genetic similarity among hosts, the effect of geographic distance was stronger. Overall, our findings identify a major role for spatial context in structuring oak-associated herbivore communities-but we still trace detectable imprints of host genotype at multiple spatial scales even in this peripheral, genetically impoverished oak population.