Genetic variation in plant traits can influence the structure of associated multi-trophic arthropod communities. Accordingly, plant adaptation to the abiotic environment is likely to have consequences for associated herbivores and predators, but such dynamics largely unstudied. To explore these dynamics, we conducted common garden studies of Artemisia californica sourced from populations along the steep aridity gradient characterizing its distribution along the California coast. Analyzing results from these common garden studies we characterized (i) population genetic variation in plant traits underlying adaptation to both the abiotic environment and herbivory; (ii) which aspects of the abiotic environment best predicted population variation in plant traits; and (iii) which traits best predicted genetic plant variation in herbivore and predator density. In addition to these common garden studies, we sampled arthropod densities from wild-grown plants along the coast.
Results/Conclusions
Within our common garden we found clinal genetic variation in most plant traits. With respect to water relations, southern populations (relative to northern) had lower specific leaf area, smaller leaves and lower leaf nitrogen and water content. With respect to herbivore resistance, southern populations had higher C:N ratios and increased terpene concentrations, as well as lower herbivore and predator densities. Sampling of wild-grown plants revealed a parallel cline of southward decreasing herbivore and predator density, demonstrating that the genetic variation documented within our common garden was expressed and relevant within a natural setting. Water relations traits were best predicted by the mean annual temperature of population source sites, whereas resistance herbivore traits were not correlated with variation in any aspect of the population source site abiotic environment. Plant genetic variation in herbivore (and predator) density was best predicted not by resistance traits, but rather by water relations traits, possibly because of variation in leaf nitrogen. In summary, plant adaptation to a coastal aridity cline results in variation in plant water relations traits and herbivore and predator abundance. In this way, the abiotic environment indirectly drives arthropod community structure through direct effects on plant traits.